什麼是代謝

現生生物每個細胞內發生的化學反應總和，供應生命所需能量併合成新的細胞物質。「中間代謝」論及龐大網狀的互相聯繫的化學反應，藉此產生或消滅所有細胞的組成，許多成分在細胞外面極為罕見。同化反應（合成代謝作用）利用能量從較簡單的有機化合物建造複雜的分子（如氨基酸變成蛋白質，糖變成碳水化合物，脂肪酸和丙三醇變成脂肪）；異化反應（分解代謝作用）將複雜的分子分解成較簡單的分子，釋放出化學能。對大多數生物而言，能量總歸是來自太陽，不管是由光合作用取得儲存在有機化合物之間，或是吃這些光合作用的生物消耗其儲存的有機化合物。有些特殊環境的細菌，例如在深海裂口，能量是由化學鍵結。在細胞內或生物體內，能量是由三磷酸腺來傳遞；同化反應消耗三磷酸腺，異化作用則生成。每個細胞化學反應是由特定的居間協調。物質分解作用通常是不可逆的作用，因此需要不同的。亦請參閲digestion、fermentation、tricarboxylic acid cycle。

metabolism

Sum of all the chemical reactions that take place in every cell of a living organism, providing energy for the processes of life and synthesizing new cellular material. “Intermediary metabolism” refers to the vast web of interconnected chemical reactions by which all the cell's constituents, many rarely found outside it, are created and destroyed. Anabolic reactions use energy to build complex molecules from simpler organic compounds (e.g., proteins from amino acids, carbohydrates from sugars, fats from fatty acids and glycerol); catabolic reactions break complex molecules down into simpler ones, releasing chemical energy. For most organisms, the energy comes ultimately from the sun, whether they obtain it by photosynthesis and store it in organic compounds or consume those that photosynthesize for the energy stored in their organic compounds. In some bacteria in special environments such as deep-sea vents, the energy comes from chemical bonds instead. Energy is transferred within the cell and the organism by ATP; anabolic reactions consume it, and catabolic reactions generate it. Every cellular chemical reaction is mediated by a specific enzyme. The process that breaks down a substance is not usually the reverse of the process that makes it and uses a different enzyme. See also digestion, fermentation, tricarboxylic acid cycle.