The Biology Project: Biochemistry

Photosynthesis Problem Set 1

Problem 3 Tutorial: Overall redox reaction

During noncyclic photophosphorylation, water is oxidized and the electrons from water pass through photosystem II and photosystem I before reducing:

Pathway of electron transfer

The pathway of electrons in photosynthesis is shown with red arrows in the diagram below. Each step in this pathway is a coupled oxidation-reduction reaction. Water is oxidized as a result of the light reaction of photosystem II. From photosystem II, electrons pass to plastoquinone (PQ) to cytochromes (Cyt) to plastocyanin (PC). Another light reaction at photosystem I activates electrons for transfer to ferredoxin, and finally to NADP+ .

The overall equation for non-cyclic electron transport

Water is oxidized to oxygen, releasing protons. The protons become part of the energy source for ATP synthesis. The electrons from water eventually reduce NADP+ to NADPH.

The Biology Project
University of Arizona
Thursday, October 3, 1996
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