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.
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 cellular respiration, the process of oxidation is broken down into two basic metabolic pathways:
- Glycolysis
- Anaerobic Respiration or aerobic respiration
Glycolysis is a metabolic pathway that is found in all living organisms and does not require oxygen. The process converts one molecule of glucose into two molecules of pyruvate, and releases energy in the form of two molecules of ATP. It takes place in the cytoplasm of the cell.
Conversion of glucose into pyruvate:
(1)A glucose molecule from the hydrolysation of starch or glycogen is phosphorylated using 1 ATP molecule to give glucose-6-phosphate. (2)The glucose-6-phosphate is converted to fructose-6-phosphate by isomerisation. (3)Fructose-6-phosphate is again phosphorylated to give fructose-1,6-diphosphate with the use of another ATP molecule. (4)Next, the fructose-1,6-diphosphate is then lysed into 2 molecules of 3-Carbon sugar (dihydroxyacetone phosphate and glyceraldehyde-3-phosphate) which are interconvertible. (5)The 3-Carbon sugars are dehydrogenated and inorganic phosphate is added to them, forming 2 molecules of 1,3 diphosphoglycerate. (6)The hydrogen is used to oxidise 2 molecules of NAD, a hydrogen carrier, to give NADH2. NADH2 later proceeds to the mitochondria for use in the electron transport chain. (7)The 2 molecules of 1,3 diphosphoglycerate loose 2 phosphate groups to form 2 molecules of glycerate-3-phosphate (3-phosphoglycerate), converting 2 molecules of ADP to ATP. (8)The 2 molecules of glycerate-3-phosphate again loose phosphate forming 2 molecules of pyruvate, with the production of another 2 ATP molecules.
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 glycolysis.
- 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).
See Also
