Compare and contrast cyclic and noncyclic photophosphorylation.

Both cyclic photophosphorylation and noncyclic photophosphorylation are aspects of the light-dependent reactions of photosynthesis. They both involve the excitation of electrons by solar energy; these excited electrons then participate in an electron transport pathway to create a proton gradient that can be used to produce ATP through chemiosmosis. However, as its name implies, cyclic photophosphorylation occurs when electrons return to the chlorophyll molecules whence they came. Excited electrons in noncyclic photophosphorylation are also used in the same way to produce ATP; however, these electrons are donated to an NADP+ molecule at the end of their transport chain, producing NADPH as a product of the system. Because electrons are constantly leaving the chlorophyll molecules where they are being excited, and not returning, there must also be a steady source of electrons for the photosystem. These electrons can come from molecules such as water or hydrogen sulfide.