Metabolism [muh-tab-uh-liz-uhm] is the name given to the chemical reactions which keep an organism alive. A chemical reaction is the transformation of one set of chemical substances to another. Organisms require myriad reactions to grow, reproduce, maintain their structures, and respond to their environments. These reactions are catalyzed by enzymes (aided by reusable proteins that change the rate of chemical reactions).
Most of the structures that make up animals, plants, and microbes are made from three basic classes of molecule: amino acids (the building blocks of proteins), carbohydrates (sugars), and lipids (fats). As these molecules are vital for life, metabolic reactions either focus on making these molecules during the construction of cells and tissues (anabolism), or by breaking them down and using them as a source of energy, by their digestion (catabolism).
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The Daily Omnivore 