is, in its broadest definition, the process in which the chemical bonds
such as glucose
are converted into energy
usable for life
processes. All forms of life except viruses
carry out respiration. Oxidation
of organic material — in a bonfire, for example — releases a large amount of energy rather quickly. The overall equation for the oxidation of glucose is:
- C6H12O6 + 6O2 ⇒ 6CO2 + 6H2O + energy
In respiration, the process of oxidation is broken down into a large number of steps. These steps are catalysed by enzymes and coenzymes; each step releases a small amount of energy in the form of ATP. This process consists of two main steps: glycolysis, and pyruvate breakdown.
Glycolysis does not need oxygen in any of its steps. It is a metabolic pathway that is found in all living organisms and it probably evolved billions of years ago before the Earth's atmosphere contained oxygen.
- It is the process that converts one molecule of glucose into two molecules of pyruvate.
- It releases energy in the form of two molecules of ATP.
- It takes place in the cytoplasm of the plant or animal cell.
Breakdown of Pyruvate
There are now two ways to break down the resulting pyruvate:
. It is the preferred method of pyruvate breakdown. It yields 36 ATP molecules, as well as carbon dioxide
, and water
. This makes for a total gain of 38 ATP molecules during cellular respiration. This takes place in the mitochondria
of the cells.
doesn't require oxygen. In this process, the pyruvate is only partially broken down.
- Fermentation (done by yeast and some types of bacteria) breaks the pyruvate down into ethanol, carbon dioxide, and water. It is important in bread making, brewing, and wine making.
- Lactic acid fermentation breaks the pyruvate down into lactic acid, carbon dioxide, and water. It occurs in the muscles of animals when they need energy faster than the blood can supply oxygen. It also occurs in some bacteria. It is this type of bacteria that convert lactose into lactic acid in yoghurt giving it it's sour taste.
Both ethyl alcohol and lactic acid contain chemical energy that can't be used by anaerobic respiration, making this an inefficient process. Anaerobic respiration releases a total of two ATP molecules (compare to the 38 of aerobic respiration).
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