The animal breaks this organic material down into components it can use for energy and growth. Animals are heterotrophs, or other-feeders. Every biological activity, such as reproduction, growth, movement and bodily functions, requires energy. Whether organisms produce food themselves or ingest it from other sources, they all must break down organic molecules to use the carbon and energy contained within. This process is called respiration.
Aerobic respiration uses oxygen and releases carbon in the form of carbon dioxide.
It compliments photosynthesis, which uses carbon dioxide and produces oxygen. Together, aerobic respiration and photosynthesis compose the carbon-oxygen cycle. All living things respire, but autotrophs carry out photosynthesis as well. Plants usually release more oxygen than they consume and animals use that excess oxygen for respiration. In turn, animals release carbon dioxide, which plants require for photosynthesis.