Photosynthesis

Photosynthesis converts energy from light into chemical energy, which organisms store in energy-rich organic molecules. In oxygen-producing (oxygenic) photosynthesis, carbon dioxide is used to build organic compounds and water is split, releasing molecular oxygen. A simplified net equation commonly used for plants is: 6 CO₂ + 6 H₂O + light energy yields C₆H₁₂O₆ + 6 O₂. This equation summarizes the overall inputs and outputs; photosynthesis occurs through many linked reactions, and glucose is not the immediate product of every step.

Plants, algae, and cyanobacteria perform oxygenic photosynthesis. In plants and algae, the process takes place in chloroplasts; cyanobacteria carry it out using internal photosynthetic membranes rather than chloroplasts. Pigments such as chlorophyll absorb particular wavelengths of light. Some other bacteria also use light to drive carbon metabolism, but in anoxygenic photosynthesis they draw electrons from substances other than water, such as hydrogen sulfide, and do not produce oxygen. Photosynthesis is therefore not a single identical pathway shared by every photosynthetic organism.

In chloroplasts, the light-dependent reactions take place in the thylakoid membranes. Pigments absorb light, and the resulting electron transfers help make ATP and NADPH, molecules that temporarily store usable chemical energy. Water is split during oxygenic light reactions; its electrons enter the reaction system and oxygen is released. The Calvin–Benson cycle occurs in the surrounding fluid, the stroma. It fixes carbon dioxide into organic molecules, using ATP and NADPH from the light-dependent reactions. One of its first stable carbohydrate products is glyceraldehyde-3-phosphate (G3P), which can be used to build sugars and other compounds. Although the cycle does not use photons directly in each reaction, it depends on products of the light-driven stage.

Photosynthetic organisms are primary producers: they introduce energy-rich organic carbon into food webs, directly or through organisms that eat them. Photosynthesis also removes carbon dioxide from local and global carbon cycles while biomass is formed, and oxygenic forms replenish atmospheric oxygen. Over geological time, oxygen released by photosynthetic cyanobacteria contributed to the oxygenation of Earth's atmosphere, a major environmental change associated with the Great Oxidation Event roughly 2.4 billion years ago. Photosynthesis still underpins much of the food and oxygen supply on which people and other organisms depend, although some ecosystems rely on chemical energy rather than sunlight.

The rate of photosynthesis depends on interacting conditions, including light, carbon dioxide availability, water, temperature, and the organism's internal structure and chemistry. More light does not increase the rate indefinitely: once other requirements become limiting, adding light alone may have little effect. Plants also balance carbon dioxide intake through leaf pores called stomata against water loss, so environmental conditions can influence both gas exchange and photosynthesis. Different plant lineages have evolved biochemical or anatomical adaptations that alter how they capture and use carbon dioxide under particular conditions.