Anaerobic respiration creates the byproduct lactic acid. Accumulation of lactic acid in the muscles due to lack of oxygen results in the pain we experience during exercise. Remember that aerobic respiration creates 36 ATP molecules per glucose, while anaerobic repiration forms only 2 ATP molecules per glucose. Since both processes begin with glycolysis, pyruvate is still generated.

Note that while lactic acid is responsible for the "burn" in muscles during exercise, other agents are responsible for muscle soreness after exercise.

In the absence of available oxygen, the body conducts metabolism anaerobically in a process known as fermentation. During strenuous exercise, like running a marathon, the body needs to generate ATP at a rate faster than oxygen is becoming available.

To combat this issue, the body converts pyruvate and NADH, generated in glycolysis, into lactic acid and NAD⁺, respectively. This regenerated NAD⁺ can participate in further glycolysis to generate more ATP, even in the absence of oxygen. Oxygen only becomes a necessary reactant in the electron transport chain; thus, glycolysis can continue to generate limited amounts of ATP in an anaerobic environment as long as NAD⁺ is present.

In an anaerobic environment, two net ATP are produced from glycolysis. Since glycolysis requires an investment of two ATP and produces four ATP, it has a total net yield of two ATP. In an aerobic environment, however, the cell performs glycolysis, pyruvate decarboxylation, the citric acid cycle, and oxidative phosphorylation. These processes together yield a net of 36 ATP.

Fermentation occurs in the absence of oxygen, and reduces pyruvate to the end product of either ethanol or lactic acid. Since pyruvate is being reduced, NADH is oxidized to NAD⁺, which is needed for the initial glycolysis reaction to produce pyruvate. During anaerobic respiration, glycolysis is still able to function, but only if NAD⁺ is available; thus, fermentation allows the regeneration of NAD⁺ in order for glycolysis to proceed in the absence of oxygen.

Pyruvate can be decarboxylated to make acetyl-CoA. This is the process that initiates the citric acid cycle. Pyruvate can also undergo fermentation, and be reduced to either lactic acid or acetaldehyde. Acetaldehyde can then be reduced to ethanol, however, pyruvate cannot directly be converted to ethanol.

Glycolysis occurs in the cytosol and does not require oxygen. The pyruvate dehydrogenase complex (PDC) and Kreb's cycle require oxygen indirectly, while the electron transport chain and oxydative phosphorylation require oxygen directly. After glycolysis produces pyruvate, either aerobic respiration or anaerobic respiration can proceed depending on the availability of oxygen.

Two net molecules of ATP are produced via anaerobic cellular respiration.

During anaerobic conditions only glycolysis occurs. Glycolysis alone produces 4 ATP per glucose, but requires an input of 2 ATP per glucose. Thus, 2 ATP per glucose are yielded through glycolysis.

The brain is unable to perform beta-oxidation of free fatty acids in the event of a prolonged fasting state. It is important to know this aspect of metabolism. In a fasting state, the liver beta-oxidizes free fatty acids into ketone bodies for the brain to use. Additionally, when energy demands are high, muscles can break down fat for additional ATP.

Unlike other organs in the body, the heart relies almost entirely on beta-oxidation for its energy needs. 

Free fatty acids are chains of acetyl-CoA molecules linked together. When a free fatty acid undergoes beta-oxidation, it is returned to its component parts of acetyl-CoA. It is important to know that free fatty acids cannot be used to make glucose; they can only be fed into the Krebs cycle.