What is photosynthesis?

Photosynthesis uses light energy to drive biological chemical transformations. Oxygenic photosynthesis in plants, algae and cyanobacteria involves producing organic matter from carbon dioxide and using water. It is fundamental to understanding much of Earth’s food webs and the biological source of atmospheric oxygen.

Structures and stages of photosynthesis

1. Capturing light Chlorophyll and accessory pigments absorb particular wavelengths. In plants and algae, photosynthesis occurs in chloroplasts. Chlorophyll appearing green does not mean all green light is entirely useless: absorption and light distribution within leaves are more complex. 2. Light-dependent reactions Systems in thylakoid membranes link light energy to electron transfer. Water is split, oxygen is released, and carriers including ATP and NADPH are produced. Released oxygen comes from water rather than carbon dioxide in oxygenic photosynthesis. 3. Carbon fixation The Calvin cycle incorporates carbon dioxide into organic molecules, using carriers generated by the light-dependent stage. Not directly requiring a light reaction does not mean this cycle runs only at night. Sugar production involves multiple reactions.

Environment, growth and ecosystems

4. Limiting conditions Light, carbon dioxide, temperature and water availability affect photosynthetic rates. Increasing one factor does not yield unlimited growth; another may become limiting. Excessive heat or drought can suppress the system, with responses differing among species. 5. Relationship with respiration Plants also perform cellular respiration. Photosynthesis contributes organic matter while respiration makes energy in organic molecules available for use. Plants do not simply absorb carbon dioxide without ever using oxygen. Growth depends on the balance of production and consumption. 6. Global importance Photosynthetic organisms supply organic matter to land and aquatic food webs, including important microscopic ocean producers. Photosynthesis participates in the carbon cycle, but long-term storage depends on what happens to the resulting material. Not every carbon atom incorporated by a leaf is permanently removed from the atmosphere.