Learning Objectives for this Section

A substance called adenosine triphosphate (ATP) links most cellular exergonic (def) and endergonic (def) chemical reactions. To obtain energy to do cellular work, organisms take energy-rich compounds such as glucose into the cell and enzymatically break them down to release their potential energy. Therefore, the organism needs a way to trap some of that released energy and store the energy in a form that can be utilized by the cell to do cellular work. Principally, energy is trapped and stored in the form of adenosine triphosphate (def) or ATP.

A tremendous amount of ATP is needed for normal cellular growth. For example,a human at rest uses about 45 kilograms (about 99 pounds) of ATP each day but at any one time has a surplus of less than one gram. It is estimated that each cell will generate and consume approximately 10,000,000 molecules of ATP per second. As can be seen, ATP production is an ongoing cellular process.

To trap energy released from exergonic catabolic chemical reactions (def), the cell uses some of that released energy to attach an inorganic phosphate group on to adenosine diphosphate (ADP) to make adenosine triphosphate (ATP). Thus, energy is trapped and stored in what are known as high-energy phosphate bonds. To obtain energy to do cellular work during endergonic anabolic chemical reactions (def), the organism enzymatically removes the third phosphate from ATP thus releasing the stored energy and forming ADP and inorganic phosphate once again (see Fig. 1).

Animation illustrating the formation of ATP from ADP and phosphate.

Animation illustrating the hydrolysis of ATP to provide energy for cellular work.

Depending on the type of organism, cells transfer energy and generate ATP by photophosphorylation, by substrate-level phosphorylation, and/or by oxidative phosphorylation. (Phosphorylation (def) refers to the attachment of a phosphate group to a molecule.)

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Updated: June 26, 2001