23. Energy Transformations: Respiration and Photosynthesis   Previous PageNext Page
       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.

  Page 07 of 40 HomeGlossary