They are the primary food producers. All other organisms must feed, directly or indirectly, on organic material produced by plants. Animals cannot process carbon via photosynthesis. Instead, they acquire carbon by eating the organic matter contained in plant and animal tissue or dissolved in water. 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.