Fermentation

When oxygen is not available to a respiring cell -- as may happen, for a plant, in waterlogged root tissue, in densely packed germinating seeds, or in fleshy fruit and storage tissue with restricted internal aeration -- the pyruvic acid produced by glycolysis cannot be sent onward into the mitochondria for complete oxidation. Instead, many plant cells (and numerous microorganisms, most notably yeast) fall back on fermentation, an anaerobic pathway that converts pyruvic acid into a simpler organic end-product entirely within the cytoplasm, without ever using molecular oxygen.

Fermentation is best understood, in energetic terms, not as an alternative energy-releasing pathway of its own, but as a way of allowing glycolysis to keep running under anaerobic conditions. Glycolysis, as the previous section explained, oxidises glyceraldehyde-3-phosphate using NAD+, converting it to NADH; but a cell holds only a small, finite pool of NAD+, and unless this NADH is promptly reoxidised back to NAD+, the entire glycolytic pathway grinds to a halt for lack of the oxidised coenzyme it needs at that particular step. Fermentation solves this problem by using the pyruvic acid itself, rather than the mitochondrial electron transport system, as the final acceptor that reoxidises NADH back to NAD+, so that glycolysis -- and the small, direct ATP yield it provides -- can continue.

Two principal types of fermentation are recognised, differing in the end-product to which pyruvic acid is converted. In alcoholic fermentation, most familiar from the metabolism of yeast (and occurring in some plant tissues, such as fruit stored under low-oxygen conditions), pyruvic acid is first decarboxylated by the enzyme pyruvate decarboxylase, releasing one molecule of carbon dioxide and forming acetaldehyde. The acetaldehyde is then reduced by the enzyme alcohol dehydrogenase, using the hydrogen carried by NADH (regenerating NAD+ in the process), to form ethanol. Alcoholic fermentation is thus the pathway industrially exploited in brewing and baking, where the carbon dioxide released causes bread dough to rise and the ethanol produced is the basis of fermented beverages.

In lactic acid fermentation, by contrast, pyruvic acid is directly reduced to lactic acid by the enzyme lactate dehydrogenase, again using the hydrogen carried by NADH and regenerating NAD+ -- but without any decarboxylation step, so no carbon dioxide is released in this pathway. Lactic acid fermentation occurs in certain bacteria (used industrially to sour milk into curd and other fermented dairy products) and can also occur transiently in oxygen-starved plant tissue, playing broadly the same NAD+-regenerating role as alcoholic fermentation.

Whichever route it takes, fermentation is an inherently wasteful process in energetic terms: because pyruvic acid is only partially, not completely, oxidised, only the modest net two ATP generated during glycolysis itself is actually captured as usable energy, while the great majority of the chemical energy that was originally present in the glucose molecule remains locked up, unreleased, within the ethanol or lactic acid that fermentation leaves behind. This stands in sharp contrast to the far larger energy yield of complete, aerobic oxidation described later in this chapter. …