All living organisms partake in some form of cellular respiration. This includes bacteria, fungi, protists, plants and animals. Cellular respiration is sometimes called aerobic respiration because it requires oxygen to occur, although a few organisms are capable of anaerobic respiration, respiration without oxygen.
All life-forms require energy to move, grow, repair damage and reproduce. Many organisms, such as plants and algae, are able to produce their own energy from sunlight. These photosynthesizing organisms form the base of their respective food chains, and all subsequent organisms in these food chains obtain their energy by consuming this photosynthesizing life or consuming other organisms that rely on photosynthesizing life.
The chemical energy that all organisms acquire through photosynthesis or consumption provide the cells with nutrients, such as complex and simple sugars, fats and amino acids. These nutrients are broken down in the cellular respiration process in the presence of oxygen. The final product of the cellular respiration process is an energy-rich molecule called ATP. This ATP is the simplest form of energy used by all life.
Anaerobic life-forms use other oxidizing agents besides oxygen to break down complex nutrients into simpler ones. These oxidizing agents include sulfates and nitrates, but they are not as efficient at oxidization as oxygen.