4.17–4.18: There are alternative pathways to energy acquisition.

Unlike cars, living organisms (such as this cheetah) use multiple types of fuel.

Every beer brewery, the entire wine industry, and all distilleries of whiskey, vodka, tequila, and other alcoholic beverages owe their existence to microscopic yeast cells scrambling to break down their food for energy under stressful conditions. To better understand how yeast metabolism produces alcohol, it helps to begin by investigating what happens when humans and other animals try to metabolize energy from sugar molecules under some stressful conditions.

If you run or swim as fast as you can, you soon feel a burning sensation in your muscles. Why? Your muscle cells are becoming very acidic. This acid buildup occurs when we demand of our bodies bursts of energy beyond what they can sustain (FIGURE 4-36). (The next-day muscle soreness is not due to acid buildup, which goes away in a matter of minutes or hours; this soreness is caused by damage to the muscle fibers. They break down a bit before growing stronger.)

Figure 4.36: Energy production without oxygen. Organisms have a backup method for breaking down sugar when oxygen is not present.

With rapid, strenuous exertion, our bodies soon fall behind in delivering oxygen from the lungs to the bloodstream to the cells and finally to the mitochondria. Oxygen deficiency then limits the rate at which the mitochondria can break down fuel and produce ATP (FIGURE 4-37). This slowdown occurs because the electron transport chain requires oxygen as the final acceptor of all the electrons generated during glycolysis and the Krebs cycle. If oxygen is in short supply, the electrons from NADH (and FADH₂) have nowhere to go. Consequently, the regeneration of NAD⁺ (and FAD) in the electron transport chain is halted, leaving no recipient molecules for the high-energy electrons harvested from the breakdown of glucose and pyruvate, and the whole process of cellular respiration can grind to a stop. Organisms don’t let this interruption last long, though; most have a backup method for breaking down sugar.

Among animals, there is an acceptor for the NADH electrons in the absence of oxygen: pyruvate, the end product of glycolysis. When pyruvate accepts the electrons, it forms lactic acid (see Figure 4-37). Once the NADH gives up its electrons, NAD⁺ is regenerated, and glycolysis can continue. But as lactic acid builds up, it causes a burning feeling in our muscles. It’s not ideal, but to escape a predator or to exercise strenuously, the two ATP molecules generated from each glucose molecule during glycolysis—which can serve as an immediate energy source—are better than nothing.

Figure 4.37: Energy production with and without oxygen.

Like humans, yeast normally use oxygen during their breakdown of food. And like humans, they have a backup method when oxygen is not available. But yeast make use of a different electron acceptor, and the resulting reaction leads to the production of all drinking alcohol.

After glycolysis in these single-celled organisms, pyruvate is usually converted to a molecule called acetaldehyde, releasing bubbles of CO₂ in the process (which, when yeast is used in baking, allows bread to rise). In the absence of oxygen, acetaldehyde accepts the electrons released from NADH, allowing glycolysis to resume. Acetaldehyde’s acceptance of NADH’s electrons results in the production of ethanol, the molecule that gives beer, wine, and spirits their kick. Ethanol can also be used as a fuel source because it can be combusted in much the same way as the biofuels generated from animal fats and plant oils described earlier in this chapter.

Fermentation is the process by which cells obtain energy in the absence of oxygen. It occurs when, following glycolysis, alternative molecules are used as electron acceptors. Interestingly, although ethanol is always the alcohol produced by fermentation, the flavor of the output of fermentation depends on the type of sugar metabolized by the yeast. Fruits, vegetables, and grains all give different results. If the sugar comes from grapes, wine is produced. If the sugar comes from a germinating barley plant, beer is produced. Potatoes, on the other hand, are the sugar source usually used to produce vodka.

Because yeast prefer the more efficient process of aerobic respiration, they produce alcohol only in the absence of oxygen. Fermentation tanks used in producing wine, beer, and other spirits are built to keep oxygen out so that yeast cells are forced to use their backup pathway of fermentation (FIGURE 4-38).

Figure 4.38: Just a by-product of metabolism under stressful conditions. Beer, wine, and spirits are by-products of cellular metabolism in the absence of oxygen.