Glycolysis is the first step in cell respiration, and it requires no oxygen to proceed. Glycolysis converts a molecule of sugar into two molecules of pyruvate, also producing two molecules each of adenosine triphosphate (ATP) and nicotinamide adenine dinucleotide (NADH). When oxygen is absent, a cell can metabolize the pyruvates through the process of fermentation.
ATP is the cell’s energy storage molecule, while NADH and its oxidized version, NAD+, take part in the cell reactions that involve the transfer of electrons, known as the redox reactions. If oxygen is present, the cell can extract substantial chemical energy by breaking down pyruvate through the citric acid cycle, which converts NADH back to NAD+. Without oxidation, the cell must use fermentation to oxidize NADH before it builds up to unhealthy levels.
Pyruvate is a three-carbon molecule that the enzyme lactate dehydrogenase converts into lactate through the process known as homolactic fermentation. In the process, NADH is oxidized into NAD+ that is needed for glycolysis to proceed. In the absence of oxygen, homolactic fermentation prevents NADH from accumulating, which would halt glycolysis and rob the cell of its energy source. Fermentation doesn't yield any ATP molecules, but it does allow glycolysis to continue and producing a small trickle of ATPs. In homolactic fermentation, lactate is the sole product.
In the absence of oxygen, certain organisms such as yeast can convert pyruvate into carbon dioxide and ethanol. Brewers capitalize on this process to convert grain mash into beer. Heterolactic fermentation proceeds in two steps. First, the enzyme pyruvate dehydrogenase converts pyruvate to acetaldehyde. In the second step, the enzyme alcohol dehydrogenase transfers hydrogen from NADH to the acetaldehyde, converting it into ethanol and carbon dioxide. The process also regenerates NAD+, which enables glycolysis to continue.
Feeling the Burn
If you've ever felt your muscles burn during heavy physical activity, you are experiencing the effect of homolactic fermentation in your muscle cells. Strenuous exercise temporarily depletes a cell’s oxygen supply. Under these conditions, muscles metabolize pyruvate into lactic acid, which produces the familiar burning sensation. However, this is a stopgap reaction to low oxygen levels. Without oxygen, cells can quickly die.
Cabbage and Yogurt
Anaerobic fermentation is used to create several foodstuffs besides beer. For example, cabbage benefits from fermentation to yield delicacies such as kimchee and sauerkraut. Certain strains of bacteria, including Lactobacillus bulgaricus and Streptococcus thermophiles, convert milk to yogurt through homolactic fermentation. The process congeals the milk, gives the yogurt flavor and increases the milk’s acidity, which makes it unpalatable to many harmful bacteria.