|MadSci Network: Biochemistry|
Most of the oxygen that is used in respiration is to obtain chemical energy from the fats and carbohydrates in our food. These fats and sugars are essentially burned in a controlled way (being oxidized to water and carbon dioxide). The energy is trapped in the form of ATP that is used by our bodies to synthesize proteins, move muscles etc. Breakdown of food by glycolysis, fatty acid oxidation and the citric acid cycle result in the formation of NADH and FADH2. These are energy-rich molecules because they contain a pair of electrons that have a high transfer potential. When these electrons are transferred to oxygen (to make water) a large amount of energy is liberated. This process is called oxidative phosphorylation. These reactions are performed by a series of electron carrier proteins (the respiratory chain) in the mitochondria. As elecron pass down the respiratory chain, protons are moved from one side of the mitochondrial membrane to the other. This results in an electrical potential across the membrane that ultimately provides the energy to make ATP. (The ATP is made by another complex of proteins in the membrane called the F1 ATPase). The last electron carrier in the respiratory chain is called cytochrome oxidase. It contains heme and copper ions that add 4 electrons and 4 protons to an oxygen molecule to produce 2 molecules of water. Although most of the oxygen is used in this way, a small amount is also needed for the synthesis and breakdown of other metabolically important molecules (such as steroid hormones).
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