Most of the structures that make up animals, plants and microbes are made from four basic classes of molecules: amino acids, carbohydrates, nucleic acid and lipids (often called fats). As these molecules are vital for life,

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Catabolism is the set of metabolic processes that break down large molecules. These include breaking down and oxidizing food molecules. The purpose of the catabolic reactions is to provide the energy and components needed by anabolic

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Oxidative phosphorylation
In oxidative phosphorylation, the electrons removed from organic molecules in areas such as the citric acid

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Anabolism is the set of constructive metabolic processes where the energy released by catabolism is used to synthesize complex

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Living organisms must obey the laws of thermodynamics, which describe the transfer of heat and work. The second law of thermodynamics states

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As the environments of most organisms are constantly changing, the reactions of metabolism must be finely regulated to maintain a constant set of conditions within cells, a condition

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All organisms are constantly exposed to compounds that they cannot use as foods and that would be harmful if they accumulated in cells, as they have no metabolic function. These potentially damaging compounds are called xenobiotics.

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