Why can our muscle cells respire anaerobically?
When oxygen is used to release energy from sugar it is called aerobic respiration. The word equation for aerobic respiration is:
Glucose (sugar) + oxygen = carbon dioxide + water + energy
The glucose is broken down in two stages. The second stage uses oxygen and yields much energy (18 times more compared with anaerobic respiration).
When muscles are working hard, they use up a lot of oxygen. If oxygen does not get to the muscles fast enough for aerobic respiration, the muscles change to anaerobic respiration by processing only the first stage of glucose breakdown and making lactic acid instead.
Glucose (sugar) only = carbon dioxide + lactic acid + some energy
In anaerobic respiration only the first stage of sugar breakdown occurs. Lactic acid builds up in the muscles and stops them working. The person breathes faster to take in more oxygen so that lactic acid can be broken down to carbon dioxide and water.
When you run very quickly, the oxygen in your muscle cells gets used up very quickly. This means that if you do not get any more oxygen you cannot respire using aerobic respiration. This can happen when the amount of oxygen being supplied in the blood does not meet the demand for oxygen the muscles need. If there were no other way of respiring without oxygen, then all activity would come to a sudden stop. Luckily we can respire for a short time without oxygen. This is called anaerobic respiration. Oxygen is needed to full break down the glucose and release all the energy stored in it. In anaerobic respiration glucose cannot be fully broken down and only a small amount of energy is released. Lactic acid stops muscle from working if it builds up. This leads to cramp. This is why anaerobic respiration is used only when necessary. Aerobic respiration is more efficient. After a period of anaerobic respiration, there is often a period of heavy breathing & panting. This brings more oxygen into the blood. The lactic acid needs to be burnt off and the oxygen is used to do this. When we respire anaerobically we build up what is called an oxygen debt due to the lack of oxygen. Panting helps to repays that debt.
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Plants and fungi can respire anaerobically also but they do not release lactic acid, they produce ethanol (alcohol) and carbon dioxide. Some organisms like yeast respire this way all the time.