Photosynthesis & Cellular Respiration

How living things make and use energy

Photosynthesis and cellular respiration are complementary processes that cycle energy and matter through ecosystems. Photosynthesis occurs in chloroplasts (green organelles in plant cells): light energy (absorbed by chlorophyll) drives the conversion of CO₂ + H₂O into glucose (C₆H₁₂O₆) and oxygen. The oxygen is released as a byproduct — this is the source of Earth's atmospheric oxygen. Cellular respiration occurs in mitochondria of all living cells: glucose is broken down through glycolysis, the Krebs cycle, and the electron transport chain, releasing ATP (the cell's energy currency), CO₂, and water. The two processes are essentially the reverse of each other. The carbon atoms in every glucose molecule have been cycled through the atmosphere countless times.