Photosynthesis: Light, Energy, and Carbon Fixation

Photosynthesis and Chemical Energy

Photosynthesis is the biological process by which plants, algae, cyanobacteria, and some bacteria convert light energy into chemical energy. In oxygenic photosynthesis, light energy is absorbed by pigments such as chlorophyll, water is split to release oxygen, and the resulting electrons help produce ATP and NADPH. These energy-rich molecules are then used to convert atmospheric carbon dioxide into carbohydrates, including glucose, starch, sucrose, and cellulose. The stored chemical energy supports the organism’s metabolism and provides energy to other organisms through food chains. Photosynthesis occurs in two major stages. Light-dependent reactions take place in the thylakoid membranes of chloroplasts and use photosystems, electron transport chains, proton gradients, and ATP synthase to produce ATP and NADPH. Light-independent reactions, primarily the Calvin cycle, occur in the chloroplast stroma and use ATP and NADPH to fix carbon dioxide into three-carbon sugars that are later formed into carbohydrates and other organic molecules. Some organisms perform anoxygenic photosynthesis, using electron donors such as hydrogen sulfide instead of water and producing sulfur rather than oxygen. Photosynthesis is essential for Earth’s oxygen supply, global carbon cycling, and the energy needs of complex life.

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Photosynthetic Organisms and Autotrophy

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