METABOLISM OF CARBOHYDRATES

 METABOLISM OF CARBOHYDRATES

Carbohydrates are the major source of energy for living cells. As such, carbohydrates are the first cellular constituents, synthesized by green plants during photosynthesis from carbon dioxide and water, on the absorption of light. Thus, light is the ultimate source of energy for all biological processes. The monosaccharide glucose is the central molecule in carbohydrate metabolism since all the major pathways of carbohydrate metabolism are connected with it. Glucose is utilized as a source of energy, it is synthesized from non-carbohydrate precursors and stored as glycogen to release glucose as and when the need arises. The other monosaccharides important in carbohydrate metabolism are fructose, galactose, and mannose. The fasting blood glucose level in normal individuals is 70-100 mg/dl (4.5-5.5 mmol/l) and it is very efficiently maintained at this level. The liver plays a key role in monitoring and stabilizing blood glucose levels. Thus liver may be appropriately considered as a glucose monitor.

Major pathways of carbohydrate metabolism 

The important pathways of carbohydrate metabolism are listed

1. Glycolysis (Embden-Meyerhof pathway): The oxidation of glucose to pyruvate and lactate.

 2. Citric acid cycle (Krebs cycle or tricarboxylic acid cycle): The oxidation of acetyl CoA to CO2. The Krebs cycle is the final common oxidative pathway for carbohydrates, fats, or amino acids, through acetyl CoA. 3. Gluconeogenesis: The synthesis of glucose from non-carbohydrate precursors (e.g. amino acids, glycerol, etc.).

4. Glycogenesis: The formation of glycogen from glucose.

5. Glycogenolysis: The breakdown of glycogen into glucose.

6. Hexose monophosphate shunt (pentose phosphate pathway or direct oxidative pathway): This pathway is an alternative to glycolysis and the TCA cycle for the oxidation of glucose (directly to carbon dioxide and water).

7. Uronic acid pathway: Glucose is converted to glucuronic acid, pentoses, and, in some animals, to ascorbic acid (not in man). This pathway is also an alternative oxidative pathway for glucose.

8. Galactose metabolism: The pathways concerned with the conversion of galactose to glucose and the synthesis of lactose.

9. Fructose metabolism: The oxidation of fructose to pyruvate and the relation between fructose and glucose metabolism.

10. Amino sugar and mucopolysaccharide metabolism: The synthesis of amino sugars and other sugars for the formation of mucopolysaccharides and glycoproteins.

Entry of glucose into cells

 Glucose concentration is very low in the cells compared to plasma (for humans < 100 mg/dl). However, glucose does not enter the cells by simple diffusion. Two specific transport systems are recognized for the entry of glucose into the cells

1. Insulin-independent transport system of glucose: This is a carrier-mediated uptake of glucose that is not dependent on the hormone insulin. This is operative in hepatocytes, erythrocytes, and brain

2. Insulin-dependent transport system: This occurs in muscle and adipose tissue. Glucose transporters: In recent years, at least six glucose transporters (GLUT-1 to GLUT-5 and GLUT-7) in the cell membranes have been identified. They exhibit tissue specificity. For instance, GLUT-1 is abundant in erythrocytes whereas GLUT-4 is abundant in skeletal muscle and adipose tissue. Insulin increases the number and promotes the activity of GLUT-4 in skeletal muscle and adipose tissue. In type 2 diabetes mellitus, insulin resistance is observed in these tissues. This is due to the reduction in the quantity of GLUT-4 in insulin deficiency