Photosynthesis is the process by which green plants prepare food using sunlight, carbon dioxide, and water, converting light energy into chemical energy. During the light reaction, ATP and NADPH are produced in the thylakoid membranes of chloroplasts through a process called photophosphorylation. Photophosphorylation is the synthesis of ATP from ADP and inorganic phosphate using light energy absorbed by chlorophyll pigments.
Based on the pathway of electron flow, photophosphorylation is of two types: cyclic photophosphorylation and non-cyclic photophosphorylation.
Cyclic Photophosphorylation
Cyclic photophosphorylation is a process in which the excited electrons released from Photosystem I return back to the same photosystem after passing through a series of electron carriers. In this process, only Photosystem I participates, and the electron movement occurs in a cyclic pathway.
The process mainly occurs in the stroma lamellae or intergranal thylakoid membranes of chloroplasts. It generally takes place when the plant requires extra ATP or when NADP⁺ is not available for accepting electrons.
Features of Cyclic Photophosphorylation
- Only Photosystem I participates in the process.
- Electrons move in a cyclic pathway.
- ATP is produced during the process.
- NADPH is not formed.
- Oxygen is not released because water splitting does not occur.
- It occurs mainly in the stroma lamellae of chloroplasts.
- The electrons return to the original chlorophyll molecule.
Mechanism of Cyclic Photophosphorylation
In cyclic photophosphorylation, light energy is absorbed by the pigment molecule P700 present in Photosystem I. As a result, electrons become excited and leave the reaction centre.
- These high-energy electrons are transferred to a series of electron carriers, including Ferredoxin (Fd), Cytochrome complex, and Plastocyanin (PC).
- During their movement through the electron transport chain, energy is released.
- This energy is used for the synthesis of ATP molecules from ADP and inorganic phosphate.
- Finally, the electrons return to the original reaction centre P700 of Photosystem I.
- Since the electrons return to the same photosystem from which they originated, the process is called cyclic photophosphorylation.
Non-cyclic Photophosphorylation
Non-cyclic photophosphorylation is the process in which the excited electrons do not return to the original chlorophyll molecule after passing through the electron transport chain. Instead, the electrons are transferred to NADP⁺ to form NADPH. This process involves both Photosystem II and Photosystem I.
Non-cyclic photophosphorylation occurs mainly in the grana thylakoid membranes of chloroplasts and is the major pathway of ATP production during photosynthesis.
Features of Non-cyclic Photophosphorylation
- Both Photosystem I and Photosystem II participate.
- Electrons move in a non-cyclic or linear pathway.
- Both ATP and NADPH are produced.
- Oxygen is released due to the photolysis of water.
- Water molecules are split during the process.
- Electrons do not return to the original chlorophyll molecule.
- It mainly occurs in the grana thylakoid membranes.
Mechanism of Non-cyclic Photophosphorylation
The process begins when Photosystem II absorbs light energy through its reaction centre chlorophyll molecule called P680. The absorbed energy excites electrons, which leave the reaction centre and move through a series of electron carriers.
- The electrons pass through Plastoquinone (PQ), Cytochrome b6f complex, and Plastocyanin (PC). As electrons move through these carriers, energy is released and used for ATP synthesis.
- The electrons finally reach Photosystem I, where they again absorb light energy and become re-excited. These high-energy electrons are then transferred to ferredoxin and finally to NADP⁺ reductase enzyme, which reduces NADP⁺ into NADPH.
- Since the electrons lost from Photosystem II are not returned, they are replaced by electrons obtained from the splitting of water molecules. This process is called photolysis of water.
- The splitting of water produces Electrons, Protons (H⁺ ions), and Oxygen gas (O₂). The oxygen produced is released into the atmosphere.
Importance of Photophosphorylation
The importance of photophosphorylation is explained below:
- Photophosphorylation produces ATP molecules, which act as the immediate source of energy for various cellular and metabolic activities in plants. ATP generated during this process is required for carbon fixation and sugar synthesis during the dark reaction of photosynthesis.
- During non-cyclic photophosphorylation, NADP+ is reduced to NADPH. NADPH provides reducing power required for the reduction of carbon dioxide into carbohydrates in the Calvin cycle.
- Photophosphorylation converts light energy from the sun into stable chemical energy stored in ATP and NADPH. This conversion is essential for sustaining all biological activities in living organisms.
- Without photophosphorylation, photosynthesis cannot proceed efficiently because ATP and NADPH are necessary for the synthesis of glucose and other organic compounds.
- The energy produced during photophosphorylation ultimately helps plants synthesise carbohydrates, which serve as food for plants themselves and for all other living organisms directly or indirectly.