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The chemical processes by which cells produce the substances and energy needed to sustain life. As part of metabolism, organic compounds are broken down to provide heat and energy in the process called catabolism . Simpler molecules are also used to build more complex compounds like proteins for growth and repair of tissues as part of anabolism . Many metabolic processes are brought about by the action of enzymes. The overall speed at which an organism carries out its metabolic processes is termed its metabolic rate (or, when the organism is at rest, its basal metabolic rate). Birds, for example, have a high metabolic rate, since they are warm-blooded, and their usual method of locomotion, flight, requires large amounts of energy. Accordingly, birds usually need large amounts of high-quality, energy-rich foods such as seeds or meat, which they must eat frequently.

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