Respiration is how cells convert food into energy. During the first stage of this process, glucose molecules break down into molecules of a carbon-based substance called pyruvate. If oxygen is not present, the respiration cycle does not continue past the glycolysis stage. This type of respiration--without oxygen--is known as anaerobic respiration.
Glycolysis occurs in both aerobic respiration and anaerobic respiration. During this stage, each glucose molecule splits in half, forming two molecules of pyruvate. If oxygen is present, the pyruvate breaks down further into carbon dioxide, hydrogen ions and an electron transport molecule called NADH (the reduced form of nicotinamide-adenine dinucleotide). If oxygen is not present, pyruvate ferments. In plants, fermentation creates ethanol alcohol; in animals, fermentation creates lactic acid.
Chemical Makeup of Pyruvate
Each molecule of pyruvate is made up of three molecules of carbon and six molecules of hydrogen. When pyruvate is created, a small amount of energy is released.
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According to the Royal Society of Chemistry's document "Chemistry for Biologists, cells need to continuously generate an electron transport molecule called NAD+ (the oxidized form of nicotinamide-adenine dinucleotide) to continue performing glycolysis. Without oxygen, this molecule is not generated in sufficient amounts to keep the body going. Thus, cells must undergo a different chemical reaction in order to generate NAD+. This reaction generates yeast in plants or lactic acid in humans.
Anaerobic Respiration and Exercise
When people exercise, sometimes the oxygen supply needed to power muscles is depleted more quickly than it can be restored. When this happens, muscle cells temporarily switch to anaerobic respiration to generate energy. Lactic acid builds in the muscles as a result of this process. Oversaturation of lactic acid leads to muscle cramps and fatigue.
Anaerobic Respiration and Energy
All respiration generates energy for cells so they can perform vital functions. However, anaerobic respiration provides far less energy than aerobic respiration. According to "Chemistry for Biologists," anaerobic respiration provides only 10 percent of the energy provided by aerobic respiration. A small amount of energy is generated by the breakdown of glucose into pyruvate, but the pyruvate requires further breakdown and combination with oxygen to generate a large amount of energy.