Biology – Photosynthesis as an energy transfer process | e-Consult

Cyclic Photophosphorylation: This pathway involves only photosystem I (PSI) and does not require light energy to split water. Electrons are excited in PSI and passed to an electron carrier, which then returns to PSI via the electron transport chain. This cyclic flow of electrons generates a proton gradient across the thylakoid membrane, which drives ATP synthesis via ATP synthase. Oxygen is not produced. NADPH is not produced. The primary output is ATP.

Non-Cyclic Photophosphorylation: This pathway involves both photosystem II (PSII) and photosystem I (PSI). Light energy is absorbed by PSII, leading to the splitting of water molecules (photolysis). This releases electrons, protons (H+), and oxygen. The electrons are passed along the electron transport chain to PSI. Light energy also excites electrons in PSI, which are then passed to NADP+ reductase, reducing NADP+ to NADPH. The proton gradient generated by PSII drives ATP synthesis. Oxygen is a byproduct. NADPH is produced.

Key Differences Summarized:

• Photosystems Involved: Cyclic: PSI only; Non-cyclic: PSII and PSI
• Water Splitting: Cyclic: No; Non-cyclic: Yes
• Oxygen Production: Cyclic: No; Non-cyclic: Yes
• NADPH Production: Cyclic: No; Non-cyclic: Yes
• ATP Production: Both produce ATP, but the ratio of ATP to NADPH differs.