Citric Acid Cycle

Carbohydrate Metabolism Overview
Overview Metabolism &
Energy Summary
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Hormone Control of Carbohydrate Metabolism

Introduction:

The metabolism of carbohydrates is regulated by a variety of hormones and other molecules. Some of these have already been mentioned in previous sections. The proper functions of the body are dependent on precise control of the glucose concentration in the blood. The normal fasting level of glucose in the blood is 70-90 mg/100 ml.

If the concentration of glucose in blood is too high (above 120 mg/100 mL) a condition known as hyperglycemia results. Hyperglycemia may temporarily exist as a result of eating a meal rich in carbohydrates.

If the concentration of glucose is too low (below 70 mg/100 ml) a condition of hypoglycemia exists. Hypoglycemia is characterized by general weakness, trembling, drowsiness, headache, profuse perspiration, rapid heart beat, and possible loss of consciousness. These definitions are illustrated in Figure 9. In addition, the hormone regulators are shown which will be discussed.


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Insulin:

Insulin, a polypeptide, is secreted from the pancreas in response to a hyperglycemia condition which usually results shortly after ingesting a meal. The structure is shown on the left.

Insulin Protein - Chime in new window

The major effect of insulin is to promote the transport of sugar across the cell membrane of fat and muscle cells. In addition, insulin promotes anabolic processes such as increasing the rate of synthesis for glycogen (glycogenesis), fatty acids, and proteins. Insulin inhibits the catabolic processes such as the breakdown of glycogen and fat.

A deficiency of insulin (hypoinsulinism) results in a permanent hyperglycemic condition known as diabetes mellitus. If little or no insulin is present, glucose cannot be utilized properly by the cells and accumulates in the blood. Fatty acid metabolism is also upset. For this reason, a detailed study of diabetes mellitus must wait until the next chapter.

Hyperinsulinism (too much insulin) leads to the hypoglycemic condition. Excessive amounts of glucose are removed from the blood. Severe hypoglycemia may result when a diabetic injects too much insulin. A severe insulin shock may result in a coma since glucose does not reach the brain. A diabetic usually carries a glucose rich food, such as candy, to provide a quick supply of glucose to replenish depleted glucose levels caused by too much insulin.

A functional type of hypoglycemia results in some individuals from an over stimulation of insulin. The causes of hypoglycemia are not completely understood, but it occurs in some people after eating heavily sugared food such as heavily sugared cereal and/or coffee and sweet rolls. The initial high glucose levels over stimulates the pancreas to produce too much insulin. The excess insulin causes blood sugar levels to drop below normal after 2-3 hours which may cause the person to feel sleepy, irritable, and generally tired. The condition is only exacerbated by a "quick fix" of more sweetened coffee, pastry, or candy since more insulin is produced again. A protein rich breakfast would correct the condition by allowing glucose to enter the blood stream more slowly.

Glucagon:

If one hormone, insulin, controls the excess of glucose in the blood by stimulating synthesis of glycogen, then other hormones must respond to low levels of glucose. The liver is more responsive to glucagon, a peptide also secreted by the pancreas.

Glucagon increases glucose levels in the blood by stimulating the breakdown of glycogen (glycogenolysis) in the liver into glucose which leaves the liver cells and enters the blood stream. The method of hormone stimulation is a complex cascade effect. The exact sequence has been worked out in the most detail for epinephrine (adrenalin) although glucagon works in a similar fashion.

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