Plants carry out both photosynthesis and respiration, but the amount of oxygen released during photosynthesis is far greater than the amount consumed during respiration. In photosynthesis, plants release large volumes of oxygen as a by-product while producing glucose. Although they use some oxygen for their own energy needs during respiration, this is comparatively very small. Additionally, the vast number of plants on Earth constantly replenish the atmosphere with oxygen. Hence, the overall balance ensures no shortage of oxygen in the atmosphere.