Why are the light reactions of photosynthesis sometimes referred to as “noncyclic electron transfer”?
There are two possible routes the noncyclic and cyclic. The noncyclic is the predominant route. The photons of light hit the p680 (photosystem II) to begin the process. Here an electron is excited to a higher energy level and captured by the primary electron acceptor. This electron hole must be filled so water is split releasing oxygen to the atmosphere and the hydrogen replaces the excited electrons. These photo excited electrons are passed to photosystem I (p700) through a cytochrome complex. Here the downward movement produces ATP. Light energy is absorbed by photosystem I and reenergizes the electrons moving them to a higher energy level. Here these electrons pass through an electron transport chain producing NADPH from NADP. This along with the ATP is passed out into the stroma to be used in the Calvin cycle. This is a one way process so it does not cycle. Cyclic flow uses photosystem I only. It is used when additional ATP is needed but no NADPH is produced.
