light reactions

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(light-dependent stage)

During photosynthesis, those reactions which require the presence of light (e.g. photophosphorylation and photolysis). During the light reactions, light strikes chlorophyll molecules, which absorb its energy. This allows one electron to escape for each photon of light absorbed. The electron attaches to a neighbouring molecule, thereby ejecting another electron and causing electrons to move along an electron-transport chain of molecules. Some of the transported energy is used to attach phosphate groups to molecules of adenosine diphosphate (ADP) converting them to adenosine triphosphate (ATP); this reaction is called photophosphorylation. ATP is used to carry energy to wherever it is needed in the organism. Energy not used in the ADP→ATP reaction is used to split a water molecule into H+ and OH ions; the reaction is called photolysis. The H+ attaches to nicotinamide adenine dinucleotide phosphate (NADP), converting it to NADPH. The OH passes one electron to the chlorophyll molecule, restoring the neutrality of both chlorophyll and hydroxyl. Hydroxyls then combine to form water (4OH→2 H2O + O2 ↑).

Subjects: Plant Sciences and Forestry — Ecology and Conservation.

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