Two molecules of NADH are formed per molecule of glucose in the course of glycolysis. In aerobic organisms, these NADH molecules donate their electrons to the electron-transport chain described in Chapter 14, and the NAD+ formed from the NADH is used again for glycolysis (see step 6 in Panel 2–8, pp. 104–105).
fermentations produce aTp in the absence of oxygen
For most animal and plant cells, glycolysis is only a prelude to the final stage of the breakdown of food molecules. In these cells, the pyruvate formed by glycolysis is rapidly transported into the mitochondria, where it is converted into CO2 plus acetyl CoA, whose acetyl group is then completely oxidized to CO2 and H2O.
In contrast, for many anaerobic organisms—which do not utilize molecular oxygen and can grow and divide without it—glycolysis is the principal source of the cell’s ATP. Certain animal tissues, such as skeletal muscle, can also continue to function when molecular oxygen is limited. In these anaerobic conditions, the pyruvate and the NADH electrons stay in the cytosol. The pyruvate is converted into products excreted from the cell—for example, into ethanol and CO2 in the yeasts used in brewing and breadmaking, or into lactate in muscle. In this process, the NADH gives up its electrons and is converted back into NAD+. This regeneration of NAD+ is required to maintain the reactions of glycolysis (Figure 2–47).
Energy-yielding pathways like these, in which organic molecules both donate and accept electrons (and which are often, as in these cases, anaerobic), are called one molecule of glucose
OH to be fructose 1,6 six-carbon sugar to two three-carbon sugarsSTEP 5 two molecules of glyceraldehyde
Figure 2–46 an outline of glycolysis.
each of the 10 steps shown is catalyzed by a different enzyme. note that step 4 cleaves a six-carbon sugar into two three-carbon sugars, so that the number of molecules at every stage after this doubles. as indicated, step 6 begins the energy-generation phase of glycolysis. Because two molecules of aTp are hydrolyzed in the early, energy-investment phase, glycolysis results in the net synthesis of 2 aTp and 2 naDh molecules per molecule of glucose (see also panel 2–8).
Figure 2–47 Two pathways for the anaerobic breakdown of pyruvate.
(a) When there is inadequate oxygen, for example, in a muscle cell undergoing vigorous contraction, the pyruvate produced by glycolysis is converted to lactate as shown. This reaction regenerates the naD+ consumed in step 6 of glycolysis, but the whole pathway yields much less energy overall than complete oxidation.
(B) in some organisms that can grow anaerobically, such as yeasts, pyruvate is
C regeneration C converted via acetaldehyde into carbon dioxide and ethanol. again, this pathway CO HCOH regenerates naD+ from naDh, as required to enable glycolysis to continue. Both (a) and (B) are examples of fermentations.
fermentations. Studies of the commercially important fermentations carried out by yeasts inspired much of early biochemistry. Work in the nineteenth century led in 1896 to the then startling recognition that these processes could be studied outside living organisms, in cell extracts. This revolutionary discovery eventually made it possible to dissect out and study each of the individual reactions in the fermentation process. The piecing together of the complete glycolytic pathway in the 1930s was a major triumph of biochemistry, and it was quickly followed by the recognition of the central role of ATP in cell processes.
Glycolysis illustrates how enzymes Couple oxidation to energy storage