Cellular respiration

Cellular respiration

Cellular respiration is, in its broadest definition, the process in which the chemical bonds of energy-rich molecules 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.

Table of contents

1 Glycolysis
2 Breakdown of Pyruvate

2.1 Aerobic Respiration
2.2 Anaerobic Respiration

3 See Also
4 External links

Glycolysis

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:

Aerobic Respiration

Aerobic respiration requires oxygen. It is the preferred method of pyruvate breakdown. In this process, an electron is transferred from an energy-rich atom (such as a carbon atom in an organic molecule) to an oxygen atom, via an electron transport chain. Oxygen serves as the "terminal electron acceptor" in the electron transport chain. In the process, 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 in eukaryotic cells, and at the cell membrane in prokaryotic cells.

Anaerobic Respiration

"Anaerobic respiration" doesn't require oxygen. True anaerobic respiration involves an electron acceptor other than oxygen. Bacteria are capable of using a wide variety of compounds as terminal electron acceptors in respiration: nitrogenous compounds (such as nitrates and nitrites), sulfur compounds (such as sulfates, sulfites, sulfur dioxide, and elemental sulfur), carbon dioxide, iron compounds, manganese compounds, cobalt compounds, and uranium compounds.

However, none of these alternative electron acceptors yields as much energy from respiration as does oxygen. In environments where oxygen is present, typically only aerobic respiration will occur.

Fermentation is a process in which pyruvate is partially broken down, but there is no Krebs cycle and no production of ATP by an electron transport chain. Fermentations of various kinds produce a number of different compounds. Textbook examples of fermentation products are ethanol (drinkable alcohol), lactic acid, and hydrogen. However, more exotic compounds can be produced by fermentation, such as butyric acid and acetone.

Although fermentation produces no ATP, it is useful to the cell because it regenerates nicotinamide adenine dinucleotide (NAD), which is consumed by glycolisis.

Ethanol 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 its sour taste.

Fermentation products contain chemical energy that can't be further broken down by fermentation, making fermentation less efficient than respiration. Fermentation releases a total of two ATP molecules per molecule of glucose (compare to the 38 of aerobic respiration).

External links

A detailed diagram of glycolysis