Mechanism of Photosynthesis: Definition and Process

The mechanism of photosynthesis mainly occurs via two phases: The light-dependent phase and the light-independent phase. The liberation of oxygen occurs by water oxidation, which occurs at the thylakoid membrane in a photochemical reaction. Reduction of CO2CO2 happens at the stroma of chloroplasts in dark reactions. Let us explore more to know about the mechanism of photosynthesis, the process, types and examples of photosynthesis.

Definition of Photosynthesis

The synthesis of organic compounds like carbohydrates or glucose by the cells of green plants in the presence of sunlight with the help of \(C{O_2}\) and \({H_2}O\) is called photosynthesis. The chemical equation of this process is as follows:

Types of Photosynthesis

According to Cornelius Van Neil, there are two types of photosynthesis

1. Oxygenic Photosynthesis: This type of photosynthesis is mainly found in green plants, algae and cyanobacteria. In this type, \({H_2}O\) is the electron donor, and \({O_2}\) is evolved.
2. Non Oxygenic Photosynthesis- This type of photosynthesis is the characteristic of bacteria like Rhodospirillum, Chlorobium and Chromatium. In this type, \({H_2}S\) is the electron donor, and \({O_2}\) is not evolved.

Mechanism of Photosynthesis

Photosynthesis is the ultimate source of food on earth. It is also required for the release of oxygen. The part of the spectrum that is used in photosynthesis has a wavelength between \(400 – 700\,{\rm{nm}}\). It is called photosynthetically active radiation (PAR). Photosynthesis takes place in green parts of plants like leaves, stems, etc. The mesophyll cells of leaves have many chloroplasts that contain a specialised light-absorbing green pigment called chlorophyll pigments. Chloroplast is a double membrane-enclosed organelle. Photosynthesis is a complicated redox process. In this process, oxidation of water to oxygen and reduction of carbon in carbon dioxide to glucose takes place.

The photosynthesis process takes place in two stages:

1. Light-dependent or photochemical reaction or Hill reaction
2. Light-independent phase or biosynthetic phase or dark reaction

1. Light-dependent or Photochemical Reaction or Hill reaction

The light reaction takes place inside the grana of the chloroplast. Light is captured by photosynthetic pigments present in the quantasomes of grana.

(i) The ‘light-dependent or photochemical’ phase involves light absorption, water splitting, the release of oxygen, and the formation of high-energy chemical intermediates, ATP and \(NADPH\).
(ii) Several protein complexes, pigments are present in two discrete photochemical light-harvesting complexes, LHC within the photosystem-I (PS-I) and photosystem-II (PS-II).
(iii) Light-harvesting system, also called antennae, comprises antenna pigments associated with proteins. The antenna pigments are mainly chlorophyll-b, xanthophylls and carotenoids molecules, which absorb light of different wavelengths and transfer their energy to the reaction centre.
(iv) The reaction centre forms by a single chlorophyll-a molecule where the energy is trapped. In PS-I, the reaction centre has an absorption peak at \(700\,{\rm{nm}}\), hence P700; in PS-II, the reaction centre has an absorption maxima at \(600\,{\rm{nm}}\) and P680.
The Electron Transport

Fig: Electron Transport

(i) The chlorophyll of PS-II absorbs light at \(680\,{\rm{nm}}\) (red light), gets excited and causes electrons to jump.
(ii) These electrons are picked up by a primary electron acceptor and pass them downhill to an electrons transport system consisting of cytochromes through oxidation-reduction or redox reactions.
(iii) The electrons are passed on to the pigments of photosystem PS I.
(iv) PS-I are also excited after receiving a red light of wavelength \(700\,{\rm{nm}}\) and are transferred downhill again to reduce \(NAD{P^ + }\) to \(NADPH\) and \({H^ + }\).
(v) Bendall and Hill discovered this whole scheme of transfer of electrons and was called a Z-scheme due to its peculiar shape.

Fig: Electron Transport Chain in Chloroplast

Splitting of Water or Photolysis of Water

(i) The supply of electrons continuously is achieved by the splitting of water involving PS-II located on the inner side of the thylakoid membrane.
(ii) Water is split into \(2{H^ + },\left[ O \right]\) and electrons. This liberates oxygen (one of the net products of photosynthesis).

\(2{H_2}O \to 4{H^ + } + {O_2} + 4{e^ – }\)

Cyclic and noncyclic photo-phosphorylation

(i) Living organisms extract energy from oxidisable substances and store them as ATP.
(ii) The process through which ATP is synthesised from ADP and Pi (inorganic phosphate) by the cell organelles like mitochondria and chloroplast is called phosphorylation.
(iii) Photophosphorylation is the synthesis of ATP from ADP and inorganic phosphate in the presence of light.
(iv) The two photosystems are connected through an electron transport chain to produce ATP and \(NADPH\) and \({H^ + }\) in noncyclic photophosphorylation.
(v) In PSI, the electron is circulated within the photosystem, and the phosphorylation occurs in a cyclic manner in the stroma lamellae, only synthesising ATP, but not \(NADPH\) and \({H^ + }\).

Fig: Cyclic and Noncyclic Photo-Phosphorylation

Chemiosmotic Hypothesis

i) This explains how ATP is synthesised in the chloroplast.
ii) ATP synthesis occurs due to the development of a proton gradient across the thylakoid membrane.
iii) The proton or hydrogen ion accumulation occurs inside the membrane. In the lumen, the splitting of the water molecule and protons are transported across the membrane from the stroma due to the movement of electrons through the photosystems.
iv) The \(NADP\) reductase collects protons from the stroma. The \(NADP\) reductase along with electrons that come from the acceptor of electrons of PS-I and reduces \(NAD{P^ + }\) to \(NADPH\) and \({H^ + }\).
a) Hence, within the chloroplast, protons in the stroma decrease in number, while in the lumen (decrease in pH), there is an accumulation of protons creating a proton gradient across the thylakoid membrane.
(v) This gradient leads to the synthesis of ATP, with the movement of protons across the membrane to the stroma.
(vi) The transmembrane channel of the \(C{F_0}\) of the ATP synthase carries out facilitated diffusion of protons from lumen to stroma.
(vii) The \(C{F_1}\) faces the stroma and undergoes the conformational change due to the gradient breakdown, which provides energy to synthesise ATP.
(viii) The ATP will be used immediately in the biosynthetic reaction.

Fig: Chemiosmosis in chloroplast

2. Light-Independent or Dark Reaction or Biosynthetic Phase

i) This process is light-independent, or it does not require direct sunlight but depends on the products of the light reaction, i.e., ATP and \(NADPH\), besides \(C{O_2}\) and \({H_2}O\).
ii) The \(C{O_2}\)  is combined with \({H_2}O\) to produce \({\left( {C{H_2}O} \right)_n}\) or sugars.
iii) Melvin Calvin used radioactive  \(14C\) in algal photosynthesis and discovered that the first \(C{O_2}\) fixation product is a \(3\)-carbon organic acid, \(3\)-phosphoglyceric acid PGA in \({C_3}\) plants. Calvin also explained the complete biosynthetic pathway; hence it is called the Calvin cycle.
(iv) There is another group of plants (\({C_4}\) plants), where the first stable product of \(C{O_2}\) fixation is an organic acid, with \(4\) carbon atoms, identified as oxaloacetic acid or \(OAA\). The first \(C{O_2}\) acceptor molecule is a \(5\)-carbon ketose sugar, ribulose bisphosphate \(\left( {RuBP} \right)\).

The Calvin Cycle (\({C_3}\) Pathway)

i) Calvin and his co-workers explained that the pathway operated in a cyclic manner with the regeneration of \(RuBP\).
ii) The three stages of the Calvin Cycle are carboxylation, reduction and regeneration.
iii) Carboxylation
(a) The most crucial step.
(b) Fixation of \(C{O_2}\) into a stable organic intermediate.
(iv) \(C{O_2}\) is utilised for the carboxylation of \(RuBP\) by the enzyme \(RuBP\) carboxylase-oxygenase or \(RuBisCO\), resulting in the formation of two \(3\)-PGA.
(v) Reduction:
(a) This is the series of reactions that lead to the formation of glucose.
(b) Involves \(2\) molecules of ATP for phosphorylation and two molecules of \(NADPH\) for reduction per \(C{O_2}\) molecule fixed.
(c) The fixation of six molecules of \(C{O_2}\) and \(6\) turns of the cycle are required to form one molecule of glucose from the pathway.
(vi) Regeneration:
(a) The \(C{O_2}\) acceptor molecule RuBP is regenerated to continue the cycle.
(b) These steps require one ATP for phosphorylation to form \(RuBP\).
(vii) For every \(C{O_2}\) molecule which enters the Calvin cycle, \(3\) molecules of ATP and \(2\) of NADPH are required.
(viii) To make one glucose molecule, \(6\) turns of the cycle with \(18\) ATP and \(12\) \(NADPH\) are required.

Fig: Calvin Cycle

Summary

Photosynthesis is the ultimate source of food on earth. It is also required for the release of oxygen. Plants complete photosynthesis in two stages: Light-dependent and light-independent phases.  The light-dependent phase involves light absorption, water splitting, the release of oxygen, and the formation of high-energy chemical intermediates, ATP and \(NADP\). Several protein complexes, pigments are present in two discrete photochemical light-harvesting complexes, LHC within the photosystem-I (PS-I) and photosystem-II (PS-II). After absorbing light, electrons are excited and carried through PS-II and PS-I and at last to \(NADP\), forming \(NADPH\). Splitting of water molecules is linked with PS-II, resulting in the evolution of \({O_2}\), protons and transfer of electrons to PS II. The light-independent does not require direct sunlight but depends on the products of the light reaction, i.e., ATP and \(NADPH\), besides \(C{O_2}\) and \({H_2}O\). The \(C{O_2}\)  is combined with \({H_2}O\) to produce \({\left( {C{H_2}O} \right)_n}\) or sugars.

FAQs

Q.1. What do you mean by oxygenic photosynthesis?
Ans: Oxygenic photosynthesis is mainly found in green plants, algae and cyanobacteria. In this type, \({H_2}O\) is the electron donor, and \({O_2}\) is evolved.

Q.2. What are the two mechanisms of photosynthesis?
Ans: The two mechanisms of the photosynthesis process takes place in two stages:
1. Light-dependent or photochemical reaction or Hill reaction
2. Light-independent phase or biosynthetic phase or dark reaction.

Q.3. Who discovered the mechanism of photosynthesis?
Ans: The mechanism of photosynthesis is discovered by Jan Baptist van Helmont.

Q.4. What is the difference between cyclic and noncyclic photophosphorylation?
Ans: The difference between cyclic and noncyclic photophosphorylation:
1. In cyclic photophosphorylation, photosystem-I is involved, whereas in noncyclic photophosphorylation, photosystem-I and photosystem-II both are involved.
2. Photolysis of water does not occur in cyclic photophosphorylation, whereas photolysis of water occurs in noncyclic photophosphorylation.

Q.5. What is the difference between light and dark reactions?
Ans: The difference between light and dark reactions:
1. The light reaction is the initial stage of photosynthesis which traps light energy to produce ATP and \(NADPH\), whereas dark reaction is the second step of photosynthesis which utilises the energy from ATP and \(NADPH\) to produce glucose.
2. The light reaction occurs in the grana of the chloroplast, whereas the dark reaction occurs in the stroma of the chloroplast.

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