The metabolic process in which substances, usually organic compounds, are broken down to simpler products with the release of energy, which is incorporated into special energy-carrying molecules (see atp) and subsequently used for other metabolic processes. In plants, animals, fungi, and many protists, respiration requires oxygen, and carbon dioxide is an end product. It involves a series of enzyme-catalysed reactions that release the energy of the chemical bonds in a food source and store it as ATP, the cell’s usable source of fuel. It can be divided into four stages. In the first, glycolysis, glucose is partially oxidized to pyruvate. This stage uses the coenzyme NAD+ as the electron acceptor and produces some ATP by substrate-level phosphorylation; it does not require oxygen. In the second stage, the pyruvate produced by glycolysis is oxidized to form two molecules of acetyl coenzyme A. In the third stage, the acetyl groups enter the Krebs cycle in which they are completely oxidized via a sequence of reactions that yields carbon dioxide, a high-energy compound called GTP, and the reduced coenzymes NADH and FADH2. In the final stage, these reduced coenzymes donate electrons to a series of protein complexes and associated molecules called an electron transport chain (ETC or respiratory chain). This further releases energy in small increments and generates ATP by the chemiosmotic mechanism. In aerobic organisms, oxygen is the final electron acceptor, and the end products are water and carbon dioxide. In anaerobic bacteria and archaea, a range of other molecules or ions can be utilized as the terminal electron acceptor in the ETC, and various end products are formed. In cells of eukaryotic organisms, all stages except glycolysis take place in the mitochondria. Biochemists often reserve the term ‘cellular respiration’ for stages two to four as outlined above. Compare fermentation.
2. (external respiration) The exchange of oxygen and carbon dioxide between the body tissues and the environment (see ventilation). In many animals, the exchange of gases takes place at respiratory organs (e.g. lungs in air-breathing vertebrates) and is assisted by respiratory movements (e.g. breathing). In plants, oxygen enters through pores on the plant surface and diffuses through the tissues via intercellular spaces or dissolved in tissue fluids.
https://www.thoughtco.com/cellular-respiration-process-373396 Overview of cellular respiration, with illustrations, from About.com.