The effects of nutrients on living organisms and the biochemical mechanisms involved in bringing them about; also, the subdivision of biochemistry that deals specifically with these effects. In plant nutrition the essential requirements are carbon dioxide and water, from which the plant forms carbohydrates by photosynthesis; nitrogen, which is essential for the synthesis of proteins by the plant, with the aid of nitrogen-fixing bacteria; as well as phosphorus, calcium, potassium, and a number of trace elements (micronutrients). Besides proteins and carbohydrates, plants also synthesize vitamins and various fats and oils. Thus they provide a basis for human nutrition, both directly (grain and other vegetables) and indirectly (meats and dairy products), though the conversion to protein values for human nutrition is only approximately 10% for meats.Human diet requires proteins (milk, eggs, fish, and some vegetables), carbohydrates (plants), fats (oils) from both plants and animals, minerals from milk and meats, salt (chloride), vitamins from green vegetables and citrus fruits, and water. Micronutrients are furnished by seafood, cereals, vegetables, and fruit.Human digestive processes involve primarily the hydrolysis of complex carbohydrates to simple sugars, of proteins to a mixture of amino acids, and of fats to glycerol and higher fatty acids. Hydrolysis is catalyzed by various enzymes in the saliva and digestive tract. The end products of digestion are absorbed across a semipermeable membrane in the intestine and thus enter the bloodstream, unusable products being eliminated. The efficiency of digestion plus absorption is approximately 92% for protein, 95% for fat, and 98% for carbohydrates.See Metabolism; Digestion; Plant; Nutrient; RDA.
