Carbohydrates Metabolism Biochemistry for Nurses
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
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. |
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