23.
Energy Transformations: Respiration and Photosynthesis
Glucose Metabolism: Overall Plan
The overall scheme of energy extraction in higher
organisms is shown on the right. In the first step, glucose is degraded
to pyruvic acid (CH3-CO-COOH), or pyruvate, with the
production of relatively little ATP. (Since these organic acids
are partially dissociated into anions, it is common to call them
interchangeably by the name of the acid or the ion. "Pyruvate"
is easier to say than "pyruvic acid," and "lactate"
is simpler than "lactic acid." We shall use both forms.)
If the NADH produced is reused to convert pyruvate to molecules
such as lactate (CH3-CHOH-COOH) or ethanol (CH3-CH2-OH),
then the process can stop at this point. No oxygen is required,
but relatively little energy is obtained. This inefficient first
step in the energy-extracting process is called anaerobic
(non-oxygen-using) fermentation, or glycolysis.
It is what yeasts do when they are not given an adequate supply
of oxygen, a process that the winemaker turns to his advantage.
Our version of this same anaerobic process in muscles yields lactic
acid instead of ethanol, and this lactic acid causes muscle cramps,
or charley horse, when muscles are exerted too suddenly with inadequate
oxygen. When oxygen is brought in to eliminate the lactic acid,
the cramps disappear.