Photosynthesis is a two-stage process vital for converting solar energy into chemical energy, occurring in both light-dependent and light-independent reactions. The light-dependent reactions, which take place in the thylakoid membranes of chloroplasts, harness solar energy to split water molecules, releasing oxygen and generating ATP and NADPH. These reactions involve two photosystems: Photosystem II, which absorbs light to energize electrons and split water, and Photosystem I, which further energizes these electrons to reduce NADP+ to NADPH. ATP is synthesized through chemiosmosis driven by a proton gradient created by the electron transport chain.

The light-independent reactions, or Calvin Cycle, occur in the stroma of the chloroplasts and use ATP and NADPH from the light-dependent reactions to convert carbon dioxide into glucose. This cycle includes carbon fixation, where CO2 is attached to ribulose-1,5-bisphosphate (RuBP) by the enzyme Rubisco, forming a three-carbon compound. This compound is then reduced to glyceraldehyde-3-phosphate (G3P) using ATP and NADPH. Some G3P is used to synthesize glucose, while the rest is recycled to regenerate RuBP, allowing the cycle to continue.

Different plants have adapted unique strategies for photosynthesis, including C3, C4, and CAM pathways. C3 plants fix CO2 into a three-carbon compound and are most common. C4 plants, such as maize and sugarcane, initially fix CO2 into a four-carbon compound to minimize photorespiration. CAM plants, including cacti, fix CO2 at night to conserve water, releasing it during the day for the Calvin Cycle.

Photosynthesis is crucial for life on Earth, providing oxygen, forming the basis of food chains, and contributing to carbon cycling. It plays a key role in regulating atmospheric CO2 levels, which impacts global warming and climate stability.