what is d meaning of photosynthesis

ENCYCLOPEDIC ENTRY

Photosynthesis.

Photosynthesis is the process by which plants use sunlight, water, and carbon dioxide to create oxygen and energy in the form of sugar.

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Photosynthesis (Google doc)

Most life on Earth depends on photosynthesis .The process is carried out by plants, algae, and some types of bacteria, which capture energy from sunlight to produce oxygen (O 2 ) and chemical energy stored in glucose (a sugar). Herbivores then obtain this energy by eating plants, and carnivores obtain it by eating herbivores.

The process

During photosynthesis, plants take in carbon dioxide (CO 2 ) and water (H 2 O) from the air and soil. Within the plant cell, the water is oxidized, meaning it loses electrons, while the carbon dioxide is reduced, meaning it gains electrons. This transforms the water into oxygen and the carbon dioxide into glucose. The plant then releases the oxygen back into the air, and stores energy within the glucose molecules.

Chlorophyll

Inside the plant cell are small organelles called chloroplasts , which store the energy of sunlight. Within the thylakoid membranes of the chloroplast is a light-absorbing pigment called chlorophyll , which is responsible for giving the plant its green color. During photosynthesis , chlorophyll absorbs energy from blue- and red-light waves, and reflects green-light waves, making the plant appear green.

Light-dependent Reactions vs. Light-independent Reactions

While there are many steps behind the process of photosynthesis, it can be broken down into two major stages: light-dependent reactions and light-independent reactions. The light-dependent reaction takes place within the thylakoid membrane and requires a steady stream of sunlight, hence the name light- dependent reaction. The chlorophyll absorbs energy from the light waves, which is converted into chemical energy in the form of the molecules ATP and NADPH . The light-independent stage, also known as the Calvin cycle , takes place in the stroma , the space between the thylakoid membranes and the chloroplast membranes, and does not require light, hence the name light- independent reaction. During this stage, energy from the ATP and NADPH molecules is used to assemble carbohydrate molecules, like glucose, from carbon dioxide.

C3 and C4 Photosynthesis

Not all forms of photosynthesis are created equal, however. There are different types of photosynthesis, including C3 photosynthesis and C4 photosynthesis. C3 photosynthesis is used by the majority of plants. It involves producing a three-carbon compound called 3-phosphoglyceric acid during the Calvin Cycle, which goes on to become glucose. C4 photosynthesis, on the other hand, produces a four-carbon intermediate compound, which splits into carbon dioxide and a three-carbon compound during the Calvin Cycle. A benefit of C4 photosynthesis is that by producing higher levels of carbon, it allows plants to thrive in environments without much light or water. The National Geographic Society is making this content available under a Creative Commons CC-BY-NC-SA license . The License excludes the National Geographic Logo (meaning the words National Geographic + the Yellow Border Logo) and any images that are included as part of each content piece. For clarity the Logo and images may not be removed, altered, or changed in any way.

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photosynthesis

Definition of photosynthesis

Did you know.

Photosynthesis Has Greek Roots

The Greek roots of photosynthesis combine to produce the basic meaning "to put together with the help of light". Photosynthesis is what first produced oxygen in the atmosphere billions of years ago, and it's still what keeps it there. Sunlight splits the water molecules (made of hydrogen and oxygen) held in a plant's leaves and releases the oxygen in them into the air. The leftover hydrogen combines with carbon dioxide to produce carbohydrates, which the plant uses as food—as do any animals or humans who might eat the plant.

Examples of photosynthesis in a Sentence

These examples are programmatically compiled from various online sources to illustrate current usage of the word 'photosynthesis.' Any opinions expressed in the examples do not represent those of Merriam-Webster or its editors. Send us feedback about these examples.

Word History

1898, in the meaning defined above

Dictionary Entries Near photosynthesis

photosynthate

photosynthetic ratio

Cite this Entry

“Photosynthesis.” Merriam-Webster.com Dictionary , Merriam-Webster, https://www.merriam-webster.com/dictionary/photosynthesis. Accessed 19 Mar. 2024.

Kids Definition

Kids definition of photosynthesis, medical definition, medical definition of photosynthesis, more from merriam-webster on photosynthesis.

Nglish: Translation of photosynthesis for Spanish Speakers

Britannica.com: Encyclopedia article about photosynthesis

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Biology Article

Photosynthesis

Photosynthesis is a process by which phototrophs convert light energy into chemical energy, which is later used to fuel cellular activities. The chemical energy is stored in the form of sugars, which are created from water and carbon dioxide.

Table of Contents

What is Photosynthesis?
Site of photosynthesis

Photosynthesis definition states that the process exclusively takes place in the chloroplasts through photosynthetic pigments such as chlorophyll a, chlorophyll b, carotene and xanthophyll. All green plants and a few other autotrophic organisms utilize photosynthesis to synthesize nutrients by using carbon dioxide, water and sunlight. The by-product of the photosynthesis process is oxygen.Let us have a detailed look at the process, reaction and importance of photosynthesis.

What Is Photosynthesis in Biology?

The word “ photosynthesis ” is derived from the Greek words phōs (pronounced: “fos”) and σύνθεσις (pronounced: “synthesis “) Phōs means “light” and σύνθεσις means, “combining together.” This means “ combining together with the help of light .”

Photosynthesis also applies to other organisms besides green plants. These include several prokaryotes such as cyanobacteria, purple bacteria and green sulfur bacteria. These organisms exhibit photosynthesis just like green plants.The glucose produced during photosynthesis is then used to fuel various cellular activities. The by-product of this physio-chemical process is oxygen.

A visual representation of the photosynthesis reaction

Photosynthesis is also used by algae to convert solar energy into chemical energy. Oxygen is liberated as a by-product and light is considered as a major factor to complete the process of photosynthesis.
Photosynthesis occurs when plants use light energy to convert carbon dioxide and water into glucose and oxygen. Leaves contain microscopic cellular organelles known as chloroplasts.
Each chloroplast contains a green-coloured pigment called chlorophyll. Light energy is absorbed by chlorophyll molecules whereas carbon dioxide and oxygen enter through the tiny pores of stomata located in the epidermis of leaves.
Another by-product of photosynthesis is sugars such as glucose and fructose.
These sugars are then sent to the roots, stems, leaves, fruits, flowers and seeds. In other words, these sugars are used by the plants as an energy source, which helps them to grow. These sugar molecules then combine with each other to form more complex carbohydrates like cellulose and starch. The cellulose is considered as the structural material that is used in plant cell walls.

Where Does This Process Occur?

Chloroplasts are the sites of photosynthesis in plants and blue-green algae. All green parts of a plant, including the green stems, green leaves, and sepals – floral parts comprise of chloroplasts – green colour plastids. These cell organelles are present only in plant cells and are located within the mesophyll cells of leaves.

Also Read: Photosynthesis Early Experiments

Photosynthesis Equation

Photosynthesis reaction involves two reactants, carbon dioxide and water. These two reactants yield two products, namely, oxygen and glucose. Hence, the photosynthesis reaction is considered to be an endothermic reaction. Following is the photosynthesis formula:

Unlike plants, certain bacteria that perform photosynthesis do not produce oxygen as the by-product of photosynthesis. Such bacteria are called anoxygenic photosynthetic bacteria. The bacteria that do produce oxygen as a by-product of photosynthesis are called oxygenic photosynthetic bacteria.

Structure Of Chlorophyll

The structure of Chlorophyll consists of 4 nitrogen atoms that surround a magnesium atom. A hydrocarbon tail is also present. Pictured above is chlorophyll- f, which is more effective in near-infrared light than chlorophyll- a

Chlorophyll is a green pigment found in the chloroplasts of the plant cell and in the mesosomes of cyanobacteria. This green colour pigment plays a vital role in the process of photosynthesis by permitting plants to absorb energy from sunlight. Chlorophyll is a mixture of chlorophyll- a and chlorophyll- b .Besides green plants, other organisms that perform photosynthesis contain various other forms of chlorophyll such as chlorophyll- c1 , chlorophyll- c2 , chlorophyll- d and chlorophyll- f .

Process Of Photosynthesis

At the cellular level, the photosynthesis process takes place in cell organelles called chloroplasts. These organelles contain a green-coloured pigment called chlorophyll, which is responsible for the characteristic green colouration of the leaves.

As already stated, photosynthesis occurs in the leaves and the specialized cell organelles responsible for this process is called the chloroplast. Structurally, a leaf comprises a petiole, epidermis and a lamina. The lamina is used for absorption of sunlight and carbon dioxide during photosynthesis.

Structure of Chloroplast. Note the presence of the thylakoid

“Photosynthesis Steps:”

During the process of photosynthesis, carbon dioxide enters through the stomata, water is absorbed by the root hairs from the soil and is carried to the leaves through the xylem vessels. Chlorophyll absorbs the light energy from the sun to split water molecules into hydrogen and oxygen.
The hydrogen from water molecules and carbon dioxide absorbed from the air are used in the production of glucose. Furthermore, oxygen is liberated out into the atmosphere through the leaves as a waste product.
Glucose is a source of food for plants that provide energy for growth and development , while the rest is stored in the roots, leaves and fruits, for their later use.
Pigments are other fundamental cellular components of photosynthesis. They are the molecules that impart colour and they absorb light at some specific wavelength and reflect back the unabsorbed light. All green plants mainly contain chlorophyll a, chlorophyll b and carotenoids which are present in the thylakoids of chloroplasts. It is primarily used to capture light energy. Chlorophyll-a is the main pigment.

The process of photosynthesis occurs in two stages:

Light-dependent reaction or light reaction
Light independent reaction or dark reaction

Stages of Photosynthesis in Plants depicting the two phases – Light reaction and Dark reaction

Light Reaction of Photosynthesis (or) Light-dependent Reaction

Photosynthesis begins with the light reaction which is carried out only during the day in the presence of sunlight. In plants, the light-dependent reaction takes place in the thylakoid membranes of chloroplasts.
The Grana, membrane-bound sacs like structures present inside the thylakoid functions by gathering light and is called photosystems.
These photosystems have large complexes of pigment and proteins molecules present within the plant cells, which play the primary role during the process of light reactions of photosynthesis.
There are two types of photosystems: photosystem I and photosystem II.
Under the light-dependent reactions, the light energy is converted to ATP and NADPH, which are used in the second phase of photosynthesis.
During the light reactions, ATP and NADPH are generated by two electron-transport chains, water is used and oxygen is produced.

The chemical equation in the light reaction of photosynthesis can be reduced to:

2H 2 O + 2NADP+ + 3ADP + 3Pi → O 2 + 2NADPH + 3ATP

Dark Reaction of Photosynthesis (or) Light-independent Reaction

Dark reaction is also called carbon-fixing reaction.
It is a light-independent process in which sugar molecules are formed from the water and carbon dioxide molecules.
The dark reaction occurs in the stroma of the chloroplast where they utilize the NADPH and ATP products of the light reaction.
Plants capture the carbon dioxide from the atmosphere through stomata and proceed to the Calvin photosynthesis cycle.
In the Calvin cycle , the ATP and NADPH formed during light reaction drive the reaction and convert 6 molecules of carbon dioxide into one sugar molecule or glucose.

The chemical equation for the dark reaction can be reduced to:

3CO 2 + 6 NADPH + 5H 2 O + 9ATP → G3P + 2H+ + 6 NADP+ + 9 ADP + 8 Pi

* G3P – glyceraldehyde-3-phosphate

Calvin photosynthesis Cycle (Dark Reaction)

Also Read: Cyclic And Non-Cyclic Photophosphorylation

Importance of Photosynthesis

Photosynthesis is essential for the existence of all life on earth. It serves a crucial role in the food chain – the plants create their food using this process, thereby, forming the primary producers.
Photosynthesis is also responsible for the production of oxygen – which is needed by most organisms for their survival.

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Photosynthesis

Photosynthesis n., plural: photosyntheses [ˌfŏʊ.ɾoʊ.ˈsɪn̪.θə.sɪs] Definition: the conversion of light energy into chemical energy by photolithorophs

Table of Contents

Photosynthesis is a physio-chemical process carried out by photo-auto-lithotrophs by converting light energy into chemical energy . Among the endless diversity of living organisms in the world, producers are a unique breed.

Unlike consumers ( herbivores , carnivores , omnivores , or decomposers ) that rely upon other living organisms for their nutritional requirements and nourishment, producers have been distinguished by their ability to synthesize their own food. This is the reason that we call producers “autotrophic or self-reliable” in nature while consumers of all the different categories are called “heterotrophic or dependent” in nature.

Now among producers, there are different categories of producers, i.e. different mechanisms via which they produce their own food.

Photo-auto-litho-trophs: Since these organisms tend to derive their nutrition by channeling the sun’s light energy, they are termed phototrophic in nature. Also, since they utilize inorganic carbon and translate it into organic carbon atoms, i.e. their means of deriving food becomes autotrophic. Additionally, since the source of electrons (electron donors) here are inorganic compounds, they are specified as lithotrophic . In totality, they can be called photo-auto-litho-trophic in nature. Example : Green plants are nature’s brilliant entities that come under this category. They carry out a photosynthesis cycle by taking in carbon dioxide and fixing it into carbohydrates (energy storage molecule). Some of them also give out oxygen gas that’s vital for the other life forms to survive in the earth’s atmosphere.
Chemo-auto-lithotrophs: Many of us might be unaware of the fact that there are some autotrophs that don’t utilize sunlight. Rather they derive their energy stored from a different energy source like oxidation of inorganic compounds.

The scope of today’s discussion is limited to photosynthesis and photoautotrophs. So, let’s get started and get to know the answers to these common questions: what is the photosynthesis process, what are the 3 stages of photosynthesis, what does photosynthesis produce, what is a byproduct of photosynthesis, what is the purpose of photosynthesis, is photosynthesis a chemical change, the various inputs and outputs of photosynthesis, which organisms perform photosynthesis , and many other more questions!!!

photolithotroph pioneer species features

What is Photosynthesis?

Photosynthesis definition: Photosynthesis is a physio-chemical process carried out by photo-auto-lithotrophs . In simpler language, photosynthesis is the process by which green plants convert light energy into ‘chemical energy’.

This energy transformation is only possible due to the presence of the miraculous pigment molecule chlorophyll in photosynthesis. The chemical energy as referred to before is the fixed carbon molecules generated during photosynthesis.

Green plants and algae have the ability to utilize carbon dioxide molecules and water and produce food (carbohydrates) for all life forms on Earth. There’s no doubt in the fact that life is impossible and unimaginable without green plants that photosynthesize and sustain the cycles of life.

Let’s give you a brief outline of the topic before we head forward.

Etymology: The photosynthesis process finds its origin in 2 Greek words, firsts one being “phōs (φῶς)” meaning ‘light’ and the second one being “sunthesis (σύνθεσις)” meaning ‘putting together’ . The process of photosynthesis aids the conversion of light energy to chemical energy in varied forms of carbohydrate molecules like sugar molecules and starches.
Organisms that perform photosynthesis: The organisms are called photo-auto-litho-trophs or simply photoautotrophs.
Atmospheric gas consumed: Photosynthesizing organisms utilize carbon dioxide in photosynthesis (CO 2 ).
Atmospheric gas released by “some” photosynthetic organisms (MIND IT-Not all): Some photosynthesizing organisms convert carbon dioxide and aid the process of producing oxygen gas (O2).
Examples of photosynthesizing organisms: Green plants, cyanobacteria (earlier termed as blue-green algae), and different types of algae that essentially carry out phytoplankton photosynthesis.
Why is photosynthesis important? The important function of photosynthesis: Food supply for the organisms on Earth, Oxygen supply for the survival of all organisms.
Site of photosynthesis: Leaves and green tissues. (So when asked where photosynthesis takes place, we can tell that it is this site.)
What are the reactants of photosynthesis: Carbon dioxide molecules + Water molecules + Light energy
Products of photosynthesis: Fixed carbon (carbohydrates) + Oxygen (some cases) + Water

Watch this vid about photosynthesis:

Biology Definition: Photosynthesis is the synthesis of complex organic material using carbon dioxide , water , inorganic salts , and light energy (from sunlight) captured by light-absorbing pigments , such as chlorophyll and other accessory pigments . Photosynthesis may basically be simplified via this equation: 6CO 2 +12H 2 O+energy=C 6 H 12 O 6 +6O 2 +6H 2 O, wherein carbon dioxide (CO 2 ), water (H 2 O), and light energy are utilized to synthesize an energy-rich carbohydrate like glucose (C 6 H 12 O 6 ). Other products are water and oxygen .

Photosynthesis occurs in plastids (e.g. chloroplasts ), which are membrane-bounded organelles containing photosynthetic pigments (e.g. chlorophyll ), within the cells of plants and algae .
In photosynthetic bacteria ( cyanobacteria ) that do not have membrane-bounded organelles, photosynthesis occurs in the thylakoid membranes in the cytoplasm .

Etymology: from the Greek photo-, “light”, and synthesis, “putting together” Related forms: photosynthetic (adjective) Compare: chemosynthesis See also: photoautotroph

Types of Photosynthesis

Plant photosynthesis and photosynthetic organisms can be classified under different categories on the basis of some characteristic features. They are:

Types of organisms that carry out photosynthesis on the basis of “cellular structure” Both prokaryotic and eukaryotic organisms carry out photosynthesis.
Photosynthetic prokaryotes: for example, cyanobacteria
Eukaryotic: for example, protists ( diatoms , dinoflagellates , Euglena) and green plants. In particular, algae photosynthesis can be observed in green algae , red algae , brown algae , & land plants, like bryophytes , pteridophytes, gymnosperms , and angiosperms .
Prokaryotic ONLY (anoxygenic photosynthetic bacteria, green sulfur bacteria and purple bacteria)

Cyanobacteria photosynthesis and nitrogen fixation diagram

Photosynthesis: a two-stage process

Photosynthesis is an example of a metabolic process with 2 stages. Both the stages need light (direct or indirect sunlight). Hence, the long-claimed notion of the 2 processes being ‘absolute LIGHT and DARK reactions’ isn’t apt.

Scientific studies have pointed out that even the 2nd stage of photosynthesis requires indirect sunlight. Therefore, rather than classifying the stages as light and dark photosynthesis reactions, we’ll like to classify the 2 stages as follows:

Photochemical Reaction Process: Light energy is converted to ATP ; photophosphorylation process (light-dependent reactions)
Through Calvin cycle: In oxygenic photosynthesis as well as anoxygenic photosynthesis
Through Non-Calvin cycle: Only is some anoxygenic photosynthesis

Evolution of Photosynthesis Process

It is postulated that the very first photosynthetic beings and photosynthesis evolved quite early down the evolutionary timescale of life.

It is also believed that the first photosynthetic beings would have initially resorted to other available reducing agents like hydrogen ions or hydrogen sulfide in contrast to the modern-day photosynthetic organisms that utilize water as the “prime and only sources of electrons”.

It is believed that cyanobacteria would have appeared on the surface of Earth much later than the first photosynthetic beings. Once appeared they must have saturated the Earth’s atmosphere with oxygen gas and led to its oxygenation. Only after the Earth was oxygenated, the more complex forms of life would have later evolved.

When we compare photosynthesis to other metabolic processes like respiration, we can clearly notice that these two processes are almost opposite to each other. But another point to note is that both the processes in synchrony sustain life on Earth.

You cannot separate respiration from photosynthesis or photosynthesis from respiration and expect life to run normally. It is not possible that way. Let’s try to compare and list some characteristic features of photosynthesis and cellular respiration processes.

Photosynthesis vs. Respiration

Photosynthesis: Anabolic process
Cellular respiration: Catabolic process By anabolic, we mean the photosynthesis process “utilizes energy to build biomolecules” like carbohydrates, starch, and sugars. These biomolecules are further utilized by both the plants and the organisms dependent on plants for their nutritional needs. On the other hand, respiration is a catabolic process. This energy is utilized to break down complex molecules to derive nutrition out of them.
Photosynthesis: In the chloroplasts of the eukaryotic phototrophic cells.
Respiration: Primarily in the mitochondria of the cell.
Photosynthesis: Carbon dioxide molecules + Water molecules + Light energy
Respiration: Glucose + Oxygen
Photosynthesis: Fixed carbon (carbohydrates) + Oxygen (some cases) + Water
Respiration: Carbon dioxide + Water +energy (ATP)
Photosynthesis: Endergonic and endothermic
Respiration: Exergonic and exothermic Just note that these terms endergonic and endothermic both convey the same meaning of “absorbing heat”. And the terms exergonic and exothermic also convey the same meaning of “releasing heat”. The only difference is that –gonics relates to “the relative change in the free energy of the system” while –thermic relates to “the relative change in enthalpy of the system”.
Photosynthesis: 6CO 2 + 6H 2 O → C6H 12 O 6 + 6O 2
Respiration: C 6 H 12 O 6 6 + 6O 2 → 6CO 2 + 6H 2 O

Photosynthetic Membranes and Organelles

When we begin the discussion on this topic, it’s important that we know that no photosynthesis is possible without the pigment molecules that absorb light. The absorption of sunlight is the most vital step of photosynthesis.

We should also note that the energy of photons is different for every light of different wavelengths. And the energy needed for the photosynthesis to be conducted is of “a very specific wavelength range”.

For the absorption of lights of desired wavelengths, phototrophs organize their pigment molecules in the form of reaction center proteins . These proteins are located in the membranes of the organisms. Let’s learn how these pigment molecules reside inside the organism and how they make the membranes photosynthetic in nature.

Prokaryotic photosynthetic organisms: These organisms have their pigment systems or photosystems located in the cell membranes or the thylakoid membranes in the cytosol itself. There are no special organelles called chloroplasts in the prokaryotes.

Eukaryotic photosynthetic organisms (like green plants): These organisms have their pigment systems or photosystems located in the thylakoids of the chloroplast membranes. Eukaryotes have specialized organelles called chloroplasts (chlorophyll-containing plastids) in their cells.

Photosynthetic Pigments

There are 2 types of photosynthetic pigments in the oxygenic photosynthesizing organisms . They are as follows:

Porphyrin-derivatives (Chlorophyll in plants and Phycobilin)

Carotenoids

Chlorophyll.

Chlorophyll is the green-colored pigment essential for photosynthesis. Let’s try to list its major characteristic features and roles of it.

Nature: Lipid
Location: Embedded in the thylakoid membrane
Types: 9 types as identified by Arnoff and Allen in 1966 (chlorophyll-a, b, c, d, e, bacteriochlorophyll a, b, chlorobium chlorophyll-650,666). Bacteriochlorophylls are present in the anoxygenic photosynthetic organisms.
Primary photosynthetic pigment: Chlorophyll-a
Presence: In all oxygenic photosynthetic organisms
Absorption range: Visible (blue and red) and IR (Infra-red)
Ion important for its biological functioning: Magnesium ion (Mg 2+ )
Structure: Chlorophyll-a, b, and d are “ chlorin ” derivatives; c is a “ porphyrin ” derivative.
Chlorophyll Tail: Oxygenic photosynthetic organisms have a “ phytol ” tail in their chlorophyll; anoxygenic photosynthetic organisms have a “ geranyl ” tail in their bacteriochlorophylls.
Main pigment for capturing and storing solar energy
Photochemical reaction (chlorophyll-a is present in the photochemical reaction center i.e. PCRC. Chlorophyll a, b, c, and d play a role in resonance energy transfer.)

Carotenoid is the photosynthetic pigment essential for working in conjunction with chlorophyll. Let’s try to list its major characteristic features and roles of it.

Nature: Lipid-soluble
Types: More than 150
Absorption range: 400-500nm
Forms: Carotene (simple hydrocarbon, for example, beta carotene) and xanthophyll (oxygenated hydrocarbon, for example, lutein)
In excitation and resonance energy transfer
Photo-protection (work as a free-radical scavenger as well as a quencher)

Phycobilins

Phycobilins aren’t present in all the oxygenic photosynthetic organisms. They have a tetrapyrrole structure (no need for magnesium ion).

Types: Phycoerythrobilin, Phycocyanobilins, Allophycocyanobilins When these pigment molecules combine with a water-soluble protein, they form the pigment-protein complex (phycobiliproteins, like phycoerythrin and phycocyanin).
Location: Since these phycobiliproteins are water-soluble, they can’t exist in the membranes like chlorophyll and carotenoids. Therefore, phycobilin pigments as their pigment-protein complex aggregate into clusters and adhere to the membrane. These clusters are called phycobilisomes .
Exceptional Note: These are the only pigments that are associated with protein molecules.
Role: Resonance energy transfer

Organelle for Photosynthesis

What is chloroplast? In eukaryotes, photosynthesis occurs in chloroplasts as they are the designated organelles for the photosynthesis process. There are nearly 10-100 chloroplasts in a typical plant cell .

Inside chloroplasts are the thylakoids; the very specific site for the light capturing. The structure of this very unique part of the chloroplasts is briefly discussed here.

Thylakoid is a membrane-bound compartment in the chloroplasts of eukaryotic organisms. They are also present as such in the cytosol of cyanobacteria (cyanobacteria don’t have chloroplasts but they have simply thylakoids).

These thylakoids are the “primary site of the 1st stage of photosynthesis. i.e. “photochemical reaction” or popularly called “light-dependent reactions of photosynthesis”. The main components of the thylakoid are membrane, lumen, and lamellae. The chlorophyll molecules are present inside these thylakoid membranes.

Light-dependent Reactions

The first stage of photosynthesis is popularly called “light-dependent reactions” . We choose to call this stage the “1st stage: PHOTOCHEMICAL REACTION STAGE”. It is also called the “thylakoid reaction stage” or “hill’s reaction” .

This stage is marked by 3 essential steps of photosynthesis: Oxidation of water , reduction of NADP + , and ATP formation . The site where these reactions occur is the lamellar part of the chloroplast. The units of light-dependent reactions are quantosomes .

Let’s discuss this stage under some subheadings:

Wavelengths of light involved and their absorption

The white light that reaches Earth has subparts of different wavelengths together constituting the visible spectrum (390-760nm). But the photosynthetic organisms specifically use a subpart called PAR ( P hotosynthetically A ctive R adiation).

PAR ranges from 400-760nm. Blue light is 470-500nm while red light is 660-760nm). The green light (500-580nm) is reflected back by the plants and this is the reason that plants appear green in color. Blue-green light is not used, only blue light is used.

Absorption spectrum and action spectrum

Absorption Spectrum: This is a pigment-specific entity or terminology. To find the absorption spectrum of a pigment, you need to plot “the amount of absorption of different wavelengths of light by that particular pigment” . The graph has the “wavelengths of light (in nanometers/nm)” on the X-axis and the “percentage of light absorption” on the Y-axis.

Action Spectrum: To find the action spectrum of a pigment, you need to plot the “effectiveness of the different wavelengths of light in stimulating photosynthesis process” . The graph has the “wavelengths of light (in nanometers/nm)” on the X-axis and the “rate of photosynthesis (measured as oxygen released)” on the Y-axis. When you superimpose the action spectrum of photosynthesis with the absorption spectrum of the specific pigment, you can find the contribution of each different wavelength in the photosynthesis rate, photosynthetic efficiency, and photosynthetic productivity.

IMPORTANT NOTE: The absorption spectrum is calculated for any of the many pigments involved in photosynthesis. Contrastingly, the action spectrum is calculated only for the photochemical reaction performing pigment i.e. chlorophyll-a present at the reaction center. We identify the progress of photochemical reactions as the “evolution of oxygen gas” that primarily happens at the reaction center where only chlorophyll-a is present. Since the action is directly correlated to the specific excitation of chlorophyll-a molecule, the action spectrum is scientifically calculated only for this chlorophyll-a.

Absorption spectrum of chlorophyll- a : 430 nm (blue), 660nm (red) {more absorbance at 660 nm)
Absorption spectrum of chlorophyll-b: 430 nm (blue), 660nm (red) {more absorbance at 430 nm)

What actually happens in the Light-dependent reaction

Let’s briefly describe what actually happens here.

1 photon is absorbed by 1 molecule of the chlorophyll (P680) and simultaneously 1 electron is lost here.
The electron flow of the photochemical reaction begins here.
The electron is transferred to D1/D2 protein, then to a modified form of chlorophyll and “pheophytin”.
After that, it’s transferred to plastoquinone A and then B.
Initiates an electron flow down an electron transport chain.
Ultimately aids the NADP reduction to NADPH.
Creation of a proton gradient across the chloroplast membrane.
Further on this proton gradient is exploited by the ATP synthase for the generation of ATP molecules.

Water photolysis

Now, if you are wondering how the first electron lost by the 1st chlorophyll is replenished to keep this cycle going, read on. The answer to this query is “photolysis of water molecules” . The chlorophyll molecule regains the lost electron when the “oxygen-evolving complex” in the thylakoid membrane carries out the photolysis of water. The chlorophyll molecule ultimately regains the electron it lost when a water molecule is split in a process called photolysis, which releases oxygen.

Many scientists had a doubt about the source of oxygen in photosynthesis. Some speculated the oxygen atom of the CO 2 gas is the source of oxygen post-photosynthesis. But it was the collective contribution of some 4 scientists that gave clarity on this topic.

C.B. Van Niel worked on purple photosynthetic bacteria ( Chromatium vinosum ) and found out that the source of oxygen is the oxidation of water molecules (‘indirect evidence’). While Ruben, Hassid, and Kamen carried out an isotopic study that gave ‘direct evidence’ of oxygen-evolving from H 2 O molecules and not CO 2 molecules.

Hydrolysis of 2 molecules of water leads to the evolution of 1 molecule of oxygen gas. The photosynthesis equation for light-dependent reactions (non-cyclic electron flow) or the chemical formula for photosynthesis:

2 H 2 O + 2 NADP+ + 3 ADP + 3 Pi + light → 2 NADPH + 2 H+ + 3 ATP + O 2

The photochemical reaction (or the light-dependent reactions) can be classified as:

Cyclic reaction: Only 1 photosystem ( PS1 ) is involved. (Photon excites P700 in PS1, electron reaches Fe-S, then Ferredoxin, then Plastoquinone and then Cyt b6f complex and then Plastocyanin). Since in the solo involvement of PS1 here, the electron flow becomes cyclic. And this phosphorylation process is called cyclic phosphorylation. It happens in the stroma lamellae when light beyond 680nm is available.
Non-cyclic reaction: Both photosystems (PS1 and PS2 ) are involved. (Photon excites P680 in PS2, the electron is lost and transferred to pheophytin, then sent on a roller coaster (Z-scheme). Within the z-scheme, the final redox reaction enables the reduction of NADP+ to NADPH. And the chemiosmotic potential generation via proton pumping proton across the membrane and into the thylakoid lumen ensures ATP synthesis.

Data Source: Akanksha Saxena of Biology Online

Light-Independent Reactions (Carbon-fixation Reaction)

Also called the carbon fixation process, the “light-independent reactions” is a misnomer as Science has now already proved that the second stage of photosynthesis isn’t really light-independent reactions. Though it doesn’t need direct light, indirect light is involved even in this process. We choose to label this stage of photosynthesis as the “2nd stage: CARBON-FIXATION REACTION STAGE ”, which is also called:

Calvin Cycle or “stromal reaction” as it manifests in the stroma part of the chloroplast
“C3 Cycle” or the “reductive pentose phosphate cycle”

Calvin cycle

The inputs for the Calvin cycle in most plants come from the previously occurred photochemical reaction. In this cycle, the carbon dioxide produced is fixed to a glucose molecule. To be very specific, the Calvin cycle directly doesn’t produce glucose, rather it produces glyceraldehydes-5-phosphate (G-3-P). Glucose is formed after these G-3-P molecules move into the cytosol from the chloroplast .

It consists of primarily 3 steps as follows:

Carboxylation: Acceptance of CO 2 by RuBP which is a 5-carbon compound and the CO2-acceptor). 2 molecules of 3-phosphoglycerate are generated as the result of the carboxylation process.
Reduction: Generation of 3C/4C/5C/6C/7C molecules.
Regeneration of RUBP: 3 molecules of RuBP are regenerated.

In totality, 3 molecules of CO 2 produce 1 molecule of G-3-P. This uses 9 ATPs and 6 NADPHs. And, 6 molecules of CO 2 produce 2 molecules of G-3-P which further produce 1 molecule of glucose. This uses 18ATPs and 12 NADPHs.

The main enzyme is RuBisCo . It’s a multi-enzyme complex with 8 large and 8 small subunits. The substrates for this enzyme are CO 2 , O 2 , and RuBP. An essential ion for the biological functioning of this enzyme: Mg 2+ . The role of RuBisCo is that it captures carbon dioxide gas from the atmosphere and utilizes the NADPH from the 1st stage (photochemical reaction/light-dependent reaction stage) to fix the CO 2 .

The equation of dark reaction of photosynthesis/light-independent reaction stage/2nd stage is: 3 CO 2 + 9 ATP + 6 NADPH + 6 H + → C 3 H 6 O 3 -phosphate + 9 ADP + 8 Pi + 6 NADP+ + 3 H 2 O

The simple carbon sugars formed via the C3 cycle are utilized by the biological systems to form complex organic compounds like cellulose, precursors for amino acids synthesis and thereby proteins, precursors for lipids, and the source of fuel for respiration.

Important Point To Note: It happens in all the photosynthetic organisms as the basic carbon-fixation step.

Carbon concentrating mechanisms

There are many carbon concentrating mechanisms to increase the carbon dioxide levels and the carbon fixation process like C4, CAM, etc.

Doesn’t happen in all photosynthetic organisms. Rather it happens in conjunction with the C3 cycle in some 4% of angiosperm families.
Most commonly angiosperm families that witness C4 cycle: Poaceae, Cyperaceae.
First explained by: Hatch and Slack (hence also called the Hatch and Slack cycle). They worked on the maize plant.
Role: Endow the ability to efficiently conduct photosynthesis in plants of the semi-arid regions by making them well adapted.
Mechanism: By separation of photosynthesis stages in 2 types of cells (mesophyll cells and bundle sheath cells). The light reaction is restricted to the mesophyll cells and the CO 2 fixation happens in the bundle sheath cells. This phenomenon is also termed as “chloroplast dimorphism” in C4 plants. The Kranz anatomy is visible here.
Why does the need arise in the first place? – In semi-arid regions or regions with very hot and dry environmental conditions, plants are forced to close their stomata in order to limit water loss. Under such harsh conditions, the intake of CO 2 decreases during the day as the stomata are forced closed. This might lead to no CO 2 intake and hence no CO 2 fixation (2nd stage of photosynthesis). But the 1st stage of photosynthesis keeps running as it doesn’t depend on stomata opening or closure. This means that a continuous oxygen evolution happens which can lead to oxygen saturation. As we know that RuBisCo enzymes use O 2 gas as substrate too, and this can lead to an increased rate of photorespiration by the oxygenase activity of RuBisCo. This further decreases the carbon fixation. This is a very big issue if not resolved. Hence, for situations like these, carbon concentrating mechanisms have evolved in some families of plants to concentrate and enrich the CO 2 concentration in the leaves of these plants under such conditions.
Important enzyme for CO 2 concentration: PEP carboxylase
CO 2 is first added to a three-carbon compound called phosphoenolpyruvate (PEP) in this cycle. This leads to the formation of a four-carbon (4C) molecule called oxaloacetic acid or malate. This step happens in the mesophyll cells of the leaves.
After that, these 4C compounds are transferred to the bundle sheath cells where the normal C3 cycle fixes them into glucose molecules.
This CO 2 concentrating mechanism works on the “principle of separating the RuBisCo enzyme from the O 2 -generating photochemical reactions” in order to reduce the rates of photorespiration and simultaneously increase the rates of CO 2 fixation.
This increases the photosynthetic capacity of the leaf/leaf photosynthesis.
When the high light and high-temperature conditions are dominant, C4 plants prove more photosynthetically efficient than C3 plants as they produce more sugar molecules in such conditions.
Examples of C4 plants: Many crop plants like wheat, maize, rice, sorghum, millet, and sugarcane.
Number of ATPs required: 12 (for C-enrichment) + 18 (for C-fixation)= 30 ATPS for 1 glucose production
Number of NADPH required: 18 NADPH for 1 glucose production
Some plants resort to another mechanism called the CAM cycle in conjunction with the C3 cycle to fix carbon dioxide.
Examples: xerophytes like cactus photosynthesis, and most succulents.
Around 16,000 species of plants utilize the CAM mechanism
Mechanism: Utilize PEP carboxylase to capture carbon dioxide. In contrast to the C4 cycle where there is a “spatial separation of the 2 processes of CO 2 reduction to PEP and PEP fixation to glucose”, CAM plants display a “temporal separation of the 2 listed processes”.

Land plants display different types of photosynthesis based on their requirements and environmental constraints. They are C3, C4 +C3, and CAM+ C3 types of photosynthesis.

Aquatic plants and algae display some extra features in the photosynthetic machinery. These features further refine and define the smooth functioning and efficiency of photosynthesis.

Example: Cyanobacteria photosynthesis – cyanobacteria have carboxysomes that help in enriching the concentration of carbon dioxide around the RuBisCO enzyme. This directly increases the photosynthetic rates. The distinguished and specially enabled enzyme in the carboxysomes is called “carbonic anhydrase”. The carbonic anhydrase possesses the ability to evolve and release CO 2 from the dissolved hydrocarbonate ions (HCO-). As soon as the CO 2 is released, RuBisCo takes care that it doesn’t go to waste.

Order and Kinetics

There are innumerable reactions and processes involved in the biological mechanism of photosynthesis. Besides the normal flow of photosynthesis, there are some plant-specific and condition-specific additional steps that further complicate the mechanism.

Since every biological mechanism has a lot of enzymes, factors, cofactors, substrates, and entities involved, photosynthesis is no different.

Let’s try to list some kinetics-specific pointers that may help.

As discussed in the overview and starting of this article, the early photosynthetic organisms must have been primarily “anoxygenic” in nature. These bacteria used some other source than water molecules as their primary electron donors. Even the geological evidence aligns with this fact as the early atmosphere of Earth was highly reducing in nature. Some speculated organisms of the early evolutionary phase are :

Green sulfur bacteria (Electron donor= hydrogen and sulfur)
Purple sulfur bacteria (Electron donor= hydrogen and sulfur)
Green nonsulfur bacteria (Electron donor= various amino and other organic acids)
Purple nonsulfur bacteria (Electron donor= variety of nonspecific organic molecules)

After this, some filamentous photosynthetic organisms are expected to have evolved. This is scaled to be an occurrence of some 3.4 billion years old timeline. It is around 2 million years ago that oxygenic photosynthesis is believed to have evolved.

The modern and more commonly known photosynthesis in plants and most of the photosynthetic prokaryotes= Oxygenic (Electron donor= Water molecules)

Symbiosis and the origin of chloroplasts

There are some animal groups that have the ability to form and establish symbiotic relationships with photosynthetic organisms. By establishing such a relationship, these organisms can directly rely upon their photosynthetic partner for energy and food requirements. Some examples of such animal groups are:

Sea anemones
Marine mollusks (example: Elysia viridis & Elysia chlorotica )
Fungi photosynthesis (Lichens)

When such symbiotic relationships are established, it’s sometimes observed that some genes of the plant cell’s nucleus get transferred to the animal cell . (Observed in some slugs).

Origin of Chloroplasts

Such symbiosis is popularly claimed to be the source of chloroplast evolution. As we notice many similarities between the photosynthetic bacteria and chloroplasts, the evolution of chloroplasts is often hinted to have occurred from these bacteria. Some of the common features between the 2 are:

Circular chromosome
Prokaryotic-type ribosome
A similar set of proteins in the photosynthetic reaction center

It is for all these commonalities the “ endosymbiotic theory ” had been proposed for the evolution of chloroplasts and mitochondria in the eukaryotic cells. According to the endosymbiotic theory, the early eukaryotic cells are believed to have acquired the photosynthetic bacteria by the process of endocytosis). Those early eukaryotic cells after acquiring the photosynthetic bacteria transformed to be self-sustainable and became the “first plant cells”. (Mitochondria photosynthesis is true, they are associated with respiration!)

Photosynthetic eukaryotic lineages

Photosynthetic eukaryotic lineages include:

Glaucophytes
Chlorophytes
Rhodophytes
Cryptophytes (some clades)
Haptophytes (some clades)
Dinoflagellates & chromerids
Euglenids—clade Excavata (unicellular)

Cyanobacteria and the evolution of photosynthesis

Almost all the prokaryotes carry out anoxygenic photosynthesis in contrast to cyanobacteria, which perform oxygenic photosynthesis. This ability to carry out oxygenic photosynthesis is speculated to have evolved at least 2450–2320 million years ago. The first photosynthetic cyanobacteria might not have been oxygenic as Earth’s atmosphere had no oxygen then.

This topic still requires more scientific study to bring out conclusive results. From the paleontological evidence, it is claimed that the 1st cyanobacteria evolved around 2000 Ma.

For the initial years of the Earth’s oxygen-rich environment (after the oxygen-evolving mechanism evolved), cyanobacteria are claimed to be the “principal primary producers of oxygen”. Even to date, cyanobacteria have been proven vital for marine ecosystems. They’re the primary producers of oxygen in oceans.

Cyanobacteria also fix nitrogen electrons fixation and play a role in biological nitrogen cycles.

Experimental History

We will list the long experimental history in deciphering the extensive photosynthesis process through the ages.

Discovery, Refinements, and Development of the concept

Find out the discovery, refinements, and development of photosynthesis as summarized in the table below:

C3 : C4 photosynthesis research

Several studies were conducted using isotopes of radioactive elements to identify the various aspects of the photosynthetic process. A number of organisms like Chlorella , Stellaria media, Cladophora, Spirogyra, Rhodopseudomonas , sulfur bacteria, green plants like maize, etc have been used to understand the photosynthesis process over the years. Gas exchange studies, isotopic studies, light spectrum studies, radioactive studies, plant anatomical and physiological studies, studies involving roles of carbon dioxide and water, etc have all together opened the gates for our deeper understanding of this topic.

The 3 main factors that directly affect the photosynthesis process are:

Light irradiance and wavelength
Carbon dioxide concentration

Temperature

Although there are many more corollary factors, these 3 are the most important ones.

Light intensity (irradiance), wavelength, and temperature

Light is an essential factor for photosynthesis. It directly affects the rate of it. There are 3 different parameters that we should look into:

Sciophytes : Grow under “diffuse” light. Example: Oxalis
Heliophytes: Grow under “direct: light. Example: Dalbergia
Light quality: PAR as previously discussed is the quality or the fraction of light energy that is ‘photosynthetically active’ in nature. It ranges from 400-700nm in wavelength.
Duration of light: This parameter doesn’t affect the rate of photosynthesis but affects the total photosynthetic output.

Carbon dioxide levels and photorespiration

Carbon dioxide concentration is the major factor in determining the rate of photosynthesis. There is no carbon-dioxide enriching system in C3 plants like the C4 plants. So, if you increase the concentration of CO 2 in the system, the photosynthetic rate of C3 plants will increase as the CO 2 concentration increases. On the other hand, the photosynthetic yield of the C4 plant won’t increase in such a scenario.

CO 2 Compensation Point: A stage in CO 2 concentration when there’s no absorption of CO 2 by the illuminated plant part.

Featuring… “The curious case of RuBisCO and PEP Carboxylase”

Imagine an equal concentration (50-50%) of the two isotopes of carbon, C-12 and C-13, in the form of 12CO 2 and 13CO 2 , made available to both C3 and C4 plants. Now, can you tell which isotope of the carbon will be fixed more or less by the two types of photosynthetic organisms? Can you guess if there would be a “preferable” isotope between the two? Do you think C3 plants will fix the 12CO 2 and 13CO 2 equally or unequally? Or do you think the 12CO 2 and 13CO 2 incorporation would have a biased ratio in any of the two (C3/C4 plants)????

The answer to this lies in the major carbon fixing enzyme involved.

C3 plants: Major C-fixing enzyme is RuBisCo and RuBisCo has a “discriminatory ability” to preferably fix 12CO 2 and not 13CO2. Hence, you will find more 12CO 2 fixed than 13CO 2 in the C3 plants.
C4 plants: Major C-fixing enzyme is not RuBisCo but PEP Carboxylase . PEP Carboxylase has “no discriminatory ability”. So, you’ll find an almost equal proportion of 12CO 2 and 13CO 2 getting fixed in C4 plants. So, in comparison to C3 plants, the chances of getting 13CO 2 fixed are more in C4 plants.

Choose the best answer.

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Time is Up!

Rutherford, A.W., Faller, P. (Jan 2003). “Photosystem II: evolutionary perspectives”. Philosophical Transactions of the Royal Society of London. Series B, Biological Sciences. 358 (1429): 245–253. doi:10.1098/rstb.2002.1186. PMC 1693113. PMID 12594932.
Arnon, D.I., Whatley, F.R., Allen, M.B. (1954). “Photosynthesis by isolated chloroplasts. II. Photophosphorylation, the conversion of light into phosphate bond energy”. Journal of the American Chemical Society. 76 (24): 6324–6329. doi:10.1021/ja01653a025.
Ehrenberg, R. (2017-12-15). “The photosynthesis fix”. Knowable Magazine. Annual Reviews. doi:10.1146/knowable-121917-115502. Retrieved 2018-04-03.
El-Sharkawy, M.A., Hesketh, J.D. (1965). “Photosynthesis among species in relation to characteristics of leaf anatomy and CO 2 diffusion resistances”. Crop Sci. 5 (6): 517–521. doi:10.2135/cropsci1965.0011183x000500060010x.
Earl, H., Said Ennahli, S. (2004). “Estimating photosynthetic electron transport via chlorophyll fluorometry without Photosystem II light saturation”. Photosynthesis Research. 82 (2): 177–186. doi:10.1007/s11120-004-1454-3. PMID 16151873. S2CID 291238.

©BiologyOnline.com. Content provided and moderated by Biology Online Editors.

Last updated on July 15th, 2022

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Photosynthesis

What is photosynthesis.

It is the process by which green plants, algae, and certain bacteria convert light energy from the sun into chemical energy that is used to make glucose. The word ‘photosynthesis’ is derived from the Greek word phōs, meaning ‘light’ and synthesis meaning ‘combining together.’

Jan Ingenhousz, the Dutch-born British physician and scientist, discovered the process of photosynthesis.

Where does Photosynthesis Occur

Photosynthesis takes place mainly in the leaves of green plants and also in the stems of herbaceous plants as they also contain chlorophyll. Sometimes it also occurs in roots that contain chlorophyll like in water chestnut and Heart-leaved moonseed. Apart from plants, photosynthesis is also found to occur in blue-green algae.

What Happens During Photosynthesis

It involves a chemical reaction where water, carbon dioxide, chlorophyll, and solar energy are utilized as raw materials (inputs) to produce glucose, oxygen, and water (outputs).

Stages of the Process

Photosynthesis occurs in two stages:

1) The Light-dependent Reaction

Takes place in the thylakoid membranes of chloroplasts only during the day in the presence of sunlight
High-energy phosphate molecules adenosine triphosphate ( ATP ) and the reducing agent NADPH are produced with the help of electron transport chain

2) The Light-independent or Dark Reaction ( Calvin cycle )

Takes place in the stroma of chloroplast in the absence of light that helps to fix carbon
ATP and NADPH produced in the light reaction are utilized along with carbon dioxide to produce sugar in the form of glucose

Factors Affecting the Rate of Photosynthesis

Intensity of Light: The higher intensity of light increases the rate of photosynthesis
Temperature: Warmer the temperature, higher the rate of photosynthesis. The rate is highest between the temperatures of 25° to 35° C, after which it starts to decrease
Concentration of Carbon dioxide: Higher concentration of carbon dioxide increases the rate of photosynthesis until it reaches a certain point, beyond which no further effects are found

Although all the above factors together interact to affect the rate of photosynthesis, each of them individually is also capable of directly influencing the process without the other factors and thus called limiting factors.

Importance of Photosynthesis

It serves two main purposes that are essential to support life on earth:

Producing food for organisms that depend on others for their nutrition such as humans along with all other animals
Synthesizing oxygen by replacing carbon dioxide in the atmosphere

Ans. Photosynthesis is an endothermic reaction because it absorbs the heat of the sun to carry out the process.

Ans. The oxygen in photosynthesis comes from splitting the water molecules.

Ans. Chlorophyll is the main light-absorbing pigment in photosynthesis.

Ans. The role of water is to provide oxygen in the form of oxygen gas to the atmosphere.

Ans. Sunlight is the source of energy that drives photosynthesis.

Ans. The easiest way to measure the rate of photosynthesis is to quantify the carbon dioxide or oxygen levels using a data logger. The rate of photosynthesis can also be measured by determining the increase in the plant ’s biomass (weight).

Ans. Photosynthesis is an energy-requiring process occurring only in green plants, algae, and certain bacteria that utilizes carbon dioxide and water to produce food in the form of carbohydrates. In contrast, cellular respiration is an energy-releasing process found in all living organisms where oxygen and glucose are utilized to produce carbon dioxide and water.

Ans. Glucose produced in photosynthesis is used in cellular respiration to make ATP.

Article was last reviewed on Tuesday, April 21, 2020

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photosynthesis

Quick reference.

The chemical process by which green plants synthesize organic compounds from carbon dioxide and water in the presence of sunlight. It occurs in the chloroplasts (most of which are in the leaves) and there are two principal types of reactions. In the light-dependent reactions, which require the presence of light, energy from sunlight is absorbed by photosynthetic pigments (chiefly the green pigment chlorophyll) and used to bring about the photolysis of water:H 2 O → 2H + +2e − +½O 2 . The electrons released by this reaction pass along a series of electron carriers (see electron transport chain); as they do so they lose their energy, which is used to convert ADP to ATP in the process of photophosphorylation. The electrons and protons produced by the photolysis of water are used to reduce NADP:2H + +2e − +NADP + → NADPH+H + . The ATP and NADPH produced during the light-dependent reactions provide energy and reducing power, respectively, for the ensuing light-independent reactions (formerly called the ‘dark reaction’), which nevertheless cannot be sustained without the ATP generated by the light-dependent reactions. During these reactions carbon dioxide is reduced to carbohydrate in a metabolic pathway known as the Calvin cycle. Photosynthesis can be summarized by the equation:CO 2 +2H 2 O → [CH 2 O]+H 2 O+O 2 . Since virtually all other forms of life are directly or indirectly dependent on plants for food, photosynthesis is the basis for all life on earth. Furthermore virtually all the atmospheric oxygen has originated from oxygen released during photosynthesis.

H 2 O → 2H + +2e − +½O 2 .

2H + +2e − +NADP + → NADPH+H +

CO 2 +2H 2 O → [CH 2 O]+H 2 O+O 2

Photosynthesis

From: photosynthesis in A Dictionary of Chemistry »

Subjects: Science and technology — Chemistry

What is photosynthesis?

Photosynthesis is the process plants, algae and some bacteria use to turn sunlight, carbon dioxide and water into sugar and oxygen.

Photosynthetic processes
Photosynthesis equation
The carbon exchange
How do plants absorb sunlight?
Location of photosynthesis

Light-dependent reactions

The Calvin cycle

Types of photosynthesis

Additional resources.

Photosynthesis is the process used by plants, algae and some bacteria to turn sunlight into energy. The process chemically converts carbon dioxide (CO2) and water into food (sugars) and oxygen . The chemical reaction often relies on a pigment called chlorophyll, which gives plants their green color. Photosynthesis is also the reason our planet is blanketed in an oxygen-rich atmosphere.

Types of photosynthetic processes

There are two types of photosynthesis: oxygenic and anoxygenic. They both follow very similar principles, but the former is the most common and is seen in plants, algae and cyanobacteria.

During oxygenic photosynthesis, light energy transfers electrons from water (H2O) taken up by plant roots to CO2 to produce carbohydrates . In this transfer, the CO2 is "reduced," or receives electrons, and the water is "oxidized," or loses electrons. Oxygen is produced along with carbohydrates.

This process creates a balance on Earth, in which the carbon dioxide produced by breathing organisms as they consume oxygen in respiration is converted back into oxygen by plants, algae and bacteria.

Anoxygenic photosynthesis, meanwhile, uses electron donors that are not water and the process does not generate oxygen, according to "Anoxygenic Photosynthetic Bacteria" by LibreTexts . The process typically occurs in bacteria such as green sulfur bacteria and phototrophic purple bacteria.

The Photosynthesis equation

Though both types of photosynthesis are complex, multistep affairs, the overall process can be neatly summarized as a chemical equation.

The oxygenic photosynthesis equation is:

6CO2 + 12H2O + Light Energy → C6H12O6 + 6O2 + 6H2O

Here, six molecules of carbon dioxide (CO2) combine with 12 molecules of water (H2O) using light energy. The end result is the formation of a single carbohydrate molecule (C6H12O6, or glucose) along with six molecules each of oxygen and water.

Similarly, the various anoxygenic photosynthesis reactions can be represented as a single generalized formula:

CO2 + 2H2A + Light Energy → [CH2O] + 2A + H2O

The letter A in the equation is a variable, and H2A represents the potential electron donor. For example, "A" may represent sulfur in the electron donor hydrogen sulfide (H2S), according to medical and life sciences news site News Medical Life Sciences .

How is carbon dioxide and oxygen exchanged?

Plants absorb CO2 from the surrounding air and release water and oxygen via microscopic pores on their leaves called stomata.

When stomata open, they let in CO2; however, while open, the stomata release oxygen and let water vapor escape. Stomata close to prevent water loss, but that means the plant can no longer gain CO2 for photosynthesis. This tradeoff between CO2 gain and water loss is a particular problem for plants growing in hot, dry environments.

How do plants absorb sunlight for photosynthesis?

Plants contain special pigments that absorb the light energy needed for photosynthesis.

Chlorophyll is the primary pigment used for photosynthesis and gives plants their green color, according to science education site Nature Education . Chlorophyll absorbs red and blue light and reflects green light. Chlorophyll is a large molecule and takes a lot of resources to make; as such, it breaks down towards the end of the leaf's life, and most of the pigment's nitrogen (one of the building blocks of chlorophyll) is resorbed back into the plant, When leaves lose their chlorophyll in the fall, other leaf pigments such as carotenoids and anthocyanins begin to show. While carotenoids primarily absorb blue light and reflect yellow, anthocyanins absorb blue-green light and reflect red light, according to Harvard University's The Harvard Forest .

Pigment molecules are associated with proteins, which allow them the flexibility to move toward light and toward one another. A large collection of 100 to 5,000 pigment molecules constitutes an "antenna," according to an article by Wim Vermaas , a professor at Arizona State University. These structures effectively capture light energy from the sun, in the form of photons.

The situation is a little different for bacteria. While cyanobacteria contain chlorophyll, other bacteria, for example, purple bacteria and green sulfur bacteria, contain bacteriochlorophyll to absorb light for anoxygenic photosynthesis, according to " Microbiology for Dummies " (For Dummies, 2019).

Related: What if humans had photosynthetic skin?

Where in the plant does photosynthesis take place?

Photosynthesis occurs in chloroplasts, a type of plastid (an organelle with a membrane) that contains chlorophyll and is primarily found in plant leaves.

Chloroplasts are similar to mitochondria , the energy powerhouses of cells, in that they have their own genome, or collection of genes, contained within circular DNA. These genes encode proteins that are essential to the organelle and to photosynthesis.

Inside chloroplasts are plate-shaped structures called thylakoids that are responsible for harvesting photons of light for photosynthesis, according to the biology terminology website Biology Online . The thylakoids are stacked on top of each other in columns known as grana. In between the grana is the stroma — a fluid containing enzymes, molecules and ions, where sugar formation takes place.

Ultimately, light energy must be transferred to a pigment-protein complex that can convert it to chemical energy, in the form of electrons. In plants, light energy is transferred to chlorophyll pigments. The conversion to chemical energy is accomplished when a chlorophyll pigment expels an electron, which can then move on to an appropriate recipient.

The pigments and proteins that convert light energy to chemical energy and begin the process of electron transfer are known as reaction centers.

When a photon of light hits the reaction center, a pigment molecule such as chlorophyll releases an electron.

The released electron escapes through a series of protein complexes linked together, known as an electron transport chain. As it moves through the chain, it generates the energy to produce ATP (adenosine triphosphate, a source of chemical energy for cells) and NADPH — both of which are required in the next stage of photosynthesis in the Calvin cycle. The "electron hole" in the original chlorophyll pigment is filled by taking an electron from water. This splitting of water molecules releases oxygen into the atmosphere.

Light-independent reactions: The Calvin cycle

The Calvin cycle is the three-step process that generates sugars for the plant, and is named after Melvin Calvin , the Nobel Prize -winning scientist who discovered it decades ago. The Calvin cycle uses the ATP and NADPH produced in chlorophyll to generate carbohydrates. It takes plate in the plant stroma, the inner space in chloroplasts.

In the first step of this cycle, called carbon fixation, an enzyme called RuBP carboxylase/oxygenase, also known as rubiso, helps incorporate CO2 into an organic molecule called 3-phosphoglyceric acid (3-PGA). In the process, it breaks off a phosphate group on six ATP molecules to convert them to ADP, releasing energy in the process, according to LibreTexts.

In the second step, 3-PGA is reduced, meaning it takes electrons from six NADPH molecules and produces two glyceraldehyde 3-phosphate (G3P) molecules.

One of these G3P molecules leaves the Calvin cycle to do other things in the plant. The remaining G3P molecules go into the third step, which is regenerating rubisco. In between these steps, the plant produces glucose, or sugar.

Three CO2 molecules are needed to produce six G3P molecules, and it takes six turns around the Calvin cycle to make one molecule of carbohydrate, according to educational website Khan Academy.

There are three main types of photosynthetic pathways: C3, C4 and CAM. They all produce sugars from CO2 using the Calvin cycle, but each pathway is slightly different.

C3 photosynthesis

Most plants use C3 photosynthesis, according to the photosynthesis research project Realizing Increased Photosynthetic Efficiency (RIPE) . C3 plants include cereals (wheat and rice), cotton, potatoes and soybeans. This process is named for the three-carbon compound 3-PGA that it uses during the Calvin cycle.

C4 photosynthesis

Plants such as maize and sugarcane use C4 photosynthesis. This process uses a four-carbon compound intermediate (called oxaloacetate) which is converted to malate , according to Biology Online. Malate is then transported into the bundle sheath where it breaks down and releases CO2, which is then fixed by rubisco and made into sugars in the Calvin cycle (just like C3 photosynthesis). C4 plants are better adapted to hot, dry environments and can continue to fix carbon even when their stomata are closed (as they have a clever storage solution), according to Biology Online.

CAM photosynthesis

Crassulacean acid metabolism (CAM) is found in plants adapted to very hot and dry environments, such as cacti and pineapples, according to the Khan Academy. When stomata open to take in CO2, they risk losing water to the external environment. Because of this, plants in very arid and hot environments have adapted. One adaptation is CAM, whereby plants open stomata at night (when temperatures are lower and water loss is less of a risk). According to the Khan Academy, CO2 enters the plants via the stomata and is fixed into oxaloacetate and converted into malate or another organic acid (like in the C4 pathway). The CO2 is then available for light-dependent reactions in the daytime, and stomata close, reducing the risk of water loss.

Discover more facts about photosynthesis with the educational science website sciencing.com . Explore how leaf structure affects photosynthesis with The University of Arizona . Learn about the different ways photosynthesis can be measured with the educational science website Science & Plants for Schools .

This article was updated by Live Science managing editor Tia Ghose on Nov. 3, 2022.

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8.3: Overview of Photosynthesis - The Two Parts of Photosynthesis

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Learning Objectives

Distinguish between the two parts of photosynthesis

Light-Dependent Reactions

Just as the name implies, light-dependent reactions require sunlight. In the light-dependent reactions, energy from sunlight is absorbed by chlorophyll and converted into stored chemical energy, in the form of the electron carrier molecule NADPH (nicotinamide adenine dinucleotide phosphate) and the energy currency molecule ATP (adenosine triphosphate). The light-dependent reactions take place in the thylakoid membranes in the granum (stack of thylakoids), within the chloroplast.

Photosystems

The process that converts light energy into chemical energy takes place in a multi-protein complex called a photosystem. Two types of photosystems are embedded in the thylakoid membrane: photosystem II ( PSII) and photosystem I (PSI). Each photosystem plays a key role in capturing the energy from sunlight by exciting electrons. These energized electrons are transported by “energy carrier” molecules, which power the light-independent reactions.

Photosystems consist of a light-harvesting complex and a reaction center. Pigments in the light-harvesting complex pass light energy to two special chlorophyll a molecules in the reaction center. The light excites an electron from the chlorophyll a pair, which passes to the primary electron acceptor. The excited electron must then be replaced. In photosystem II, the electron comes from the splitting of water, which releases oxygen as a waste product. In photosystem I, the electron comes from the chloroplast electron transport chain.

The two photosystems oxidize different sources of the low-energy electron supply, deliver their energized electrons to different places, and respond to different wavelengths of light.

Light-Independent Reactions

In the light-independent reactions or Calvin cycle, the energized electrons from the light-dependent reactions provide the energy to form carbohydrates from carbon dioxide molecules. The light-independent reactions are sometimes called the Calvin cycle because of the cyclical nature of the process.

Although the light-independent reactions do not use light as a reactant (and as a result can take place at day or night), they require the products of the light-dependent reactions to function. The light-independent molecules depend on the energy carrier molecules, ATP and NADPH, to drive the construction of new carbohydrate molecules. After the energy is transferred, the energy carrier molecules return to the light-dependent reactions to obtain more energized electrons. In addition, several enzymes of the light-independent reactions are activated by light.

In light-dependent reactions, the energy from sunlight is absorbed by chlorophyll and converted into chemical energy in the form of electron carrier molecules like ATP and NADPH.
Light energy is harnessed in Photosystems I and II, both of which are present in the thylakoid membranes of chloroplasts.
In light-independent reactions (the Calvin cycle), carbohydrate molecules are assembled from carbon dioxide using the chemical energy harvested during the light-dependent reactions.
photosystem : Either of two biochemical systems active in chloroplasts that are part of photosynthesis.

Photosynthesis takes place in two sequential stages:

The light-dependent reactions;
The light-independent reactions, or Calvin Cycle.

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Photosynthesis

Matthew p. johnson.

Department of Molecular Biology and Biotechnology, University of Sheffield, Firth Court, Western Bank, Sheffield S10 2TN, U.K.

Photosynthesis sustains virtually all life on planet Earth providing the oxygen we breathe and the food we eat; it forms the basis of global food chains and meets the majority of humankind's current energy needs through fossilized photosynthetic fuels. The process of photosynthesis in plants is based on two reactions that are carried out by separate parts of the chloroplast. The light reactions occur in the chloroplast thylakoid membrane and involve the splitting of water into oxygen, protons and electrons. The protons and electrons are then transferred through the thylakoid membrane to create the energy storage molecules adenosine triphosphate (ATP) and nicotinomide–adenine dinucleotide phosphate (NADPH). The ATP and NADPH are then utilized by the enzymes of the Calvin–Benson cycle (the dark reactions), which converts CO 2 into carbohydrate in the chloroplast stroma. The basic principles of solar energy capture, energy, electron and proton transfer and the biochemical basis of carbon fixation are explained and their significance is discussed.

An overview of photosynthesis

Introduction.

Photosynthesis is the ultimate source of all of humankind's food and oxygen, whereas fossilized photosynthetic fuels provide ∼87% of the world's energy. It is the biochemical process that sustains the biosphere as the basis for the food chain. The oxygen produced as a by-product of photosynthesis allowed the formation of the ozone layer, the evolution of aerobic respiration and thus complex multicellular life.

Oxygenic photosynthesis involves the conversion of water and CO 2 into complex organic molecules such as carbohydrates and oxygen. Photosynthesis may be split into the ‘light’ and ‘dark’ reactions. In the light reactions, water is split using light into oxygen, protons and electrons, and in the dark reactions, the protons and electrons are used to reduce CO 2 to carbohydrate (given here by the general formula CH 2 O). The two processes can be summarized thus:

Light reactions:

Dark reactions:

The positive sign of the standard free energy change of the reaction (Δ G °) given above means that the reaction requires energy ( an endergonic reaction ). The energy required is provided by absorbed solar energy, which is converted into the chemical bond energy of the products ( Box 1 ).

Standard free energy change

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Photosynthesis converts ∼200 billion tonnes of CO 2 into complex organic compounds annually and produces ∼140 billion tonnes of oxygen into the atmosphere. By facilitating conversion of solar energy into chemical energy, photosynthesis acts as the primary energy input into the global food chain. Nearly all living organisms use the complex organic compounds derived from photosynthesis as a source of energy. The breakdown of these organic compounds occurs via the process of aerobic respiration, which of course also requires the oxygen produced by photosynthesis.

Unlike photosynthesis, aerobic respiration is an exergonic process (negative Δ G °) with the energy released being used by the organism to power biosynthetic processes that allow growth and renewal, mechanical work (such as muscle contraction or flagella rotation) and facilitating changes in chemical concentrations within the cell (e.g. accumulation of nutrients and expulsion of waste). The use of exergonic reactions to power endergonic ones associated with biosynthesis and housekeeping in biological organisms such that the overall free energy change is negative is known as ‘ coupling’.

Photosynthesis and respiration are thus seemingly the reverse of one another, with the important caveat that both oxygen formation during photosynthesis and its utilization during respiration result in its liberation or incorporation respectively into water rather than CO 2 . In addition, glucose is one of several possible products of photosynthesis with amino acids and lipids also being synthesized rapidly from the primary photosynthetic products.

The consideration of photosynthesis and respiration as opposing processes helps us to appreciate their role in shaping our environment. The fixation of CO 2 by photosynthesis and its release during breakdown of organic molecules during respiration, decay and combustion of organic matter and fossil fuels can be visualized as the global carbon cycle ( Figure 1 ).

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The relationship between respiration, photosynthesis and global CO 2 and O 2 levels.

At present, this cycle may be considered to be in a state of imbalance due to the burning of fossil fuels (fossilized photosynthesis), which is increasing the proportion of CO 2 entering the Earth's atmosphere, leading to the so-called ‘greenhouse effect’ and human-made climate change.

Oxygenic photosynthesis is thought to have evolved only once during Earth's history in the cyanobacteria. All other organisms, such as plants, algae and diatoms, which perform oxygenic photosynthesis actually do so via cyanobacterial endosymbionts or ‘chloroplasts’. An endosymbiotoic event between an ancestral eukaryotic cell and a cyanobacterium that gave rise to plants is estimated to have occurred ∼1.5 billion years ago. Free-living cyanobacteria still exist today and are responsible for ∼50% of the world's photosynthesis. Cyanobacteria themselves are thought to have evolved from simpler photosynthetic bacteria that use either organic or inorganic compounds such a hydrogen sulfide as a source of electrons rather than water and thus do not produce oxygen.

The site of photosynthesis in plants

In land plants, the principal organs of photosynthesis are the leaves ( Figure 2 A). Leaves have evolved to expose the largest possible area of green tissue to light and entry of CO 2 to the leaf is controlled by small holes in the lower epidermis called stomata ( Figure 2 B). The size of the stomatal openings is variable and regulated by a pair of guard cells, which respond to the turgor pressure (water content) of the leaf, thus when the leaf is hydrated, the stomata can open to allow CO 2 in. In contrast, when water is scarce, the guard cells lose turgor pressure and close, preventing the escape of water from the leaf via transpiration.

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( A ) The model plant Arabidopsis thaliana . ( B ) Basic structure of a leaf shown in cross-section. Chloroplasts are shown as green dots within the cells. ( C ) An electron micrograph of an Arabidopsis chloroplast within the leaf. ( D ) Close-up region of the chloroplast showing the stacked structure of the thylakoid membrane.

Within the green tissue of the leaf (mainly the mesophyll) each cell (∼100 μm in length) contains ∼100 chloroplasts (2–3 μm in length), the tiny organelles where photosynthesis takes place. The chloroplast has a complex structure ( Figure 2 C, D) with two outer membranes (the envelope), which are colourless and do not participate in photosynthesis, enclosing an aqueous space (the stroma) wherein sits a third membrane known as the thylakoid, which in turn encloses a single continuous aqueous space called the lumen.

The light reactions of photosynthesis involve light-driven electron and proton transfers, which occur in the thylakoid membrane, whereas the dark reactions involve the fixation of CO 2 into carbohydrate, via the Calvin–Benson cycle, which occurs in the stroma ( Figure 3 ). The light reactions involve electron transfer from water to NADP + to form NADPH and these reactions are coupled to proton transfers that lead to the phosphorylation of adenosine diphosphate (ADP) into ATP. The Calvin–Benson cycle uses ATP and NADPH to convert CO 2 into carbohydrates ( Figure 3 ), regenerating ADP and NADP + . The light and dark reactions are therefore mutually dependent on one another.

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The light reactions of photosynthesis take place in the thylakoid membrane, whereas the dark reactions are located in the chloroplast stroma.

Photosynthetic electron and proton transfer chain

The light-driven electron transfer reactions of photosynthesis begin with the splitting of water by Photosystem II (PSII). PSII is a chlorophyll–protein complex embedded in the thylakoid membrane that uses light to oxidize water to oxygen and reduce the electron acceptor plastoquinone to plastoquinol. Plastoquinol in turn carries the electrons derived from water to another thylakoid-embedded protein complex called cytochrome b 6 f (cyt b 6 f ). cyt b 6 f oxidizes plastoquinol to plastoquinone and reduces a small water-soluble electron carrier protein plastocyanin, which resides in the lumen. A second light-driven reaction is then carried out by another chlorophyll protein complex called Photosystem I (PSI). PSI oxidizes plastocyanin and reduces another soluble electron carrier protein ferredoxin that resides in the stroma. Ferredoxin can then be used by the ferredoxin–NADP + reductase (FNR) enzyme to reduce NADP + to NADPH. This scheme is known as the linear electron transfer pathway or Z-scheme ( Figure 4 ).

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The linear electron transfer pathway from water to NADP + to form NADPH results in the formation of a proton gradient across the thylakoid membrane that is used by the ATP synthase enzyme to make ATP.

The Z-scheme, so-called since it resembles the letter ‘Z’ when turned on its side ( Figure 5 ), thus shows how the electrons move from the water–oxygen couple (+820 mV) via a chain of redox carriers to NADP + /NADPH (−320 mV) during photosynthetic electron transfer. Generally, electrons are transferred from redox couples with low potentials (good reductants) to those with higher potentials (good oxidants) (e.g. during respiratory electron transfer in mitochondria) since this process is exergonic (see Box 2 ). However, photosynthetic electron transfer also involves two endergonic steps, which occur at PSII and at PSI and require an energy input in the form of light. The light energy is used to excite an electron within a chlorophyll molecule residing in PSII or PSI to a higher energy level; this excited chlorophyll is then able to reduce the subsequent acceptors in the chain. The oxidized chlorophyll is then reduced by water in the case of PSII and plastocyanin in the case of PSI.

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The main components of the linear electron transfer pathway are shown on a scale of redox potential to illustrate how two separate inputs of light energy at PSI and PSII result in the endergonic transfer of electrons from water to NADP + .

Relationship between redox potentials and standard free energy changes

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The water-splitting reaction at PSII and plastoquinol oxidation at cyt b 6 f result in the release of protons into the lumen, resulting in a build-up of protons in this compartment relative to the stroma. The difference in the proton concentration between the two sides of the membrane is called a proton gradient. The proton gradient is a store of free energy (similar to a gradient of ions in a battery) that is utilized by a molecular mechanical motor ATP synthase, which resides in the thylakoid membrane ( Figure 4 ). The ATP synthase allows the protons to move down their concentration gradient from the lumen (high H + concentration) to the stroma (low H + concentration). This exergonic reaction is used to power the endergonic synthesis of ATP from ADP and inorganic phosphate (P i ). This process of photophosphorylation is thus essentially similar to oxidative phosphorylation, which occurs in the inner mitochondrial membrane during respiration.

An alternative electron transfer pathway exists in plants and algae, known as cyclic electron flow. Cyclic electron flow involves the recycling of electrons from ferredoxin to plastoquinone, with the result that there is no net production of NADPH; however, since protons are still transferred into the lumen by oxidation of plastoquinol by cyt b 6 f , ATP can still be formed. Thus photosynthetic organisms can control the ratio of NADPH/ATP to meet metabolic need by controlling the relative amounts of cyclic and linear electron transfer.

How the photosystems work

Light absorption by pigments.

Photosynthesis begins with the absorption of light by pigments molecules located in the thylakoid membrane. The most well-known of these is chlorophyll, but there are also carotenoids and, in cyanobacteria and some algae, bilins. These pigments all have in common within their chemical structures an alternating series of carbon single and double bonds, which form a conjugated system π–electron system ( Figure 6 ).

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The chemical structures of the chlorophyll and carotenoid pigments present in the thylakoid membrane. Note the presence in each of a conjugated system of carbon–carbon double bonds that is responsible for light absorption.

The variety of pigments present within each type of photosynthetic organism reflects the light environment in which it lives; plants on land contain chlorophylls a and b and carotenoids such as β-carotene, lutein, zeaxanthin, violaxanthin, antheraxanthin and neoxanthin ( Figure 6 ). The chlorophylls absorb blue and red light and so appear green in colour, whereas carotenoids absorb light only in the blue and so appear yellow/red ( Figure 7 ), colours more obvious in the autumn as chlorophyll is the first pigment to be broken down in decaying leaves.

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Chlorophylls absorb light energy in the red and blue part of the visible spectrum, whereas carotenoids only absorb light in the blue/green.

Light, or electromagnetic radiation, has the properties of both a wave and a stream of particles (light quanta). Each quantum of light contains a discrete amount of energy that can be calculated by multiplying Planck's constant, h (6.626×10 −34 J·s) by ν, the frequency of the radiation in cycles per second (s −1 ):

The frequency (ν) of the light and so its energy varies with its colour, thus blue photons (∼450 nm) are more energetic than red photons (∼650 nm). The frequency (ν) and wavelength (λ) of light are related by:

where c is the velocity of light (3.0×10 8 m·s −1 ), and the energy of a particular wavelength (λ) of light is given by:

Thus 1 mol of 680 nm photons of red light has an energy of 176 kJ·mol −1 .

The electrons within the delocalized π system of the pigment have the ability to jump up from the lowest occupied molecular orbital (ground state) to higher unoccupied molecular electron orbitals (excited states) via the absorption of specific wavelengths of light in the visible range (400–725 nm). Chlorophyll has two excited states known as S 1 and S 2 and, upon interaction of the molecule with a photon of light, one of its π electrons is promoted from the ground state (S 0 ) to an excited state, a process taking just 10 −15 s ( Figure 8 ). The energy gap between the S 0 and S 1 states is spanned by the energy provided by a red photon (∼600–700 nm), whereas the energy gap between the S 0 and S 2 states is larger and therefore requires a more energetic (shorter wavelength, higher frequency) blue photon (∼400–500 nm) to span the energy gap.

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Photons with slightly different energies (colours) excite each of the vibrational substates of each excited state (as shown by variation in the size and colour of the arrows).

Upon excitation, the electron in the S 2 state quickly undergoes losses of energy as heat through molecular vibration and undergoes conversion into the energy of the S 1 state by a process called internal conversion. Internal conversion occurs on a timescale of 10 −12 s. The energy of a blue photon is thus rapidly degraded to that of a red photon. Excitation of the molecule with a red photon would lead to promotion of an electron to the S 1 state directly. Once the electron resides in the S 1 state, it is lower in energy and thus stable on a somewhat longer timescale (10 −9 s). The energy of the excited electron in the S 1 state can have one of several fates: it could return to the ground state (S 0 ) by emission of the energy as a photon of light (fluorescence), or it could be lost as heat due to internal conversion between S 1 and S 0 . Alternatively, if another chlorophyll is nearby, a process known as excitation energy transfer (EET) can result in the non-radiative exchange of energy between the two molecules ( Figure 9 ). For this to occur, the two chlorophylls must be close by (<7 nm), have a specific orientation with respect to one another, and excited state energies that overlap (are resonant) with one another. If these conditions are met, the energy is exchanged, resulting in a mirror S 0 →S 1 transition in the acceptor molecule and a S 1 →S 0 transition in the other.

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Two chlorophyll molecules with resonant S 1 states undergo a mirror transition resulting in the non-radiative transfer of excitation energy between them.

Light-harvesting complexes

In photosynthetic systems, chlorophylls and carotenoids are found attached to membrane-embedded proteins known as light-harvesting complexes (LHCs). Through careful binding and orientation of the pigment molecules, absorbed energy can be transferred among them by EET. Each pigment is bound to the protein by a series of non-covalent bonding interactions (such as, hydrogen bonds, van der Waals interactions, hydrophobic interaction and co-ordination bonds between lone pair electrons of residues such as histidine in the protein and the Mg 2+ ion in chlorophyll); the protein structure is such that each bound pigment experiences a slightly different environment in terms of the surrounding amino acid side chains, lipids, etc., meaning that the S 1 and S 2 energy levels are shifted in energy with respect to that of other neighbouring pigment molecules. The effect is to create a range of pigment energies that act to ‘funnel’ the energy on to the lowest-energy pigments in the LHC by EET.

Reaction centres

A photosystem consists of numerous LHCs that form an antenna of hundreds of pigment molecules. The antenna pigments act to collect and concentrate excitation energy and transfer it towards a ‘special pair’ of chlorophyll molecules that reside in the reaction centre (RC) ( Figure 10 ). Unlike the antenna pigments, the special pair of chlorophylls are ‘redox-active’ in the sense that they can return to the ground state (S 0 ) by the transfer of the electron residing in the S 1 excited state (Chl*) to another species. This process is known as charge separation and result in formation of an oxidized special pair (Chl + ) and a reduced acceptor (A − ). The acceptor in PSII is plastoquinone and in PSI it is ferredoxin. If the RC is to go on functioning, the electron deficiency on the special pair must be made good, in PSII the electron donor is water and in PSI it is plastocyanin.

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Light energy is captured by the antenna pigments and transferred to the special pair of RC chlorophylls which undergo a redox reaction leading to reduction of an acceptor molecule. The oxidized special pair is regenerated by an electron donor.

It is worth asking why photosynthetic organisms bother to have a large antenna of pigments serving an RC rather than more numerous RCs. The answer lies in the fact that the special pair of chlorophylls alone have a rather small spatial and spectral cross-section, meaning that there is a limit to the amount of light they can efficiently absorb. The amount of light they can practically absorb is around two orders of magnitude smaller than their maximum possible turnover rate, Thus LHCs act to increase the spatial (hundreds of pigments) and spectral (several types of pigments with different light absorption characteristics) cross-section of the RC special pair ensuring that its turnover rate runs much closer to capacity.

Photosystem II

PSII is a light-driven water–plastoquinone oxidoreductase and is the only enzyme in Nature that is capable of performing the difficult chemistry of splitting water into protons, electrons and oxygen ( Figure 11 ). In principle, water is an extremely poor electron donor since the redox potential of the water–oxygen couple is +820 mV. PSII uses light energy to excite a special pair of chlorophylls, known as P680 due to their 680 nm absorption peak in the red part of the spectrum. P680* undergoes charge separation that results in the formation of an extremely oxidizing species P680 + which has a redox potential of +1200 mV, sufficient to oxidize water. Nonetheless, since water splitting involves four electron chemistry and charge separation only involves transfer of one electron, four separate charge separations (turnovers of PSII) are required to drive formation of one molecule of O 2 from two molecules of water. The initial electron donation to generate the P680 from P680 + is therefore provided by a cluster of manganese ions within the oxygen-evolving complex (OEC), which is attached to the lumen side of PSII ( Figure 12 ). Manganese is a transition metal that can exist in a range of oxidation states from +1 to +5 and thus accumulates the positive charges derived from each light-driven turnover of P680. Progressive extraction of electrons from the manganese cluster is driven by the oxidation of P680 within PSII by light and is known as the S-state cycle ( Figure 12 ). After the fourth turnover of P680, sufficient positive charge is built up in the manganese cluster to permit the splitting of water into electrons, which regenerate the original state of the manganese cluster, protons, which are released into the lumen and contribute to the proton gradient used for ATP synthesis, and the by-product O 2 . Thus charge separation at P680 provides the thermodynamic driving force, whereas the manganese cluster acts as a catalyst for the water-splitting reaction.

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The organization of PSII and its light-harvesting antenna. Protein is shown in grey, with chlorophylls in green and carotenoids in orange. Drawn from PDB code 3JCU

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Progressive extraction of electrons from the manganese cluster is driven by the oxidation of P680 within PSII by light. Each of the electrons given up by the cluster is eventually repaid at the S 4 to S 0 transition when molecular oxygen (O 2 ) is formed. The protons extracted from water during the process are deposited into the lumen and contribute to the protonmotive force.

The electrons yielded by P680* following charge separation are not passed directly to plastoquinone, but rather via another acceptor called pheophytin, a porphyrin molecule lacking the central magnesium ion as in chlorophyll. Plastoquinone reduction to plastoquinol requires two electrons and thus two molecules of plastoquinol are formed per O 2 molecule evolved by PSII. Two protons are also taken up upon formation of plastoquinol and these are derived from the stroma. PSII is found within the thylakoid membrane of plants as a dimeric RC complex surrounded by a peripheral antenna of six minor monomeric antenna LHC complexes and two to eight trimeric LHC complexes, which together form a PSII–LHCII supercomplex ( Figure 11 ).

Photosystem I

PSI is a light-driven plastocyanin–ferredoxin oxidoreductase ( Figure 13 ). In PSI, the special pair of chlorophylls are known as P700 due to their 700 nm absorption peak in the red part of the spectrum. P700* is an extremely strong reductant that is able to reduce ferredoxin which has a redox potential of −450 mV (and is thus is, in principle, a poor electron acceptor). Reduced ferredoxin is then used to generate NADPH for the Calvin–Benson cycle at a separate complex known as FNR. The electron from P700* is donated via another chlorophyll molecule and a bound quinone to a series of iron–sulfur clusters at the stromal side of the complex, whereupon the electron is donated to ferredoxin. The P700 species is regenerated form P700 + via donation of an electron from the soluble electron carrier protein plastocyanin.

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The organization of PSI and its light-harvesting antenna. Protein is shown in grey, with chlorophylls in green and carotenoids in orange. Drawn from PDB code 4XK8.

PSI is found within the thylakoid membrane as a monomeric RC surrounded on one side by four LHC complexes known as LHCI. The PSI–LHCI supercomplex is found mainly in the unstacked regions of the thylakoid membrane ( Figure 13 ).

Other electron transfer chain components

Plastoquinone/plastoquinol.

Plastoquinone is a small lipophilic electron carrier molecule that resides within the thylakoid membrane and carries two electrons and two protons from PSII to the cyt b 6 f complex. It has a very similar structure to that of the molecule ubiquinone (coenzyme Q 10 ) in the mitochondrial inner membrane.

Cytochrome b 6 f complex

The cyt b 6 f complex is a plastoquinol–plastocyanin oxidoreductase and possess a similar structure to that of the cytochrome bc 1 complex (complex III) in mitochondria ( Figure 14 A). As with Complex III, cyt b 6 f exists as a dimer in the membrane and carries out both the oxidation and reduction of quinones via the so-called Q-cycle. The Q-cycle ( Figure 14 B) involves oxidation of one plastoquinol molecule at the Qp site of the complex, both protons from this molecule are deposited in the lumen and contribute to the proton gradient for ATP synthesis. The two electrons, however, have different fates. The first is transferred via an iron–sulfur cluster and a haem cofactor to the soluble electron carrier plastocyanin (see below). The second electron derived from plastoquinol is passed via two separate haem cofactors to another molecule of plastoquinone bound to a separate site (Qn) on the complex, thus reducing it to a semiquinone. When a second plastoquinol molecule is oxidized at Qp, a second molecule of plastocyanin is reduced and two further protons are deposited in the lumen. The second electron reduces the semiquinone at the Qn site which, concomitant with uptake of two protons from the stroma, causes its reduction to plastoquinol. Thus for each pair of plastoquinol molecules oxidized by the complex, one is regenerated, yet all four protons are deposited into the lumen. The Q-cycle thus doubles the number of protons transferred from the stroma to the lumen per plastoquinol molecule oxidized.

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( A ) Structure drawn from PDB code 1Q90. ( B ) The protonmotive Q-cycle showing how electrons from plastoquinol are passed to both plastocyanin and plastoquinone, doubling the protons deposited in the lumen for every plastoquinol molecule oxidized by the complex.

Plastocyanin

Plastocyanin is a small soluble electron carrier protein that resides in the thylakoid lumen. The active site of the plastocyanin protein binds a copper ion, which cycles between the Cu 2+ and Cu + oxidation states following its oxidation by PSI and reduction by cyt b 6 f respectively.

Ferredoxin is a small soluble electron carrier protein that resides in the chloroplast stroma. The active site of the ferredoxin protein binds an iron–sulfur cluster, which cycles between the Fe 2+ and Fe 3+ oxidation states following its reduction by PSI and oxidation by the FNR complex respectively.

Ferredoxin–NADP + reductase

The FNR complex is found in both soluble and thylakoid membrane-bound forms. The complex binds a flavin–adenine dinucleotide (FAD) cofactor at its active site, which accepts two electrons from two molecules of ferredoxin before using them reduce NADP + to NADPH.

ATP synthase

The ATP synthase enzyme is responsible for making ATP from ADP and P i ; this endergonic reaction is powered by the energy contained within the protonmotive force. According to the structure, 4.67 H + are required for every ATP molecule synthesized by the chloroplast ATP synthase. The enzyme is a rotary motor which contains two domains: the membrane-spanning F O portion which conducts protons from the lumen to the stroma, and the F 1 catalytic domain that couples this exergonic proton movement to ATP synthesis.

Membrane stacking and the regulation of photosynthesis

Within the thylakoid membrane, PSII–LHCII supercomplexes are packed together into domains known as the grana, which associate with one another to form grana stacks. PSI and ATP synthase are excluded from these stacked PSII–LHCII regions by steric constraints and thus PSII and PSI are segregated in the thylakoid membrane between the stacked and unstacked regions ( Figure 15 ). The cyt b 6 f complex, in contrast, is evenly distributed throughout the grana and stromal lamellae. The evolutionary advantage of membrane stacking is believed to be a higher efficiency of electron transport by preventing the fast energy trap PSI from ‘stealing’ excitation energy from the slower trap PSII, a phenomenon known as spillover. Another possible advantage of membrane stacking in thylakoids may be the segregation of the linear and cyclic electron transfer pathways, which might otherwise compete to reduce plastoquinone. In this view, PSII, cyt b 6 f and a sub-fraction of PSI closest to the grana is involved in linear flow, whereas PSI and cyt b 6 f in the stromal lamellae participates in cyclic flow. The cyclic electron transfer pathway recycles electrons from ferredoxin back to plastoquinone and thus allows protonmotive force generation (and ATP synthesis) without net NADPH production. Cyclic electron transfer thereby provides the additional ATP required for the Calvin–Benson cycle (see below).

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( A ) Electron micrograph of the thylakoid membrane showing stacked grana and unstacked stromal lamellae regions. ( B ) Model showing the distribution of the major complexes of photosynthetic electron and proton transfer between the stacked grana and unstacked stromal lamellae regions.

‘Dark’ reactions: the Calvin–Benson cycle

CO 2 is fixed into carbohydrate via the Calvin–Benson cycle in plants, which consumes the ATP and NADPH produced during the light reactions and thus in turn regenerates ADP, P i and NADP + . In the first step of the Calvin–Benson cycle ( Figure 16 ), CO 2 is combined with a 5-carbon (5C) sugar, ribulose 1,5-bisphosphate in a reaction catalysed by the enzyme ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco). The reaction forms an unstable 6C intermediate that immediately splits into two molecules of 3-phosphoglycerate. 3-Phosphoglycerate is first phosphorylated by 3-phosphoglycerate kinase using ATP to form 1,3-bisphosphoglycerate. 1,3-Bisphosphoglycerate is then reduced by glyceraldehyde 3-phosphate dehydrogenase using NADPH to form glyceraldehyde 3-phosphate (GAP, a triose or 3C sugar) in reactions, which are the reverse of glycolysis. For every three CO 2 molecules initially combined with ribulose 1,5-bisphopshate, six molecules of GAP are produced by the subsequent steps. However only one of these six molecules can be considered as a product of the Calvin–Benson cycle since the remaining five are required to regenerate ribulose 1,5-bisphosphate in a complex series of reactions that also require ATP. The one molecule of GAP that is produced for each turn of the cycle can be quickly converted by a range of metabolic pathways into amino acids, lipids or sugars such as glucose. Glucose in turn may be stored as the polymer starch as large granules within chloroplasts.

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Overview of the biochemical pathway for the fixation of CO 2 into carbohydrate in plants.

A complex biochemical ‘dance’ ( Figure 16 ) is then involved in the regeneration of three ribulose 1,5-bisphosphate (5C) from the remaining five GAP (3C) molecules. The regeneration begins with the conversion of two molecules of GAP into dihydroxyacetone phosphate (DHAP) by triose phosphate isomerase; one of the DHAP molecules is the combined with another GAP molecule to make fructose 1,6-bisphosphate (6C) by aldolase. The fructose 1,6-bisphosphate is then dephosphorylated by fructose-1,6-bisphosphatase to yield fructose 6-phosphate (6C) and releasing P i . Two carbons are then removed from fructose 6-phosphate by transketolase, generating erythrose 4-phosphate (4C); the two carbons are transferred to another molecule of GAP generating xylulose 5-phosphate (5C). Another DHAP molecule, formed from GAP by triose phosphate isomerase is then combined with the erythrose 4-phosphate by aldolase to form sedoheptulose 1,7-bisphosphate (7C). Sedoheptulose 1,7-bisphosphate is then dephosphorylated to sedoheptulose 7-phosphate (7C) by sedoheptulose-1,7-bisphosphatase releasing P i . Sedoheptulose 7-phosphate has two carbons removed by transketolase to produce ribose 5-phosphate (5C) and the two carbons are transferred to another GAP molecule producing another xylulose 5-phosphate (5C). Ribose 5-phosphate and the two molecules of xylulose 5-phosphate (5C) are then converted by phosphopentose isomerase to three molecules of ribulose 5-phosphate (5C). The three ribulose 5-phosphate molecules are then phosphorylated using three ATP by phosphoribulokinase to regenerate three ribulose 1,5-bisphosphate (5C).

Overall the synthesis of 1 mol of GAP requires 9 mol of ATP and 6 mol of NADPH, a required ratio of 1.5 ATP/NADPH. Linear electron transfer is generally thought to supply ATP/NADPH in a ratio of 1.28 (assuming an H + /ATP ratio of 4.67) with the shortfall of ATP believed to be provided by cyclic electron transfer reactions. Since the product of the Calvin cycle is GAP (a 3C sugar) the pathway is often referred to as C 3 photosynthesis and plants that utilize it are called C 3 plants and include many of the world's major crops such as rice, wheat and potato.

Many of the enzymes involved in the Calvin–Benson cycle (e.g. transketolase, glyceraldehyde-3-phosphate dehydrogenase and aldolase) are also involved in the glycolysis pathway of carbohydrate degradation and their activity must therefore be carefully regulated to avoid futile cycling when light is present, i.e. the unwanted degradation of carbohydrate. The regulation of the Calvin–Benson cycle enzymes is achieved by the activity of the light reactions, which modify the environment of the dark reactions (i.e. the stroma). Proton gradient formation across the thylakoid membrane during the light reactions increases the pH and also increases the Mg 2+ concentration in the stroma (as Mg 2+ flows out of the lumen as H + flows in to compensate for the influx of positive charges). In addition, by reducing ferredoxin and NADP + , PSI changes the redox state of the stroma, which is sensed by the regulatory protein thioredoxin. Thioredoxin, pH and Mg 2+ concentration play a key role in regulating the activity of the Calvin–Benson cycle enzymes, ensuring the activity of the light and dark reactions is closely co-ordinated.

It is noteworthy that, despite the complexity of the dark reactions outlined above, the carbon fixation step itself (i.e. the incorporation of CO 2 into carbohydrate) is carried out by a single enzyme, Rubisco. Rubisco is a large multisubunit soluble protein complex found in the chloroplast stroma. The complex consists of eight large (56 kDa) subunits, which contain both catalytic and regulatory domains, and eight small subunits (14 kDa), which enhance the catalytic function of the L subunits ( Figure 17 A). The carboxylation reaction carried out by Rubisco is highly exergonic (Δ G °=−51.9 kJ·mol- 1 ), yet kinetically very slow (just 3 s −1 ) and begins with the protonation of ribulose 1,5-bisphosphate to form an enediolate intermediate which can be combined with CO 2 to form an unstable 6C intermediate that is quickly hydrolysed to yield two 3C 3-phosphoglycerate molecules. The active site in the Rubisco enzyme contains a key lysine residue, which reacts with another (non-substrate) molecule of CO 2 to form a carbamate anion that is then able to bind Mg 2+ . The Mg 2+ in the active site is essential for the catalytic function of Rubisco, playing a key role in binding ribulose 1,5-bisphosphate and activating it such that it readily reacts with CO 2.. Rubisco activity is co-ordinated with that of the light reactions since carbamate formation requires both high Mg 2+ concentration and alkaline conditions, which are provided by the light-driven changes in the stromal environment discussed above ( Figure 17 B).

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( A ) Structure of the Rubisco enzyme (the large subunits are shown in blue and the small subunits in green); four of each type of subunit are visible in the image. Drawn from PDB code 1RXO. ( B ) Activation of the lysine residue within the active site of Rubisco occurs via elevated stromal pH and Mg 2+ concentration as a result of the activity of the light reactions.

In addition to carboxylation, Rubisco also catalyses a competitive oxygenation reaction, known as photorespiration, that results in the combination of ribulose 1,5-bisphosphate with O 2 rather than CO 2 . In the oxygenation reaction, one rather than two molecules of 3-phosphoglycerate and one molecule of a 2C sugar known as phosphoglycolate are produced by Rubisco. The phosphoglycolate must be converted in a series of reactions that regenerate one molecule of 3-phosphoglycerate and one molecule of CO 2 . These reactions consume additional ATP and thus result in an energy loss to the plant. Although the oxygenation reaction of Rubisco is much less favourable than the carboxylation reaction, the relatively high concentration of O 2 in the leaf (250 μM) compared with CO 2 (10 μM) means that a significant amount of photorespiration is always occurring. Under normal conditions, the ratio of carboxylation to oxygenation is between 3:1 and 4:1. However, this ratio can be decreased with increasing temperature due to decreased CO 2 concentration in the leaf, a decrease in the affinity of Rubisco for CO 2 compared with O 2 and an increase in the maximum rate of the oxygenation reaction compared with the carboxylation reaction. The inefficiencies of the Rubisco enzyme mean that plants must produce it in very large amounts (∼30–50% of total soluble protein in a spinach leaf) to achieve the maximal photosynthetic rate.

CO 2 -concentrating mechanisms

To counter photorespiration, plants, algae and cyanobacteria have evolved different CO 2 -concentrating mechanisms CCMs that aim to increase the concentration of CO 2 relative to O 2 in the vicinity of Rubisco. One such CCM is C 4 photosynthesis that is found in plants such as maize, sugar cane and savanna grasses. C 4 plants show a specialized leaf anatomy: Kranz anatomy ( Figure 18 ). Kranz, German for wreath, refers to a bundle sheath of cells that surrounds the central vein within the leaf, which in turn are surrounded by the mesophyll cells. The mesophyll cells in such leaves are rich in the enzyme phosphoenolpyruvate (PEP) carboxylase, which fixes CO 2 into a 4C carboxylic acid: oxaloaceatate. The oxaloacetate formed by the mesophyll cells is reduced using NADPH to malate, another 4C acid: malate. The malate is then exported from the mesophyll cells to the bundle sheath cells, where it is decarboxylated to pyruvate thus regenerating NADPH and CO 2 . The CO 2 is then utilized by Rubisco in the Calvin cycle. The pyruvate is in turn returned to the mesophyll cells where it is phosphorylated using ATP to reform PEP ( Figure 19 ). The advantage of C 4 photosynthesis is that CO 2 accumulates at a very high concentration in the bundle sheath cells that is then sufficient to allow Rubisco to operate efficiently.

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Plants growing in hot, bright and dry conditions inevitably have to have their stomata closed for large parts of the day to avoid excessive water loss and wilting. The net result is that the internal CO 2 concentration in the leaf is very low, meaning that C 3 photosynthesis is not possible. To counter this limitation, another CCM is found in succulent plants such as cacti. The Crassulaceae fix CO 2 into malate during the day via PEP carboxylase, store it within the vacuole of the plant cell at night and then release it within their tissues by day to be fixed via normal C 3 photosynthesis. This is termed crassulacean acid metabolism (CAM).

Acknowledgments

I thank Professor Colin Osborne (University of Sheffield, Sheffield, U.K.) for useful discussions on the article, Dr Dan Canniffe (Penn State University, Pennsylvania, PA, U.S.A.) for providing pure pigment spectra and Dr P.J. Weaire (Kingston University, Kingston-upon-Thames, U.K.) for his original Photosynthesis BASC article (1994) on which this essay is partly based.

Abbreviations

This article is a reviewed, revised and updated version of the following ‘Biochemistry Across the School Curriculum’ (BASC) booklet: Weaire, P.J. (1994) Photosynthesis . For further information and to provide feedback on this or any other Biochemical Society education resource, please contact [email protected]. For further information on other Biochemical Society publications, please visit www.biochemistry.org/publications .

Competing Interests

The Author declares that there are no competing interests associated with this article.

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March 13, 2024

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Surprising bacterium from Canadian lake shines new light on ancient photosynthesis

by Katie McQuaid, University of Waterloo

Sometimes an experiment doesn't go as planned. That's science. But a "failed" experiment or unexpected results can be the avenue to a discovery you could never anticipate. University of Waterloo Ph.D. student Jackson Tsuji had a poorly growing bacterial sample he wasn't ready to give up on, which ultimately led to a once-in-a-lifetime finding that could change how scientists view photosynthesis and its origins.

In 2015, Tsuji joined professor and University Research Chair Josh Neufeld's lab to hunt for unusual photosynthetic bacteria in northern Canadian lakes. Based on a hypothesis by Earth and Environmental Science professor Sherry Schiff that these lakes might harbor bacteria analogous to those on early Earth, Neufeld, Tsuji and co-op student Nicolette Shaw set out to study Lake 227 at the IISD-Experimental Lakes Area near Kenora, Ontario.

Unfortunately, the team had mixed results in their first two years of lake sampling and cultivation. They didn't manage to grow the bacteria they were looking for, even after hundreds of lake water incubations in glass bottles were exposed to light.

The team went back to the drawing board , but Tsuji kept one of the experiment bottles from Lake 227, because although the sample didn't behave as the team expected, something was happening that he was curious about.

"At the three-month mark, we saw signs that the iron in the bottle was starting to oxidize, even though our main target bacterium was not present in the bottle, which was interesting," Tsuji says. "We tried our best to keep the culture going as a side project and for nearly a year, we kept some culture samples in the back of an incubator and some in the fridge as a backup."

The team's DNA-based tests showed them that a highly unusual bacterium was growing in the bottle, only distantly related to any bacteria that had been grown in a laboratory previously. Because of that, Tsuji took samples from this culture on a research trip to Japan in 2018 to the Photosynthetic Microbial Consortium Laboratory.

The researchers in the Photosynthetic Microbial Consortium Laboratory had experience growing the closest known photosynthetic relatives of the strange bacterium in Tsuji's bottle, and after two months, Tsuji and the research team coaxed the bacterium in the bottle into growing well. They also studied its genome and found something amazing that they hadn't anticipated.

"Certain core photosynthesis genes we had expected to see weren't there, but in their absence, there was a completely novel clade of photosynthetic reaction center protein," Tsuji says. "I remember a professor emeritus in the lab, Keizo Shimada, congratulating me on discovering what we now believe is a new branch of photosynthetic life and a key piece of the puzzle for resolving how photosynthesis developed on Earth. It was surreal."

After his trip, Tsuji returned to Waterloo to complete his Ph.D. and dig deeper into what this discovery could mean.

"This breakthrough challenged current scientific knowledge of how photosynthesis came to be and explained some of the 'whys' we couldn't explain before. Yet, it also created more questions than answers—and I was excited to investigate more," Tsuji says.

When Tsuji completed his Ph.D., Neufeld gave him the samples to continue his work as a postdoctoral researcher at Hokkaido University in Japan.

"This is Jackson's project and I want him to take the lead on next steps," Neufeld says. "His persistence led to an unexpected discovery that could alter perspectives on the origins of photosynthetic life, including plants and algae. It's an exciting time for photosynthesis research, and now that our paper is published, I'm excited to see what will come next for this new research topic in Jackson's laboratory."

Tsuji's postdoctoral work at Hokkaido University, with professor Manabu Fukui and assistant professor Tomohiro Watanabe, enabled the team to gather the key data needed to validate and publish their initial discoveries—the culmination of which is the paper titled "Anoxygenic phototroph of the Chloroflexota uses a Type I reaction center," now published in Nature .

Now, as a young research fellow at the Japan Agency for Marine-Earth Science and Technology, Tsuji continues his research to answer all the questions posed by this bacterium and everything it means for how scientists view the history of photosynthesis. He is currently exploring how the bacterium harvests light and transforms nutrients in its environment, to understand afresh what photosynthetic life on early Earth may have been like.

"Our initial findings for the metabolism of this bacterium and its photosynthetic machinery make it natural to speculate that the bacterium is like a 'living fossil'—an organism that has held onto traits like those of ancient life," Tsuji says. "In the years ahead, we want to learn more about the surprising characteristics of this bacterium to gain new insights into how photosynthesis came to work the way it does today and how this process transformed the Earth throughout its history."

Provided by University of Waterloo

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Photosynthesis
In chemical terms, photosynthesis is a light-energized oxidation-reduction process. (Oxidation refers to the removal of electrons from a molecule; reduction refers to the gain of electrons by a molecule.) In plant photosynthesis, the energy of light is used to drive the oxidation of water (H 2 O), producing oxygen gas (O 2 ), hydrogen ions (H ...
Photosynthesis
The process. During photosynthesis, plants take in carbon dioxide (CO 2) and water (H 2 O) from the air and soil. Within the plant cell, the water is oxidized, meaning it loses electrons, while the carbon dioxide is reduced, meaning it gains electrons. This transforms the water into oxygen and the carbon dioxide into glucose.
Photosynthesis
Photosynthesis Definition. Photosynthesis is the biochemical pathway which converts the energy of light into the bonds of glucose molecules. The process of photosynthesis occurs in two steps. In the first step, energy from light is stored in the bonds of adenosine triphosphate (ATP), and nicotinamide adenine dinucleotide phosphate (NADPH).
Intro to photosynthesis (article)
Photosynthesis is the process in which light energy is converted to chemical energy in the form of sugars. In a process driven by light energy, glucose molecules (or other sugars) are constructed from water and carbon dioxide, and oxygen is released as a byproduct. The glucose molecules provide organisms with two crucial resources: energy and ...
Photosynthesis
Photosynthesis ( / ˌfoʊtəˈsɪnθəsɪs / FOH-tə-SINTH-ə-sis) [1] is a system of biological processes by which photosynthetic organisms, such as most plants, algae, and cyanobacteria, convert light energy, typically from sunlight, into the chemical energy necessary to fuel their activities.
Photosynthesis Definition & Meaning
The meaning of PHOTOSYNTHESIS is synthesis of chemical compounds with the aid of radiant energy and especially light; especially : formation of carbohydrates from carbon dioxide and a source of hydrogen (such as water) in the chlorophyll-containing cells (as of green plants) exposed to light. Photosynthesis Has Greek Roots
Photosynthesis
Photosynthesis definition states that the process exclusively takes place in the chloroplasts through photosynthetic pigments such as chlorophyll a, chlorophyll b, carotene and xanthophyll. All green plants and a few other autotrophic organisms utilize photosynthesis to synthesize nutrients by using carbon dioxide, water and sunlight. The by ...
Photosynthesis
Photosynthesis definition: Photosynthesis is a physio-chemical process carried out by photo-auto-lithotrophs. In simpler language, photosynthesis is the process by which green plants convert light energy into 'chemical energy'.
The Purpose and Process of Photosynthesis
photosynthesis: the process by which plants and other photoautotrophs generate carbohydrates and oxygen from carbon dioxide, water, and light energy in chloroplasts. photoautotroph: an organism that can synthesize its own food by using light as a source of energy. chemoautotroph: a simple organism, such as a protozoan, that derives its energy ...
Photosynthesis in organisms (article)
Photosynthesis is powered by energy from sunlight. This energy is used to rearrange atoms in carbon dioxide and water to make oxygen and sugars. Carbon dioxide and water are inputs of photosynthesis. These inputs come from the environment. Oxygen and sugars are outputs of photosynthesis. The oxygen is released into the environment.
8.1: Overview of Photosynthesis
Main Structures and Summary of Photosynthesis. Photosynthesis is a multi-step process that requires sunlight, carbon dioxide (which is low in energy), and water as substrates (Figure 8.1.3 8.1. 3 ). After the process is complete, it releases oxygen and produces glyceraldehyde-3-phosphate (GA3P), simple carbohydrate molecules (which are high in ...
Photosynthesis review (article)
Meaning. Photosynthesis. The process by which plants, algae, and some bacteria convert light energy to chemical energy in the form of sugars. Photoautotroph. An organism that produces its own food using light energy (like plants) ATP. Adenosine triphosphate, the primary energy carrier in living things. Chloroplast.
Photosynthesis
Photosynthesis. It is the process by which green plants, algae, and certain bacteria convert light energy from the sun into chemical energy that is used to make glucose. The word 'photosynthesis' is derived from the Greek word phōs, meaning 'light' and synthesis meaning 'combining together.'. Jan Ingenhousz, the Dutch-born British ...
Photosynthesis
Photosynthesis can be summarized by the equation:CO 2 +2H 2 O → [CH 2 O]+H 2 O+O 2. Since virtually all other forms of life are directly or indirectly dependent on plants for food, photosynthesis is the basis for all life on earth. Furthermore virtually all the atmospheric oxygen has originated from oxygen released during photosynthesis.
What is photosynthesis?
Photosynthesis is the process used by plants, algae and some bacteria to turn sunlight into energy. The process chemically converts carbon dioxide (CO2) and water into food (sugars) and oxygen ...
The Two Parts of Photosynthesis
Photosystems. Figure 8.3.1 8.3. 1: Photosystems I & II: As explained above, the photosystems manipulate electrons with energy harvested from light. The process that converts light energy into chemical energy takes place in a multi-protein complex called a photosystem. Two types of photosystems are embedded in the thylakoid membrane: photosystem ...
Light-dependent reactions (photosynthesis reaction) (article)
Light energy is converted to chemical energy during the first stage of photosynthesis, which involves a series of chemical reactions known as the light-dependent reactions. ... meaning that one of its electrons is boosted to a higher-energy orbital. Most of the pigments in a photosystem act as an energy funnel, passing energy inward to a main ...
Photosynthesis
It is the biochemical process that sustains the biosphere as the basis for the food chain. The oxygen produced as a by-product of photosynthesis allowed the formation of the ozone layer, the evolution of aerobic respiration and thus complex multicellular life. Oxygenic photosynthesis involves the conversion of water and CO 2 into complex ...
Photosynthesis
Photosynthesis is really important for the plant because it provides the plant with food: some of the glucose is used immediately, to give the plant energy in the process of respiration. some of ...
What is photosynthesis?
photosynthesis A chemical reaction that occurs in the chloroplasts of plants in which the energy in light is stored in glucose. is a process that occurs in the leaves of a plant and needs both ...
Photosynthesis in ecosystems (article)
Photosynthesis is carried out by photosynthetic organisms. Photosynthesis drives the movement of matter, or atoms, between organisms and the environment. Photosynthetic organisms take in and use carbon dioxide and water from the air and soil. Photosynthetic organisms release oxygen into the air. Organisms throughout the ecosystem use this ...
PHOTOSYNTHESIS
PHOTOSYNTHESIS definition: 1. the process by which a plant uses carbon dioxide from the air, water from the ground, and the…. Learn more.
Surprising bacterium from Canadian lake shines new light on ancient
"Certain core photosynthesis genes we had expected to see weren't there, but in their absence, there was a completely novel clade of photosynthetic reaction center protein," Tsuji says.
Light and photosynthetic pigments
The set of wavelengths absorbed by a pigment is its absorption spectrum. In the diagram below, you can see the absorption spectra of three key pigments in photosynthesis: chlorophyll a, chlorophyll b, and β-carotene. The set of wavelengths that a pigment doesn't absorb are reflected, and the reflected light is what we see as color.

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