Unit two. The Living Cell
7. How Cells Harvest Energy from Food
7.5. Cells Can Metabolize Food Without Oxygen
In the absence of oxygen, aerobic metabolism cannot occur, and cells must rely exclusively on glycolysis to produce ATP. Under these conditions, the hydrogen atoms generated by glycolysis are donated to organic molecules in a process called fermentation, a process that recycles NAD+, the electron acceptor required for glycolysis to proceed.
Bacteria carry out more than a dozen kinds of fermentations, all using some form of organic molecule to accept the hydrogen atom from NADH and thus recycle NAD+:
Often the reduced organic compound is an organic acid—such as acetic acid, butyric acid, propionic acid, or lactic acid—or an alcohol.
Ethanol Fermentation. Eukaryotic cells are capable of only a few types of fermentation. In one type, which occurs in single- celled fungi called yeast, the molecule that accepts hydrogen from NADH is pyruvate, the end product of glycolysis itself. Yeast enzymes remove a CO2 group from pyruvate through decarboxylation, producing a two-carbon molecule called acetaldehyde. The CO2 released causes bread made with yeast to rise, while bread made without yeast (unleavened bread) does not. The acetaldehyde accepts a hydrogen atom from NADH, producing NAD+ and ethanol (figure 7.7). This particular type of fermentation is of great interest to humans because it is the source of the ethanol in wine and beer. Ethanol is a by-product of fermentation that is actually toxic to yeast; as it approaches a concentration of about 12%, it begins to kill the yeast. That explains why naturally fermented wine contains only about 12% ethanol.
Lactic Acid Fermentation. Most animal cells regenerate NAD+ without decarboxylation. Muscle cells, for example, use an enzyme called lactate dehydrogenase to transfer a hydrogen atom from NADH back to the pyruvate that is produced by glycolysis. This reaction converts pyruvate into lactic acid and regenerates NAD+ from NADH (figure 7.7). It therefore closes the metabolic circle, allowing glycolysis to continue as long as glucose is available. Circulating blood removes excess lactate (the ionized form of lactic acid) from muscles. It was once thought that during strenuous exercise, when the removal of lactic acid cannot keep pace with its production, the accumulation of lactic acid induces muscle fatigue. However, as you will learn in chapter 22, muscle fatigue actually has a quite different cause, involving the leakage of calcium ions within muscles.
Figure 7.7. Fermentation.
Yeasts carry out the conversion of pyruvate to ethanol. Muscle cells convert pyruvate into lactate, which is less toxic than ethanol. In both cases, NAD+ is regenerated to allow glycolysis to continue.
Key Learning Outcome 7.5. In fermentation, which occurs in the absence of oxygen, the electrons that result from the glycolytic breakdown of glucose are donated to an organic molecule, regenerating NAD+ from NADH.
A Closer Look
Beer and Wine Products of Fermentation
Alcoholic fermentation, the anaerobic conversion of sugars into alcohol discussed on the facing page, predates human history. Like many other natural processes, humans seem to have first learned to control this process by stumbling across its benefit—the pleasures of beer and wine. Artifacts from the third millennium b.c. contain wine residue, and we know from the pollen record that domesticated grapes first became abundant at that time. The production of beer started even earlier, back in the fifth millennium B.C., making beer possibly one of the oldest beverages produced by humans.
The process of making beer is called brewing, and involves the fermentation of cereal grains. Grains are rich in starches, which you will recall from chapter 3 are long chains of glucose molecules linked together. First, the cereal grains are crushed and soaked in warm water, creating an extract called mash. Enzymes called amylases are added to the mash to break down the starch within the grains into free sugars. The mash is filtered, yielding a darker, sugary liquid called wort. The wort is boiled in large tanks, like the ones shown in the photo here, called fermentors. This helps break down the starch and kills any bacteria or other microorganisms that might be present before they begin to feast on all the sugar. Hops, another plant product, is added during the boiling stage. Hops gives the solution a bitter taste that complements the sweet flavor of the sugar in the wort.
Now we are ready to make beer. The temperature of the wort is brought down and yeast is added. This begins the fermentation process. It can be done in either of two ways, yielding the two basic kinds of beer.
Lager. The yeast Saccharomyces uvarum is a bottom- fermenting yeast that settles on the bottom of the vat during fermentation. This yeast produces a lighter, pale beer called lager. Bottom fermentation is the most widespread method of brewing. Bottom fermentation occurs at low temperatures (5-8°C). After fermentation is complete, the beer is cooled further to 0°C, allowing for the beer to mature before the yeast is filtered out.
Ale. The yeast S. cerevisiae, often called "brewer's yeast,” is a top-fermenting yeast. It rises to the top during fermentation and produces a darker, more aromatic beer called ale (also called porter or stout). In top fermentation, less yeast is used. The temperature is higher, around 15-25°C, and so fermentation occurs more quickly but less sugar is converted into alcohol, giving the beer a sweeter taste.
Most yeasts used in beer brewing are alcohol-tolerant only up to about 5% alcohol—alcohol levels above that kill the yeast. That is why commercial beer is typically 5% alcohol.
Carbon dioxide is also a product of fermentation, and gives beer the bubbles that form the head of the beer. However, because only a little CO2 forms naturally, the last step in most brewing involves artificial carbonation, with CO2 gas being injected into the beer.
Wine. Wine fermentation is similar to beer fermentation in that yeasts are used to ferment sugars into alcohol. Wine fermentation, however, uses grapes. Rather than the starches found in cereal grains, grapes are fruits rich in the sugar sucrose, a disaccharide combination of glucose and fructose. Grapes are crushed and placed into barrels. There is no need for boiling to break down the starch, and grapes have their own amylase enzymes to cleave sucrose into free glucose and fructose sugars.
To start the fermentation, yeast is added directly to the crush. Saccharomyces cerevisiae and S. bayanus are common wine yeasts. They differ from the yeast used in beer fermentation in that they have higher alcohol tolerances—up to 12% alcohol (some wine yeasts have even higher alcohol tolerances).
A common misconception about wine is that the color of the wine results from the color of the grape juice used, red wine using juice from red grapes and white wines using juice from white grapes. This is not true. The juice of all grapes is similar in color, usually light. The color of red wine is produced by leaving the skins of red or black grapes in the crush during the fermentation process. White wines can be made from any color of grapes, and are white simply because the skins that contribute red coloring are removed before fermentation.
White wines are typically fermented at low temperatures (8-19°C). Red wines are fermented at higher temperatures of 25-32°C with yeasts that have a higher heat tolerance. During wine fermentation, the carbon dioxide is vented out, leaving no carbonation in the wine. Champagnes have some natural carbonation that results from a two-step fermentation process: In the first fermentation step, the carbon dioxide is allowed to escape; in a second fermentation step, the container is sealed, trapping the carbon dioxide. But, like beer brewing, the natural carbonation is often supplemented with artificial carbonation.