The principal molecule involved in providing the energy for endergonic cellular reactions to take place is adenosine triphosphate, or ATP, the same molecule used in the example above. ATP is typically produced by joining an inorganic phosphate to adenosine diphosphate (ADP), which is an endergonic reaction. This, too, represents a characteristic of chemical reactions: If a reaction is exergonic in one direction, it will be endergonic in the opposite direction. Thus, the breakdown of ATP is exergonic, while the production of ATP is endergonic. The energy for production of most of the ATP in plant cells comes from the light reactions of photosynthesis and the electron transport system in the mitochondria.
The enigma is why ATP, and not any other molecule, is used. Although no complete justification is available, there are several points that support its significance. First, there is the high stability of the ATP molecule at the physiological pH (around 7.4) toward hydrolysis and decomposition in the absence of an enzyme catalyst. This stability allows ATP to be stored in the cell until needed. Second, ATP is one of the molecules (a nucleotide) that is used in synthesis of DNA. Finally, the magnitude of the change in free energy involved in the ATP-ADP transformation is of an amount useful for driving many of the endergonic reactions in the cell. As a result, it can play the role of an intermediate quite easily.
Paris Svoronos, updated by Bryan Ness
See also: Anaerobic photosynthesis; ATP and other energetic molecules; C4 and CAM photosynthesis; Calvin cycle; Chloroplasts and other plastids; Energy flow in plant cells; Glycolysis and fermentation; Krebs cycle; Mitochondria; Oxidative phos-phorylation; Photorespiration; Photosynthesis; Photosynthetic light absorption; Photosynthetic light reactions; Plant cells: molecular level; Respiration.
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