CoachATP (adenosine triphosphate) is a nucleotide carrying three phosphate groups in a row. Removing the last one releases about per mole under cell conditions, and every energy-requiring process of the cell — contraction, transport, synthesis, the light of a firefly — is driven by that removal. The product, ADP, is recharged into ATP by the reactions of this chapter. A cell holds a few seconds’ worth of ATP and turns it over continuously; a resting human recycles about its own body mass of ATP in a day.
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Example 31.5 (Reading an oxygen trace)
Isolated mitochondria in the chamber: the oxygen stays at ; glucose is added, nothing; pyruvate is added, the oxygen falls at per minute; ADP is added, the fall steepens to per minute for a while, then returns to ; cyanide is added, the fall stops. Four facts in one trace: the mitochondrion needs pyruvate, not glucose; oxygen use is coupled to ATP synthesis; the coupling is through the chain that cyanide blocks; and a mitochondrion without work to do idles.
Example 31.7 (The muscle’s choice)
A sprinter’s muscle fibre needs ATP faster than its mitochondria and its oxygen supply can make it; glycolysis with lactic fermentation delivers ATP three times faster, at fifteen times the glucose cost, and the lactate accumulates until the pain stops the effort. A marathon runner’s fibres run on respiration, slowly and almost indefinitely, burning fat as well as glucose; her mitochondria are larger and more numerous, her capillaries denser, and her lactate stays low. The fibre types of Chapter 9 are these two strategies built into cells.