Metabolism distributes nutrients to pathways that provide and store energy or renew and build tissues. Nutrient pools are the circulating nutrients (glucose, amino acids, and fatty acids) taken in by cells. They may undergo catabolism, the break down of simple nutrients within cells to provide energy or anabolism, the utilization of building blocks for growth and repair.
ATP is the energy currency of living systems. Addition of a phosphate group to ADP to produce ATP (ADP + phosphate –> ATP) stores energy. The reverse (ATP –> ADP + phosphate) releases energy. The energy released is harnessed to perform work (such as muscle contraction and protein synthesis).
The energy value of foods is measure in (big) Calories (kcal). Fats produce 9 kcal/g; proteins and carbohydrates produce 4 kcal/g. Weight is maintained with a balance of energy intake and expenditure.
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