Plants convert energy from sunlight into sugar in a process called photosynthesis. Photosynthesis begins with the light reactions, which convert the electromagnetic energy of light into the chemical energy of ATP. The reactions link an electron transfer process to the charging of membrane. In the plants, flattened, membrane-enclosed sacs (thylakoid disks) are the batteries. Large protein photosystems, embedded in the membrane, capture the energy of light and use it to strip electrons from water, forming oxygen gas. The electrons are then transferred down a chain of enzymes, and the membrane is charged with hydrogen ions similarly. The charged membrane then is used, as in oxidative phosphorylation, to make ATP. After a day’s work in the sun, the plant has build up a supply of ATP. In the dark reactions, the ATP formed in the light is put to work.
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