What Is Photophosphorylation?

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Action of a photosystem in passing electrons to the reaction center (chlorophyll a) so that light of the critical wavelength can excite the electrons to a primary electron acceptor and then into an electron transport chain.

Photophosphorylation is the use of excited electrons to add a phosphate group to a molecule. Light strikes a form of chlorophyll in Photosystem II. The electrons attach to a primary electron acceptor in their excited state. A sequence of redox reactions begins where the electrons pass through some membrane-bound proteins that carry out redox reactions. Energy is released, gathered and stored as ADP. It is then reduced to ATP. The electrons eventually reach a molecule in Photosystem I. The P680 molecule replaces the lost electrons by breaking water into hydrogen and oxygen ions and electrons. The process releases oxygen when the oxygen ions combine into O2.

Light excites a molecule of chlorophyll a in Photosystem I, causing electrons to be boosted to a still higher level. The electrons are attached to a different primary electron acceptor (not in Photosystem II). The electrons pass through a series of redox reactions and attach to NADP+ and H+ to form NADPH. NADPH is an energy carrier needed in the Light Independent Reaction. Electrons from Photosystem II replace the excited electrons lost by the chlorophyll a molecule. The continuous flow of electrons from water to NADPH is used to construct carbohydrates during the Light Independent Process.

This electron flow occurs in primitive bacteria (that use photosynthesis) and occasionally in some eukaryotes. Since these bacteria live in extremely acidic environments, hydrogen used in the construction of a carbohydrate comes from the environment, not from NADPH. No oxygen is formed since water is not split into ions. They produce only ATP, not NADPH.

The generation of ATP occurs by chemiosmosis. Electrons move along the transport chain next to Photosystem II. The difference in electron levels increases the hydrogen ion levels in the thylakoid membrane. This drives the conversion of ADP + P into ATP.

Living systems cannot directly use light energy. Instead, they convert the light energy stored in ATP and NADPH into the C-C bond energies of simple carbohydrates. This stored energy can then be released on demand by other metabolic processes. The reactions that use ATP and NADPH to convert carbon dioxide into carbohydrates make up the second stage of photosynthesis. These reactions are
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