Photosynthetic

Photosynthetic refers to the process by which light energy is converted into chemical energy, or to organisms that perform this process. In most organisms termed photosynthetic, light energy drives the synthesis of organic compounds from carbon dioxide and water, releasing oxygen as a byproduct in oxygenic photosynthesis. The most common sites are chloroplasts in plants and algae, and specialized photosynthetic membranes in cyanobacteria.

Photosynthesis occurs in two main stages: light-dependent reactions and the Calvin cycle (also called the light-independent reactions). In the light-dependent reactions, chlorophyll and other pigments absorb light, driving electron transport chains in the thylakoid membranes to generate ATP and NADPH and to split water, releasing oxygen. In the Calvin cycle, ATP and NADPH are used to reduce carbon dioxide to triose phosphates, which can be assembled into glucose and other carbohydrates.

Key pigments are chlorophylls, particularly chlorophyll a, along with chlorophyll b and accessory pigments such as carotenoids. In cyanobacteria, phycobiliproteins play a similar role. Some bacteria perform photosynthesis without producing oxygen (anoxygenic photosynthesis) using different pigments, such as bacteriochlorophyll, and often using electron donors other than water.

Photosynthesis is the principal source of organic matter for nearly all organisms and is responsible for most of the atmospheric oxygen. It underpins food webs, fuels plant growth, and influences global biogeochemical cycles. The study of photosynthetic pathways informs fields ranging from agriculture and bioenergy to climate science and evolution.