What is Photosynthesis?

Photosynthesis is the transformation of electromagnetic light into chemical energy utilised by green plants and photosynthetic bacteria to convert water and carbon dioxide into carbohydrates and oxygen.

• In addition to providing the required energy for energy transmission among ecosystems, the carbohydrates produced during photosynthesis also offer the carbon molecules needed to create a variety of proteins.
• Photosynthesis is a light-driven oxidation mechanism that oxidises water to produce oxygen gas as well as hydrogen ions and then reduces carbon dioxide to organic molecules by transferring electrons to it.
• Due to the fact that they can produce chemical fuels such as glucose from carbon dioxide and water, using sunlight as a source of energy, photosynthetic organisms are known as autotrophs.
• In the end, autotrophs are the only sources of energy for other species that get their energy from other organisms.
• Chlorophyll is a green pigment present inside the chloroplasts of green plants and also some bacteria, and is one of the necessary elements for photosynthesis.
• The pigment is necessary for “catching” sunlight, which subsequently powers the whole photosynthetic process.

Photosynthesis equations/reactions/formulas

• The method of photosynthesis varies between sulphur bacteria and green plants.
• Water and carbon dioxide are used by plants to liberate glucose and oxygen molecules.
• Carbon dioxide and hydrogen sulphide are both used by sulphur bacteria to release molecules of water, sulphur, and carbohydrates.

Oxygenic Photosynthesis

Following are the general effects of photosynthesis in plants:

Carbon dioxide + Water + solar energy → Glucose + Oxygen

6CO2 + 6H2O + solar energy → C6H12O6 + 6O2 \sOR

Carbon dioxide + Water + solar energy → Glucose + Oxygen + Water

6CO2 + 12H2O+ solar energy → C6H12O6 + 6O2 + 6H2O

Anoxygenic Photosynthesis

Following are the general effects of photosynthesis in sulphur bacteria:

CO2 + 2H2S + light energy → (CH2O) + H2O + 2S

Photosynthetic pigments

• In order for organisms capable of photosynthesis to produce photochemical reactions, photosynthetic pigments are the molecules responsible for collecting electromagnetic radiation and delivering the power of the acquired photons to the reaction centre.
• The chlorophyll and carotenoids that make up photosynthetic pigment molecules are relatively common.
• Pheophytin, also known as bacteriopheophytin in bacteria, is a pigment found in photosynthetic systems in addition to chlorophyll. It is essential for the transport of electrons in these systems.
• Additionally, certain photosynthetic systems include specific pigments, such as xanthophylls in plants.

Chlorophyll

• As in the chloroplasts of the majority of green plants, the pigment molecule chlorophyll functions as the primary photoreceptor.
• Chlorophylls are made up of a phytol chain joined to a porphyrin ring that is coupled to the ion Mg2+.
• Chlorophylls have networks of alternate single and double bonds, making them particularly efficient photoreceptors.
• Since the electrons in chlorophyll are not restricted to a single atomic nucleus, they may more easily absorb light energy.
• In the visible part of the spectrum, chlorophylls also exhibit strong absorption bands.
• Chlorophyll may be found within the thylakoid membranes of plant chloroplasts or the cytoplasmic membranes of photosynthetic bacteria.

Bacteriorhodopsin

• Bacteriorhodopsin is a type of photosynthetic pigment that is exclusive to halobacteria.
• It is composed of a protein linked to a retinal prosthetic group.
• This pigment is in charge of absorbing light photons, which causes a protein’s conformation to alter and the ejection of protons from the cell.

Phycobilins

• Phycobilins, including phycoerythrobilin and phycocyanobilin, are used by cyanobacteria and red algae as light-harvesting pigments.
• The extended polyene system seen in chlorophylls is present in these open-chain tetrapyrroles, but neither their cyclic structure nor their core Mg2+ are present.
• The principal light-harvesting structures in these bacteria are termed phycobilisomes, which are formed when phycobilins are covalently attached to certain binding proteins to generate phycobiliproteins.

Carotenoids

• Thylakoid membranes also contain carotenoids. They are additional or supplementary light-absorbing pigments.
• Yellow, red, or purple carotenoids are possible. The two most significant ones are the yellow carotenoid lutein and the red-orange isoprenoid-carotene.
• The carotenoid pigments act as additional light sensors by absorbing light at wavelengths that the chlorophylls cannot.

Factors affecting photosynthesis

While researching the variables influencing the rate of photosynthesis, Blackman developed the Law of Limiting Factors. According to this law, the component having the minimum supply will have the greatest effect on the rate of a physiological process. Similar to how several variables might alter the rate of photosynthesis, these include

Light

• The rate of photosynthesis and the rate of light-dependent photosynthesis processes both increase as light intensity rises.
• The quantity of photons that drop on a leaf likewise rises with rising light intensity. More chlorophyll molecules are ionised as a consequence, which increases the production of ATP and NADH.
• But at a certain point, when the light intensity rises, the rate of photosynthesis stays constant. Currently, photosynthesis is impeded by other causes.
• In addition, the wavelength of light influences the photosynthesis rate.
• Various photosynthetic systems absorb light energy very effectively at certain wavelengths.

Carbon dioxide

• As carbon dioxide concentration rises, the rate during which carbon is incorporated into carbohydrates through the light-independent stages of photosynthesis increases.
• Thus, raising the atmospheric carbon dioxide concentration causes the rate of photosynthesis to grow quickly, up to a point at which it is constrained by other variables.

Temperature

• Because they are enzyme-catalyzed, the light-dependent processes of photosynthesis are unaffected by temperature fluctuations, but the light-independent reactions are.
• Once the enzymes have reached their ideal temperature, the pace of the reactions starts to slow down as the enzymes start to denature.

Process: Steps of Photosynthesis

Four steps or processes may be used to objectively separate the total photosynthetic process:

1. Absorption of light

• As the initial step of photosynthesis, chlorophylls coupled to proteins in chloroplast thylakoids absorb light.
• After light energy is absorbed, oxygen is produced by withdrawing electrons via an electron donor like water.
• Next, the electrons are transported to quinine (Q), a CoQ-like primary electron acceptor in the electron transfer chain.

2. Electron Transfer

• With a chain of electron transfer molecules located in the thylakoid membrane, the electrons are now transferred from the primary electron acceptor towards the terminal electron acceptor, which is frequently NADP+.
• The proton gradient across the membrane is the consequence of protons being pushed out of the membrane when electrons are transported across it.

3. Generation of ATP

• ATP is produced from ADP and Pi by protons moving from the stroma to the lumen of the thylakoid via the F0F1 complex.
• This process and the ATP synthesis stage in the electron transport chain are the same.

4. Carbon Fixation

• The breakdown of carbon into six-carbon sugar molecules is powered by electrons and the NADP and ATP generated in steps 2 and 3, respectively.
• The previous three stages of photosynthesis are known as light reactions because they directly rely on light energy, while this step’s reactions are dark reactions since they don’t rely on light.

Types/Stages/Parts of photosynthesis

The light-dependent processes and also the Calvin cycle are the two stages of photosynthesis. Light-dependent mechanisms in the thylakoid membrane use light energy to make ATP and NADPH. In the stroma, the Calvin cycle uses the energy from these molecules to make GA3P from CO2.

Based on how light energy is used, photosynthesis is separated into two stages:

1. Light-dependent reactions

• Only when the plants or microorganisms are lit can the light-dependent photosynthetic processes occur.
• In the light-dependent processes, chlorophyll as well as other pigments in photosynthetic cells acquire light energy and store them like ATP and NADPH simultaneously releasing O2 gas.
• The chlorophyll collects significant amounts of energy and short-wavelength light during the light-dependent processes of photosynthesis, which stimulates the electrons in the thylakoid membrane.
• The conversion of light energy into chemical energy is now started by the excitation of electrons.
• Two photosystems that are found in chloroplast thylakoids are included in the light reactions.

2. Photosystem II

• In order to absorb light energy and transmit electrons via a series of molecules until they reach an electron acceptor, a combination of proteins and pigments called photosystem II must operate together.
• A pair of chlorophyll molecules, sometimes referred to as P680 because they are the greatest at absorbing light with a wavelength of 680 nm, are part of photosystem II.
• After absorbing light energy, the P680 gives a pair of electrons, resulting in an oxidised version of the P680.
• An enzyme finally divides a water molecule into two electrons, two hydrogen ions, and two oxygen molecules.
• Then, P680 receives this pair of electrons, which causes it to go back to its beginning state.

3. Photosystem I

• The difference between photosystem I and photosystem II is that photosystem I has a pair of chlorophyll molecules known as P700 because they optimally absorb light at a wavelength of 700 nm.
• Photosystem I gets activated and transfers electrons as it absorbs light energy.
• After receiving an electron from photosystem II, the now-oxidized form of P700 re-strings to its starting state.
• Following that, the ferredoxin protein conducts a series of redox reactions using the electrons from photosystem I.
• After eventually reaching NADP+, the electrons reduce it to NADPH.

Reaction 

2 H2O + 2 NADP+ + 3 ADP + 3 Pi + light → 2 NADPH + 2 H+ + 3 ATP + O2

1. Light independent reactions (Calvin cycle)

Photosynthesis is a set of anabolic reactions that result in the formation of the six-carbon molecule glucose in plants. As they do not directly depend on light energy but do need the byproducts of light reactions, these reactions are sometimes known as “dark reactions.”

This phase consists of three more stages leading to carbon fixation and assimilation.

Step 1: Fixation of CO2 into 3-phosphoglycerate

• In this phase, the ribulose 1,5-biphosphate carboxylase enzyme, commonly known as rubisco, catalyses the covalent attachment of one CO2 molecule to the five-carbon complex ribulose 1,5-biphosphate.
• As a consequence of the connection, an unstable six-carbon complex is created, which is subsequently broken down into two molecules of 3-phosphoglycerate.

Step 2: Conversion of 3-phosphoglycerate to glyceraldehydes 3-phosphate

• Two distinct processes transform the 3-phosphoglycerate created in step 1 into glyceraldehyde 3-phosphate.
• In order to produce 1,3-bisphosphoglycerate, the enzyme 3-phosphoglycerate kinase, which is found in the stroma, first catalyses the transfer of a phosphoryl group from ATP to 3-phosphoglycerate.
• The chloroplast-specific isozyme of glyceraldehyde 3-phosphate dehydrogenase then catalyses a process where NADPH contributes electrons to create glyceraldehyde-3-phosphate and phosphate (Pi).
• The regeneration of ribulose 1,5-bisphosphate uses the majority of the glyceraldehyde-3-phosphate that is so generated.
• Either the leftover glyceraldehyde is exported to the cytosol and converted to sucrose for transport to the plant’s development regions, or it is transformed to starch in the chloroplast and preserved for future use.

Step 3: Regeneration of ribulose 1,5-biphosphate from triose phosphates

• Using intermediates made of three-, four-, five-, six-, and seven-carbon sugars, the three-carbon molecules produced in earlier steps are subsequently converted into the five-carbon complex ribulose 1,5-biphosphate.
• When the initial molecules in the process are renewed, photosynthesis at this step creates a cycle (the Calvin cycle).

Reaction

Glyceraldehyde-3-phosphate (G3P): 3 CO2 + 9 ATP + 6 NADPH + 6 H+ 9 ADP + 8 Pi + 6 NADP+ 3 H2O

Since a G3P molecule has three fixed carbon atoms, two G3Ps are required to create a glucose molecule with six carbons. One molecule of glucose would be created after six cycles.

Products of Photosynthesis

Examples of photosynthetic processes that rely on light include: 

ATP

NADPH

O2

H+ ions

The following are the byproducts of the Calvin cycle of photosynthesis:

glucose/glyceraldehyde-3-phosphate (G3P) (carbohydrates)

H+ ions

The general outcomes of photosynthesis include:

Glucose (carbohydrates)

Water

Oxygen

Sulfur (in photosynthetic sulphur bacteria)

Photosynthesis examples

Photosynthesis in green plants or oxygenic bacteria

• Chlorophyll, a green pigment, is necessary for photosynthesis to occur in plants and oxygen-consuming microbes like cyanobacteria.
• It occurs in the chloroplasts’ thylakoids and produces oxygen gas, glucose, and water molecules as byproducts.
• In plants, the majority of the glucose molecules are connected to create starch, fructose, or even sucrose.

Photosynthesis in sulphur bacteria

• Instead of using water, hydrogen sulphur is used during photosynthesis in purple sulphur bacteria.
• Chlorophyll is present in some of these bacteria, such as the green sulphur bacteria, whereas carotenoids are the photosynthetic pigments in the purple sulphur bacteria.
• These bacteria produce carbohydrates (not necessarily glucose), sulphur gas, and water molecules as a consequence of photosynthesis.

Importance of photosynthesis

• In autotrophs, which produce their food by using carbon dioxide, sunshine, and photosynthetic pigments, photosynthesis is the main source of energy.
• Heterotrophs, who get their energy from autotrophs, are equally dependent on photosynthesis.
• For the atmosphere’s oxygen levels to remain stable, plants must engage in photosynthesis.
• In addition, the byproducts of photosynthesis play a role in the carbon cycle that affects the atmosphere, seas, plants, and animals.
• In a similar way, it supports the coexistence of people, animals, and plants.
• All other kinds of energy on earth are mostly derived from sunlight or solar energy, which is used by plants during photosynthesis.

Artificial photosynthesis

An artificial version of photosynthesis is a chemical process that uses carbon dioxide, water, and sunshine to generate oxygen and carbohydrates.

• Photocatalysts that can mimic the oxidation-reduction processes occurring during natural photosynthesis are used in artificial photosynthesis.
• The primary goal of artificial photosynthesis is to convert light energy into solar fuel that can be stored and used when sunshine is not available.
• Artificial photosynthesis may be utilised to synthesise just oxygen from water and sunshine, resulting in clean energy generation while solar fuels are being produced.
• The photocatalytic splitting of a water molecule into oxygen and significant amounts of hydrogen gas is the most crucial step in artificial photosynthesis.
• Additionally, light-driven carbon reduction may be used to mimic the natural process of carbon fixation, which yields molecules of carbohydrates.
• In order to produce solar fuels, photoelectrochemistry, engineered enzymes, and photoautotrophic microorganisms that produce microbial biofuel and biohydrogen from sunlight, artificial photosynthesis is used.

Photosynthesis vs Cellular respiration

Photosynthesis	Cellular respiration
Photosynthesis takes place in green plants, algae, and some photosynthetic bacteria.	Cellular respiration takes place in all living organisms.
The process of photosynthesis occurs in the thylakoids of chloroplasts.	The process of cellular respiration occurs in mitochondria.
The reactants of photosynthesis are light energy, carbon dioxide, and water.	The reactants of cellular respiration are glucose and oxygen.
6CO2 + 6H2O → C6H12O6 + 6O2	6O2 + C6H12O6 → 6CO2 + 6H2O
The products of photosynthesis are carbon dioxide, water, and energy.	The products of cellular respiration are glucose, oxygen, and water molecules.
Photosynthesis is an anabolic process, resulting in the production of organic molecules.	Cellular respiration is a catabolic process, resulting in the oxidation of organic molecules to release energy.
Photosynthesis is an endergonic reaction that results in the utilization of energy.	Cellular respiration is an exergonic reaction that results in the release of energy
Photosynthesis can only take place in the presence of sunlight.	Cellular respiration occurs all the time as it doesn’t require sunlight.

FAQs (Revision Questions)

Where does photosynthesis occur?

Photosynthesis occurs in the thylakoid membrane of the chloroplasts.

What are the products of photosynthesis?

The products of photosynthesis are carbohydrates (glucose), oxygen, and water molecules.

What are the reactants of photosynthesis?

The reactants of photosynthesis are carbon dioxide, water, photosynthetic pigments, and sunlight.

How are photosynthesis and cellular respiration related?

Photosynthesis and cellular respiration are essentially the reverses of one another where photosynthesis is an anabolic process resulting in the formation of organic molecules. In contrast, cellular respiration is a catabolic process resulting in the breaking down of organic molecules to release energy.

References

• Berg JM, Tymoczko JL, Stryer L. Biochemistry. 5th edition. New York: W H Freeman; 2002. Section 17.2, Entry to the Citric Acid Cycle and Metabolism Through It Are Controlled.Available from: https://www.ncbi.nlm.nih.gov/books/NBK22347/
• Nelson DL and Cox MM. Lehninger Principles of Biochemistry. Fourth Edition.
• Montero F. (2011) Photosynthetic Pigments. In: Gargaud M. et al. (eds) Encyclopedia of Astrobiology. Springer, Berlin, Heidelberg
• Lodish H, Berk A, Zipursky SL, et al. Molecular Cell Biology. 4th edition. New York: W. H. Freeman; 2000. Section 16.3, Photosynthetic Stages and Light-Absorbing Pigments.Available from: https://www.ncbi.nlm.nih.gov/books/NBK21598/

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