By taking up nourishment or nutrients (proteins, fats, carbohydrates, vitamins, minerals, and trace elements) the body can create energy by the “combustion” of nutrients to maintain its organization (structure, catabolic, or energy metabolism) and the functions derived from it (growth, renewal of cells, body temperature, mechanical and chemical work). The nutrients are degraded by the enzymes of the digestive glands in the various divisions of the digestive system, broken down into chemical combinations that can be absorbed, and taken up (resorbed) by the gastrointestinal mucosa. These energy-rich combinations (e. g., fatty acids, amino acids, or glucose) enter the bloodstream and next reach the liver by way of the portal vein (see Liver, Function below). Eventually they reach the cells of the body, where they are broken down oxidatively in the mitochondria to energy-poor combinations (CO2 and H2O) (biological oxidation). The energy thus liberated is stored in energy-rich ATP following a chain reaction (mitochondrial respiratory chain). ATP in its turn is made available to processes requiring energy (e. g., protein synthesis or muscular work). ATP splits off phosphate molecules to liberate energy (see Chapter 1: Mitochondria). _ Metabolism, Energy Requirements and Nutrients Metabolism Metabolism, which includes all the biochemical processes needed to maintain life by building, transforming, and breaking down the organism, can be divided into constructive (productive, anabolic) and energy transforming (catabolic) metabolism. Anabolism (constructive metabolism)
