Chapter 27 Nutrition and Metabolism

Overview of Nutrition and Metabolism Nutrition refers to the food (nutrients) humans eat Malnutrition—a deficiency in the consumption of food, vitamins, and minerals Categories of nutrients Macronutrients—nutrients needed in large amounts (bulk nutrients) Macromolecules such as carbohydrates, fats (lipids), and proteins Water Macrominerals - sodium, chloride, and calcium Micronutrients—nutrients needed in very small amounts Vitamins Microminerals (trace elements) - iron, iodine, zinc, Balance of nutrients is required for good health

Overview of Nutrition and Metabolism Metabolism - the use of nutrients Many chemical processes Catabolism Breaks food down into smaller molecular compounds Releases two forms of energy—heat and chemical energy Anabolism A synthesis process Both processes take place inside of cells continuously and concurrently Chemical energy released by catabolism must be transferred to ATP, which supplies energy directly to the energy-using reactions of all cells

Carbohydrates Dietary sources of carbohydrates Complex carbohydrates Polysaccharides—starches; found in vegetables and grains; glycogen is found in meat Cellulose—a component of most plant tissue; passes through the digestive system without being broken down Disaccharides—found in refined sugar; must be broken down before they can be absorbed Monosaccharides—found in fruits; move directly into the internal environment without being processed directly Glucose—carbohydrate most useful to the human cell; can be converted from other monosaccharides

Carbohydrates Carbohydrate metabolism (review pages 101-106, 321-322) Glucose transport and phosphorylation Glucose reacts with ATP to form glucose-6-phosphate This step prepares glucose for further metabolic reactions This step is irreversible except in the intestinal mucosa, the liver, and the kidney tubules Glycolysis The first process of carbohydrate catabolism Consists of a series of chemical reactions Glycolysis occurs in the cytoplasm of all human cells An anaerobic process—the only process that provides cells with energy under conditions of inadequate oxygen It breaks down chemical bonds in glucose molecules and releases about 5% of the energy stored in them It prepares glucose for the second step in catabolism—the citric acid cycle

Carbohydrates Carbohydrate metabolism (cont.) Citric acid cycle Two pyruvic acid molecules from glycolysis are converted to two acetyl molecules in a transition reaction Releases two carbon dioxide molecules Acetyl CoA is converted to a number of intermediates Citrate (C6) Isocitrate (C6) Alph-ketoglutarate (C5) Succinyl CoA (C4) Succinate (C4) Fumarate (C4) Malate (C4) Oxaloacetate (C4)

Carbohydrates Carbohydrate metabolism (cont.) Electron transport system High-energy electrons (along with their protons) removed during citric acid cycle enter a chain of molecules that are embedded in the inner membrane of the mitochondria As electrons move down the chain, they release small bursts of energy to pump protons between the inner and the outer membrane of the mitochondrion Protons move down their concentration gradient and across the inner membrane, driving ATP-synthase Oxidative phosphorylation—the joining of a phosphate group to ADP to form ATP by the action of ATP synthase

Carbohydrates Carbohydrate metabolism (cont.) The anaerobic pathway A pathway for the catabolism of glucose Transfers energy to ATP using only glycolysis Produces lactic acid Glycogenesis Glucose molecules are joined to form a strand of glucose beads Operates when the blood glucose level increases above the midpoint of its normal range Glycogenolysis The reversal of glycogenesis Gluconeogenesis The formation of new glucose Cccurs chiefly in liver

Carbohydrates Carbohydrate metabolism (cont.) Control of glucose metabolism—hormonal and neural devices maintain homeostasis of blood glucose concentration Insulin secreted by beta cells to decrease blood glucose level Glucagon secreted by alpha cell to increases blood glucose level Increases activity of enzyme phosphorylase Epinephrine Hormone secreted in times of stress Increases phosphorylase activity Adrenocorticotropic hormone Stimulates adrenal cortex to increase its secretion of glucocorticoids Glucocorticoids accelerate gluconeogenesis Growth hormone Increases blood glucose level by shifting from carbohydrate to fat catabolism Thyroid-stimulating hormone has complex effects on metabolism

Carbohydrates Carbohydrate metabolism (cont.) Hyperglycemic Hormones that cause the blood glucose level to rise Glugacon GH Epinephrine Glucocorticoids ACTH Hypoglycemic Hormones that cause the blood glucose level to decrease Insulin