Metabolism and energy exchange

Metabolism and energy exchange is the most important sign of life: the body takes nutrients, water and oxygen from the environment and releases waste products. The process has stages: digestion, absorption into blood, transport to cells by blood, metabolism inside cells and excretion. In a cell two opposite but linked processes occur: assimilation (plastic metabolism) – synthesis of complex substances (proteins) from simple ones (amino acids), using energy; dissimilation (energy exchange) – breakdown of large molecules with release of energy. The energy released is stored in the cell in ATP molecules and spent on muscle contraction, nerve impulses, temperature maintenance, synthesis and so on. Water is the medium for reactions, carries substances and regulates temperature through sweating. Mineral salts: calcium and phosphorus – for bones and teeth, iron – for haemoglobin, iodine – for thyroxine, sodium and potassium – for nerve and muscle function. Need for water depends on age, activity and weather; drink when thirsty, and more in heat and during exercise.