Cell membrane

Figure 2-14

Formation of adenosine triphosphate (ATP) in the cell, showing that most of the ATP is formed in the mitochondria. ADP, adeno-sine diphosphate.

all these digestive and metabolic functions are given in Chapters 62 through 72.

Briefly, almost all these oxidative reactions occur inside the mitochondria, and the energy that is released is used to form the high-energy compound ATP. Then, ATP, not the original foodstuffs, is used throughout the cell to energize almost all the subsequent intracellular metabolic reactions.

Functional Characteristics of ATP

Functional Characteristics of ATP

ATP is a nucleotide composed of (1) the nitrogenous base adenine, (2) the pentose sugar ribose, and (3) three phosphate radicals. The last two phosphate radicals are connected with the remainder of the molecule by so-called high-energy phosphate bonds, which are represented in the formula above by the symbol Under the physical and chemical conditions of the body, each of these high-energy bonds contains about 12,000 calories of energy per mole of ATP, which is many times greater than the energy stored in the average chemical bond, thus giving rise to the term high-energy bond. Further, the high-energy phosphate bond is very labile, so that it can be split instantly on demand whenever energy is required to promote other intracellular reactions.

When ATP releases its energy, a phosphoric acid radical is split away, and adenosine diphosphate (ADP) is formed. This released energy is used to energize virtually all of the cell's other functions, such as synthesis of substances and muscular contraction.

To reconstitute the cellular ATP as it is used up, energy derived from the cellular nutrients causes ADP and phosphoric acid to recombine to form new ATP, and the entire process repeats over and over again. For these reasons,ATP has been called the energy currency of the cell because it can be spent and remade continually, having a turnover time of only a few minutes.

Chemical Processes in the Formation of ATP—Role of the Mitochondria. On entry into the cells, glucose is subjected to enzymes in the cytoplasm that convert it into pyruvic acid (a process called glycolysis). A small amount of ADP is changed into ATP by the energy released during this conversion, but this amount

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