Carbohydrate Metabolism

The primary energy source for living cells is glucose. Since glucose is linked to all of the main pathways involved in the metabolism of carbohydrates, it is the key molecule in this process. Energy is obtained from glucose, which is produced from non-carbohydrate precursors and stored as glycogen to be released as needed. The liver is essential for controlling and regulating blood sugar levels. Liver can therefore be adequately regarded as a glucostat monitor. Major pathways of carbohydrate metabolism. The important pathways of carbohydrate metabolism are listed

                                                        Overview of glucose metabolism.                                                 (Note : For majority of the pathways, glucose participates as glucose 6-phosphate)

1. Glycolysis (Embden-Meyerhof pathway): Phosphate and lactate are produced when glucose is oxidised. 

2. Citric acid cycle (Krebs cycle or tricarboxylic acid cycle) : Acetyl CoA is oxidised to CO2 in the citric acid cycle, also known as the Krebs cycle or the tricarboxylic acid cycle. Through acetyl CoA, the Krebs cycle is the last frequent oxidative pathway for lipids, carbohydrates, or amino acids.

3. Gluconeogenesis: the process by which glucose is created from precursors that are not carbohydrates (such as glycerol, amino acids, etc.).

4. Glycogenesis: The process by which glucose is converted to glycogen.

5. Glycogenolysis: The process by which glucose is produced from glycogen.

6. Hexose monophosphate shunt, also known as the pentose phosphate route or direct oxidative pathway, is a glucose oxidation process that converts glucose directly to carbon dioxide and water, replacing glycolysis and the TCA cycle. 

7. Uronic acid pathway: Glucose is transformed into pentoses, glucuronic acid, and ascorbic acid (not in humans) in certain species. This glucose oxidative route is an additional option.

8. Galactose metabolism: The processes that result in the synthesis of lactose and the conversion of galactose to glucose.

9. Fructose metabolism: the process by which fructose is converted to pyruvate and the connection between the metabolism of fructose and glucose.

10. Amino sugar metabolism: The process by which other sugars and amino sugars are synthesized in order to create glycoproteins (Glycoproteins are molecules that comprise protein and carbohydrate chains that are involved in many physiological functions including immunity.)

Glucose's entry into cells Compared to plasma, the concentration of glucose in the cells is quite low (less than 100 mg/dl for humans). Nevertheless, glucose does not just flow into the cells. It is known that there are two distinct transport systems that allow glucose to enter cells.

 
1. Glucose transport pathway independent of insulin: This glucose absorption is carrier-mediated and is independent of the insulin hormone. This functions in the brain, erythrocytes (RBCs), and hepatocytes (cells of liver).

2. Adipose and muscular tissue have an insulin-dependent transport mechanism.
insulin transporters: At least six glucose transporters in cell membranes (GLUT-1 through GLUT-5 and GLUT-7) have been discovered in recent years. Their tissue selectivity is evident. For example, GLUT-1 is highly expressed in blood cells, while GLUT-4 is highly expressed in fatty tissue and muscle cells. In muscle and fat cells, insulin increases the quantity of GLUT-4 molecules and stimulates their activity. Insulin resistance is seen in these tissues in type 2 diabetes mellitus. This results from the decrease in GLUT-4 amount in insulin insufficiency.

References: ‘Biochemistry' Satyanarayana and Chakrapani