What is metabolism? • Metabolism is all the chemical reactions done by living organism. Virtually all chemical reactions are catalyzed by enzymes. • Catabolismis a part of metabolism– the breakdown of substances by an organism. Many important catabolic reactions occur in the mitochondria. • Anabolismis another part of metabolism– the synthesis of molecules used to build molecules for cell growth and multiplication. Nutrients are also converted to their storage forms by anabolic processes. Many anabolic reactions occur in cytoplasm. • Cells usually employ different chemical reactions for the breakdown and synthesis of the same molecule.
Carbohydrate metabolism • Carbohydrate occupies central drama of the cell. • Supporting roles are played by fats and amino acids. • The star of carbohydrate metabolism is glucose. In carbohydrate metabolism, we will see: how is glucose broken down inside the cell to produce energy; how is it stored; how is the level of glucose regulated in the blood; and, how defects in regulation of blood glucose lead to diabetes.
Catabolism of glucose • The paths that cells use to oxidize glucose completely to carbon dioxide involve many individual chemical reactions. These reactions occur in three different stages. These are: - initial break down of glucose to pyruvate in glycolysis, - further degradation of pyruvate to acetyl coenzyme A, and - finally complete oxidation of acetyl coenzyme A to water and carbon dioxide in the citric acid cycle. • Glycolysis (Greek: glyco,sugar; lysis, splitting) is the sequence of chemical reactions by which glucose, a six-carbon sugar is cleaved to two molecules of pyruvate, a three-carbon acid. Also known as glycolytic pathway or the Embden-Meyerhof pathway after the name of two German biochemist Gustav Embden and Otto Meyerhof, who proposed it in the latter part of the 19thcentury.
The reactions of glycolysis The enzymes that catalyze the steps of glycolysis are found in the cytoplasm of the cells. Hence, glycolysis occur in cytoplasm.
Formation of Acetyl coenzyme A Two carbons of pyruvate are incorporated into acetyl coenzyme A • When an aerobic cell is operating with a good supply of oxygen, pyruvate molecule flows into the mitochondria. Two of the carbons of each pyruvate ion end up as acetyl CoA, and one molecule of NAD+ is reduced to NADH. Carbon dioxide is formed as waste product. • A multi-enzyme complex is responsible for the formation of acetyl CoA from pyruvate.
Gluconeogenesis • The pathway by which lactate is converted to glucose is called gluconeogenesis (Greek: gluco, glucose; neo, new; genesis,creation). Gluconeogenesis is the process for synthesis of glucose from non-carbohydrate source (lactate). This is an example of anabolic or synthetic pathway.