However I find it fair and interesting to write something about Anaerobic respiration since I started with aerobic. If you have read about Aerobic respiration you will surely understand this one much better...
In some circumstances, however, respiration may occur without the use of oxygen. This is termed anaerobic respiration.
Anaerobic respiration/fermentation differs from aerobic respiration in the following ways.
- Anaerobic respiration breaks down glucose in the absence of oxygen.
- Anaerobic respiration does not break down glucose completely into carbon dioxide and water but into intermediate substances such as alcohol or lactic acid.
- Anaerobic respiration releases far less energy (net gain of 2 ATP) than aerobic respiration (net gain of 36 ATP) since glucose is no completely broken down.
Anaerobic respiration starts with glycolysis which occurs in the cytoplasmic matrix (cytosol) i.e. outside the mitochondria. It involves the phosphorylation (i.e. addition of energy from ATP) of the energy-rich compounds such as a 6-carbon glucose molecule.
The activated glucose moleule is then broken down step-wise to form two 3-carbon molecules of pyruvic acid. Energized hydrogen atoms and energy are released in the process. The energy ids used to form ATP. A highly specific enzyme catalyses each step in the process. What happens next tp pyruvic acid depends on whether anaerobic respiration is occuring in a plant or animal.
In plants, the pyruvic acid is converted to ethanol and carbon dioxide with the release of energy as ATP. In animals , the pyruvic acid is converted to lactic acid with the relese of energy as ATP.
Anaerobic Respiration in humans
When humans engage in strenuous activity (eg. running a long distance race), energy is first supplied through aerobic respiration. The breathing rate and heartbeat increases to increase the supply of oxygen to the tissues. When, however, the rate of oxygen supply to the tissues does not match the rate of respiration required to meet the energy demands of the body, then anaerobic respiration occurs to supplement the energy supply. The shortage of oxygen which leads to anaerobic respiration is called an oxygen debt.
The lactic acid that is formed as a by-product of anaerobic respiration accumulates in the muscle tissues of the person. When lactic acid reaches a critical level, it causes muscle cramps, preventing further contraction. As a result, the person is not able to continue with the activity, but collapses instead. Even after the activity has stopped, the breathing rate and heartbeat of the person is still high. This happens so that more oxygen can be taken to the muscles to pay off the energy debt. This oxygen is used to convert some of the lactic acid to carbon dioxide and water while some is converted to glucose.