Metabolism Of Carbohydrates Biochemistry Notes-I

Afza.Malik GDA

Carbohydrates Metabolism Biochemistry for Nurses

Metabolism Of Carbohydrates Biochemistry Notes-I

Utilization of Glucose in the Body,Withdrawal of carbohydrates from blood,Release of glucose by liver to the blood.,Oxidation,Storage,Conversion to Fats,Conversion to Other Carbohydrates,Conversion to Amino acids.

Metabolism Of Carbohydrates
Utilization of Glucose in the Body

General Outline

    After absorption of monosaccharides into the portal blood, it passes through the liver (the first ‘filter’) before entering the systemic circulation, a fact of considerable physiological and biochemical importance. In liver two mechanisms operate:

Withdrawal of carbohydrates from blood
• Release of glucose by liver to the blood.

    The amount of glucose reaching the systemic circulation at any instant, will be the resultant of operation of these two groups of opposing forces. Once glucose is in systemic circulation, it becomes available for its utilization by “extrahepatic tissues”. 

    Thus, extrahepatic tissues are presented with carbohydrates which have already been “picked over” by the liver in a selective manner. Hence functional state of the liver will be of prime importance and will have a profound influence on the carbohydrate metabolism on the entire organism. 

    Glucose is taken up by intestinal mucosal cells and kidney tubule cells by “active” transport. Hepatic cells are freely permeable to glucose. 

    Insulin increases uptake of glucose by many extrahepatic tissues as skeletal muscle, heart muscle, diaphragm, adipose tissue, lactating mammary gland, etc. Utilization of Glucose

1. Oxidation

• For Provision of Energy In response to physiological needs, human body requires energy. Oxidation of glucose or glycogen to pyruvate and lactate by EM pathway is called glycolysis. 

    Glucose is degraded by glycolysis to pyruvate, which in presence of O2 is completely oxidized to CO2 and H2O. Glycolysis occurs in all tissues.

• HMP Shunt An alternative pathway for oxidation of glucose. It is not meant for energy. The pathway provides

(i) NADPH which is used for reductive synthesis and

(ii) Pentoses which is used for nucleic acids synthesis. This pathway operates only in certain special tissues and not all tissues.

• Uronic Acid Pathway This is another alternative pathway for oxidation of glucose. It provides D-glucuronic acid which is used for synthesis of mucopolysaccharides and conjugation reactions.

2. Storage

    Excess of glucose taken is converted to glycogen in various tissues (glycogenesis) specially liver and skeletal muscle and stored there for future needs. Amount of glycogen storage in liver and muscles is limited. 

    Liver can store approx. 72 to 108 gm (4 to 6% of the weight of liver) and muscles can store approx. 245 gm (0.7% of total weight).

3. Conversion to Fats

    As mentioned above, since the amount of glycogen that can be stored is limited, excess of glucose is converted to FA and stored as triacyl glycerol (TG) in fat depots (lipogenesis). There is no fixed amount for storage of fats as is evidenced from everyday observations on human beings.

4. Conversion to Other Carbohydrates

Small amounts of glucose are used directly or indirectly, in the synthesis of certain other carbohydrates or derivatives, which play important role in the body.

Formation of ribose and deoxyribose: This is required for synthesis of nucleic acids. It is formed by HMP Shunt.

Formation of fructose from glucose: Seminal fluid is rich in fructose and it is required for the metabolism of spermatozoa. Fructose is formed from glucose in seminiferous tubular epithelial cells by ‘Sorbitol’ (polyol) pathway.

Mannose, fucose, glucosamine and neuraminic acid: Form parts of mucopolysaccharides (MPS) and glycoproteins.

Galactose A component part of glycolipids: Galactose required for synthesis of lactose (milk sugar) in lactating mammary gland is synthesised from glucose.

• D Glucuronic acid: Required in the formation of mucopolysaccharides (MPS) and in conjugation reaction for detoxication. It is produced in the body from glucose by uronic acid pathway.

5. Conversion to Amino acids Certain amino acids are not required in the diet, although they occur in tissue proteins. These amino acids are synthesized in the body. This group is called as dispensable or non-essential amino acids. The C skeletons of such amino acids are derived from glucose or its metabolites

“Withdrawal” of carbohydrates from blood

“Release” of glucose by liver to the blood

Uptake of hexoses by liver cells such as galactose, and fructose and their conversion to glucose by liver cells.

Formation of blood glucose from hexoses other than glucose by liver and its release from liver cells.


Conversion of glucose to glycogen for storage in liver (glycogenesis).

Conversion of liver glycogen to blood glucose (glycogenolysis).

Utilization of glucose, by oxidation (glycolysis) for energy production.

Formation of blood glucose by the liver from non- carbohydrate sources, viz. amino acids (glucogenic), pyruvates and lactates, glycerol and propionyl-CoA (gluconeogenesis).

Utilization of glucose for synthesis of other compounds, viz. FA and certain amino acids.

Post a Comment


Give your opinion if have any.

Post a Comment (0)

#buttons=(Ok, Go it!) #days=(20)

Our website uses cookies to enhance your experience. Check Now
Ok, Go it!