Aerobic and Anaerobic Respiration

In most cells, cellular respiration occurs in the presence of the oxygen. This is known as aerobic respiration. The largest number of ATP molecules is formed from one molecule of glucose in this process.

    In some organisms, the cell FETs energy by breaking down glucose in the complete absence of oxygen. This is known as anaerobic respiration.

     In the cells of certain bacteria, in fungi like yeast, and in cells of plant parts like germinating seeds, glucose is partially broken down to pyruvic acid. This acid is then converted to ethanol. Since the end product is an alcohol, the process is known as alcoholic fermentation.

     Alcoholic fermentation is of great importance commercially in breweries. Here, yeast breaks down materials rich in carbohydrates to produce ethanol in the absence of oxygen.

      In animal cells that are respiring anaerobically, the pyruvic acid from glucose is converted to lactic acid instead.

      In anaerobic respiration, each molecules of glucose yields only two ATP molecules. Compare this with the thirty eight molecules obtained in aerobic respiration!

      Most of the energy is still locked up in the lactic acid or ethanol molecules. In animal cells, the energy in lactic acid can be released if oxygen is supplied to the  cell. This is what happens in muscle cells. During vigorous activity, muscle cells do not get a sufficient supply of oxygen to meet their energy requirements. As a result , anaerobic respiration occurs to temporarily supply energy, and lactic acid is produced. Lactic acids accumulates in the cells up to a certain level. Beyond this level, the muscles cells cannot work, I.e. contract. This is known as muscle fatigue. The muscle cells are said to have incurred an 'oxygen debt' since they have used up more energy than was possible from the oxygen supplied to them.

     During the resting stage, the muscle cells respire aerobically and ' pay back' the oxygen debt by converting the lactic acid to pyruvic acid. The pyruvic acid may then

• be converted to glucose by Anabolic reactions; or 

• enter the Kreb's cycle to the completely broken down to carbon dioxide and water, with the accompanying release of energy.

    The muscle cells recover from fatigue and function normally once the lactic acid is removed.

    Plant cells, however,  cannot make use of ethanol that result from anaerobic respiration. In addition, ethanol is also toxic to the cells and must be excreted. This, anaerobic respiration only occurs for short periods in plant cells. This is true even for yeast cells which stop functioning once the alcohol concentration is more than 15%.