Arrays of chlorophyll molecules embedded, for example, within thylakoid membranes.
There are two types of photosystems found in chloroplasts and cyanobacteria, termed photosystem I and photosystem II. In the Z scheme, i.e., noncyclic electron flow, photosystem II preceeds photosystem I. Alternatively, only photosystem I participates in cyclic electron flow. In any case, photosystems are a component particularly of the light reaction of photosynthesis, being the location photon-excitement of electrons.
Phtosystems consist of multiple chlorophyll molecules, rather than just one, because it requires a relatively large amount of light-absorbing material to keep the post-reaction center aspects of photosynthesis, e.g., electron transport chain, running at an optimal rate. Photosystems thus have substantially more chlorophyll molecules than may donate electrons to the photosystem primary electron acceptor molecules at any one time.
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