Human Requirements Of Glucose
Human requires about 160–200 g of glucose per day as an energy source for cellular metabolism and brain functions.
Indeed, two-thirds of glucose (about 100–130g) is specifically needed by the brain to cover its high energy requirements. As a dense network of neurons, or nerve cells, which are constantly active, the brain depends
on a continuous supply of glucose from the bloodstream.
First step: Absorbation Of Glucose
After food ingestion, glucose is absorbed and released into the bloodstream by the small intestine and the stomach. Glucose cannot penetrate into the cells directly and thus circulates in the bloodstream.
Beta cells, the predominant type of cells in the islets of Langerhans, are sensitive to glucose levels and regulate the pancreas to release insulin, corresponding to the blood glucose level.
Second step: Secretion Of insulin Into The Blood
Insulin is then secreted into the blood where it travels throughout the body and helps regulate blood sugar. Beta cells also secrete amylin and C-peptide together with insulin. Thus, beta cells in the pancreas play three important tasks: producing, storing and releasing the hormone insulin.
Third step: Attachment With Insulin Receptor
Insulin attaches itself to the insulin receptor (IR), a transmembrane receptor, resulting in an ‘open’ channel that allows the passage of glucose.
The IR belongs to the large class of tyrosine kinase receptors and is activated by insulin and free insulin-like growth factors (IGF-I and
IGF-II). A ‘substrate’ protein that is phosphorylated by the insulin receptor is known as IRS-1 (insulin receptor substrate 1).
Fourth step: Phosphorylation
IRS-1 binding and phosphorylation lead to an increase in the high-affinity glucose transporter (Glut4) molecules on the outer membrane of insulin-responsive tissues, including muscle cells and adipose tissue.
Glut4 is transported from cellular vesicles to the cell surface, where it then can mediate the transport of glucose into the cell.
Three important roles of insulin :
Indeed, insulin plays three important roles:
(I) It helps muscle, fat, and liver cells absorb glucose from the bloodstream, lowering blood glucose levels,
(II) It stimulates the liver and muscle tissue to store excess glucose as glycogen,
(III) It lowers blood glucose levels by reducing glucose production in the liver
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