Pathways in Intermediary Metabolism of Carbohydrate

The different pathways in the metabolism of carbohydrate are glycogenlysis, glycogenolysis, Kerbs citric acid cycle, and gluconeogenesis and the pentose phosphate pathway.

(1) Glycogenolysis. The breakdown of glycogen is called glycogenolysis. In the liver the main end product of glycogen breadown is glucose. In muscle the main products are pyruvic acid and lactic acid. Glycogen acts as a carbohydrate reserve. Whenever glucose is required, glycogen undergoes glycogenolysis, liberating glucose into the blood stream. The hormone glucagon stimulates the conversion of glycogen to glucose in the liver, and thus causes a rise a blood sugar. Glucagon is secreted by the A-cells of the islets of Langerhans in the pancreas.

(2) Glycogenesis is the synthesis of glycogen and starch from glucose. When glucose is absorbed into the blood stream, some amount is immediately converted into glycogen in the liver. Some glucose passes from the liver into systemic circulation and is converted into glycogen in the muscles. An outline of glycogenesis is given below.



(3) Glycolysis. The metabolic pathway starting with glucose with glucose and ending with pyruvic acid called glycoslysis (see Embden-Meyerhof-Parnas scheme).

(4) Krebs citric acid cycle is the final common pathway of oxidation of carbohydrates, proteins and fats through acetyl-CoA, which is completely oxidized dioxide and water.

(5) Gluconeogenesis is the formation of glucose or glycogen from non-carbohydrate precursors. The citric acid cycle and glycolysis are the main pathways of gluconeogenesis. The substrates are certain amino acids, lactic acid, glycerol and propionate. Gluconeogenesis from amino acids and lactic acid takes place mainly in the liver. Lactic acid formed in muscle is carried to the liver. Here it is converted into glucose by a reversal of the glycolysis mechanism, and stored as glycogen.

(6) Pentose phosphate pathway or hexose menophosphate (HMP) shunt. In the liver, adipose tissue and lactating mammary glands, there is an alternative pathway to the Embden-Meyerhof-Parnas scheme of glycolysis and the Kerbs citric acid cycle for the oxidation of glucose to carbon dioxide and water. This pathway is called the pentose phosphate pathway or hexose monophosphate shunt. In the pathway for every six glucose molecules in the beginning, five hexoses are regenerated while one glucose molecule is oxidized.