Photosynthesis
Light Dependent Reactions
Photosystems
The light-dependent reactions occur on the thylokid membrane, containg photosystems I and II, which harvest light energy to generate ATP and NADPH in the stroma. The excited elecrtons are transferred in two different ways in cylic and non-cyclic photophosphorylation with different results.
Cyclic Photophosphorylation
Cyclic photophosphorylation requires photosystem I, but not photosystem II. Light-dependent electron transport occurs in the thylakoid membranes, where electrons carriers such as: ferredoxin, toquinone, cytochrome, and plastocyanin follow a cyclic pathway returning to the photosystem I reaction center. The energy of this electron transport results in a H+ gradient formation, the energy source for ATP synthesis. ATP is formed from ADP and Pi, but NADP+ is not reduced.
Non-Cyclic Photophosphorylation
During noncyclic photophosphorylation, both PS I and PS II are involved. During this process water is oxidized and the electrons from water pass through photosystem II and photosystem I before reducing. The electrons are excited and accepted by the primary electron acceptor.These electrons go down the electron transport chain, which creates a proton gradient that drives ATP synthesis. From photosystem II, electrons pass to plastoquinone (PQ) to cytochromes (Cyt) to plastocyanin (PC). Another light reaction in photosystem I activates electrons for transfer to ferredoxin, and finally to NADP+.
Overall, 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.
