Cyclic and Non-cyclic Photophosphorylation

Cyclic and non-cyclic photophosphorylation are two different pathways involved in the light-dependent reactions of photosynthesis. These pathways occur in the thylakoid membrane of chloroplasts and are responsible for generating ATP, the energy molecule used in the Calvin cycle.

Non-cyclic Photophosphorylation:
Non-cyclic photophosphorylation is the primary pathway for generating ATP in plants during photosynthesis. It involves two photosystems, known as Photosystem I (PSI) and Photosystem II (PSII), and several electron carriers.

1. Photosystem II:
- PSII absorbs light energy and transfers it to a reaction center chlorophyll called P680.
- P680 then transfers an excited electron to an electron acceptor, leaving P680 positively charged.
- The electron from PSII is replaced by the splitting of water, releasing oxygen as a byproduct.
- The excited electron is then passed through a series of electron carriers, generating ATP through chemiosmosis.

2. Photosystem I:
- The electron that leaves PSII is received by PSI, which absorbs light energy and transfers it to a reaction center chlorophyll called P700.
- P700 then transfers an excited electron to another electron acceptor, leaving P700 positively charged.
- The electron is then passed through another series of electron carriers, ultimately reducing NADP+ to NADPH.

3. ATP Synthesis:
- As the electrons pass through the electron transport chain, they release energy that is used to pump protons (H+) across the thylakoid membrane, creating a proton gradient.
- The protons then flow back across the membrane through ATP synthase, a protein complex that uses the energy from the proton flow to synthesize ATP.

Cyclic Photophosphorylation:
Cyclic photophosphorylation is an alternative pathway that can occur in certain organisms, such as bacteria and some algae. It involves only Photosystem I and does not produce NADPH or release oxygen.

1. Photosystem I:
- In cyclic photophosphorylation, the excited electron from P700 is passed back to the electron transport chain instead of being transferred to NADP+.
- This creates a cyclic flow of electrons, as the electron is continuously cycled back to PSI.

2. ATP Synthesis:
- The cyclic flow of electrons in PSI generates ATP through chemiosmosis, similar to non-cyclic photophosphorylation.
- However, since no NADPH or oxygen is produced, cyclic photophosphorylation is less efficient in terms of overall energy storage.

Key Differences between Cyclic and Non-cyclic Photophosphorylation:
1. Non-cyclic photophosphorylation involves both Photosystem I and Photosystem II, while cyclic photophosphorylation involves only Photosystem I.
2. Non-cyclic photophosphorylation produces both ATP and NADPH, whereas cyclic photophosphorylation only produces ATP.
3. Non-cyclic photophosphorylation releases oxygen as a byproduct of water splitting, while cyclic photophosphorylation does not.
4. Non-cyclic photophosphorylation is the primary