Carbohydrate Metabolism PDF

Chapter 2 Carbohydrate metabolism CARBOHYDRATE METABOLISM DIETARY CARBOHYDRATES - Mainly include sucrose (table sugar), lactose (in milk) and starch, which is present in potatoes, rice, corn and wheat. - Cellul...

Chapter 2 Carbohydrate metabolism CARBOHYDRATE METABOLISM DIETARY CARBOHYDRATES - Mainly include sucrose (table sugar), lactose (in milk) and starch, which is present in potatoes, rice, corn and wheat. - Cellulose is present in the cell wall of some plants but is not digested by humans as it contains β-glucosidic linkage not hydrolyzed by human enzymes. It increases the bulk of stools, thus preventing constipation. DIGESTION OF CARBOHYDRATES 1. In the mouth: - Salivary amylase starts digestion of the starch. 2. In the stomach: - HCL partially hydrolyzes carbohydrates to monosaccharides. 3. In the small intestines: - Pancreatic amylase completes the digestion of starch. - Intestinal disaccharidases digest disaccharides: Maltase Maltose 2 glucose Lactase Lactose glucose + galactose Sucrase Sucrose glucose + fructose ABSORPTION OF CARBOHYDRATES - Absorption by active transport; needs energy (ATP) and a transporter. 14 Chapter 2 Carbohydrate metabolism Digestion and Absorption of carbohydrates 15 Chapter 2 Carbohydrate metabolism FATE OF ABSORBED GLUCOSE 1. Uptake and utilization by different tissues for: - Oxidation to produce energy: glycolysis, then Krebs’ cycle, and electron transport chain. - Conversion to other substances: Other carbohydrates: as ribose for nucleic acids synthesis. Lipids. Proteins. 2. Storage of excess glucose: - As glycogen in liver and muscles (glycogenesis). - Excess glucose is converted to triacylglycerols in the adipose tissue (lipogenesis). 3. Excretion in urine: - Normally, there is no glucose detectable in urine. - If the blood glucose exceeds the maximum capacity of the kidneys to reabsorb glucose (>180 mg/dL), it will be excreted in urine. 16 Chapter 2 Carbohydrate metabolism GLYCOLYSIS DEFINITION: Glycolysis is a group of reactions by which glucose is converted to pyruvate (in aerobic conditions) or lactate (in anaerobic conditions). IMPORTANCE OF GLYCOLYSIS: Glucose is the major source of energy for all cells: Glycolysis itself liberates energy from glucose oxidation, however, it also provides the mitochondria with pyruvate that gives acetyl CoA to start the Krebs' cycle. Summary of Glycolysis and Krebs cycle 17 Chapter 2 Carbohydrate metabolism GLYCOGEN METABOLISM INTRODUCTION: Glycogen is the main storage form of carbohydrates in animals. It is stored mainly in the form of liver glycogen and muscle glycogen. 1. Liver glycogen: - It maintains the blood glucose, especially between meals. - It is depleted after 12-18 hours fasting. 2. Muscle glycogen: - Its supplies glucose within muscles during contraction. - It is only depleted after prolonged exercise. DEFINITIONS: 1. Glycogenesis: synthesis of glycogen from glucose; by glycogen synthase. 2. Glycogenolysis: breakdown of glycogen to glucose; by glycogen phosphorylase. 18 Chapter 2 Carbohydrate metabolism GLUCONEOGENESIS DEFINITION: - It is the synthesis of glucose from non-carbohydrate sources (mainly in liver). It occurs by reversal of glycolysis. - Gluconeogenic precursors include lactate, glycerol, some amino acids. IMPORTANCE: The main function of gluconeogenesis is to supply blood glucose in case of carbohydrate deficiency (fasting, starvation and low carbohydrate diet) for more than 18 hours (after glycogen depletion). Gluconeogenesis 19 Chapter 2 Carbohydrate metabolism SOURCES OF BLOOD GLUCOSE 20 Chapter 2 Carbohydrate metabolism TEST YOURSELF 1) Give a short note on: digestion of carbohydrates in the mouth? …………………………………………………………………………… …………………………………………………………………………… 2) Give a short note on: digestion of carbohydrates in the stomach? …………………………………………………………………………… …………………………………………………………………………… 3) Give a short note on: digestion of carbohydrates in the small intestine? …………………………………………………………………………… …………………………………………………………………………… …………………………………………………………………………… …………………………………………………………………………… 4) Give a short note on: digestion of starch? …………………………………………………………………………… …………………………………………………………………………… 5) Enumerate two disaccharides and explain their digestion? …………………………………………………………………………… …………………………………………………………………………… 6) What is the role of maltase, sucrase and lactase in carbohydrate digestion? …………………………………………………………………………… …………………………………………………………………………… …………………………………………………………………………… …………………………………………………………………………… 7) What does carbohydrates absorption need? …………………………………………………………………………… …………………………………………………………………………… 21 Chapter 2 Carbohydrate metabolism 8) Mention 3 fates of the absorbed glucose? …………………………………………………………………………… …………………………………………………………………………… …………………………………………………………………………… 9) Define glycolysis? …………………………………………………………………………… …………………………………………………………………………… 10) Mention the importance of glycolysis? …………………………………………………………………………… …………………………………………………………………………… 11) Define glycogenesis? What is the responsible enzyme for that process? …………………………………………………………………………… 12) Define glycogenolysis? What is the responsible enzyme for it? …………………………………………………………………………… 13) Enumerate the 2 main sites of glycogen storage in the body? …………………………………………………………………………… 14) Enumerate 2 differences between liver and muscle glycogen? …………………………………………………………………………… …………………………………………………………………………… 15) Define gluconeogenesis? …………………………………………………………………………… …………………………………………………………………………… 16) Enumerate 3 gluconeogenic precursors? …………………………………………………………………………… 17) What is the main function of gluconeogenesis? …………………………………………………………………………… …………………………………………………………………………… 22