Dependence between respiration and photosynthesis explains how energy flow, matter cycling or carbon transfer is represented, measured or limited within ecosystems.
Photosynthesis and aerobic respiration are interdependent through the atmosphere: photosynthesis supplies oxygen, while respiration supplies carbon dioxide.
Photosynthetic organisms release atmospheric O₂ when water is split and use atmospheric CO₂ to build carbon compounds. Aerobic organisms require O₂ as the terminal electron acceptor and release CO₂ when carbon compounds are oxidized.
Photosynthesis: CO₂ consumed, O₂ released. Aerobic respiration: O₂ consumed, CO₂ released. The annual fluxes are huge, making this a major interaction between autotrophs and heterotrophs.
A plant contributes O₂ used by animal aerobic respiration; the animal's respiratory CO₂ can later be fixed by photosynthetic organisms.
The two processes are complementary at ecosystem scale but are not exact reverse reaction pathways, and plants themselves also carry out aerobic respiration.