What is the main reason that respiration occurs in a cell?
Respiration is the process by which cells break down food source molecules into energy that can be used by the cell to continue its functions. Respiration occurs in the presence of oxygen, which is supplied by blood; the oxygen is used in the respiration process, reacting with glucose and creating carbon dioxide and adenosine triphosphate (ATP). The carbon dioxide is removed by the blood and expelled, while the ATP provides the "base unit" of energy for most biological functions in the body. The production of ATP is catabolic, meaning that the glucose molecule is "reduced" or broken down into a smaller unit, producing both wastes and energy. Cellular respiration requires oxygen; although glucose can be reduced without oxygen, it creates smaller amounts of ATP and is not biologically efficient enough for the human body.