What is Photosynthesis?

Photosynthesis uses water from soil, carbon dioxide from air, and light energy from the sun to produce glucose, which fuels plant growth and development. Oxygen gas is released as a byproduct. The overall biochemical formula is 6 CO2 + 6 H2O + Light energy → C6H12O6 + 6 O2. This process is largely responsible for maintaining Earth's atmospheric oxygen content.

Photosynthesis occurs in chloroplasts, specialized membrane-enclosed compartments in plant cells. Chloroplasts contain coin-like stacks of thylakoids called grana, where light reactions happen. The stroma, the fluid-filled space inside chloroplasts, is where light-independent reactions occur. Understanding the anatomy of chloroplasts and thylakoids is essential to comprehending how photosynthesis functions.

Chlorophyll is a green pigment enriched in thylakoid membranes that absorbs light energy in the form of photons from violet-blue, orange, and red wavelengths. This light absorption initiates a cascade that powers Photosystem II and Photosystem I, which produce ATP and NADPH molecules used to drive the Calvin Cycle.

The Calvin Cycle is a light-independent reaction that occurs in the stroma of chloroplasts. It uses ATP and NADPH produced by light reactions to convert carbon dioxide into glucose and other complex carbohydrates. This cycle is essential for storing chemical energy captured from sunlight into usable organic compounds.

C4 and CAM plants use modified carbon-fixing processes adapted to dry, hot climates. These pathways separate carbon dioxide fixation and glucose synthesis either in time or by using specialized cell compartments. This separation reduces photorespiration, a wasteful process where the enzyme rubisco binds oxygen instead of carbon dioxide, burning sugar rather than producing it.

Photosynthesis is performed by green plants, most algae, and cyanobacteria. These organisms use this biochemical process to capture energy from light and store it in chemical compounds that fuel biological functions. The ability to photosynthesize is widespread among photosynthetic organisms across terrestrial and aquatic environments.

The light-dependent reactions occur in thylakoid membranes and produce two key molecules: ATP and NADPH. These energy-rich molecules are generated when photons absorbed by chlorophyll power Photosystem II and Photosystem I. ATP and NADPH are then used to power the light-independent reactions that produce glucose and other carbohydrates.