Photosynthesis: processes, organisms, and stages

Definition and purpose of photosynthesis

Photosynthesis is a set of biological processes used by photopigment-bearing autotrophs (such as most plants, algae, and cyanobacteria) to convert light energy—usually from sunlight—into chemical energy that powers metabolism. In oxygenic photosynthesis (the most common form), light energy drives the splitting of water, releasing oxygen as a byproduct. The captured energy is stored in the chemical bonds of organic compounds, especially carbohydrates like sugars and starches. The purpose of photosynthesis is twofold: (1) to produce and maintain the oxygen content of Earth’s atmosphere and (2) to supply most of the biological energy needed for complex life. Photosynthetic organisms capture light, use it to generate energy carriers (such as ATP and NADPH), and then use those products to fix carbon dioxide into carbohydrates (e.g., via the Calvin cycle). Some organisms perform anoxygenic photosynthesis, which does not release oxygen, using other electron donors instead of water.

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Oxygenic photosynthesis and water splitting

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