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

Photosystem I (PSI) also absorbs light energy, exciting electrons in chlorophyll. These energized electrons are then passed along a second electron transport chain. Unlike the ETC in PSII, the electrons arriving at PSI have a higher energy level than those that entered the ETC in PSII.

The electrons from PSI are ultimately passed to NADP+, reducing it to NADPH. NADPH is a crucial reducing agent used in the Calvin cycle for carbon fixation. However, the movement of electrons through the PSI-linked ETC also contributes to the proton gradient across the thylakoid membrane. Electrons are passed to a carrier molecule, which then pumps protons from the stroma into the thylakoid lumen. This process is less efficient than the proton pumping in PSII, but it still adds to the overall proton gradient.

The proton gradient, established by both PSII and PSI, represents potential energy. This gradient drives the movement of protons back into the stroma through ATP synthase. As protons flow through ATP synthase, the enzyme uses the energy from this flow to phosphorylate ADP, generating ATP. Therefore, the electron transport chain associated with PSI contributes to ATP synthesis by further strengthening the proton gradient, which is then harnessed by ATP synthase to produce ATP. The combination of both PSII and PSI ETCs is essential for efficient ATP synthesis during photosynthesis.