Cholesterol content and membrane fluidity

Membrane fluidity is required for the proper movement of erythrocytes through capillaries of the spleen. Patients with spur cell anemia have increased cholesterol content (> 40%) in the membranes of their erythrocytes, resulting in decreased membrane fluidity. Thus, erythrocytes from these patients are destroyed in the spleen.

Which of the following statements is incorrect concerning the significance of cholesterol?

A. Elevated levels of cholesterol are associated with atherosclerosis.

B. The serum total cholesterol content should be less than 200 mg/dl.

C. Cholesterol is a major component of gall stones.

D. Excess cholesterol is excreted from the liver in the bile as cholesterol or bile salts.

E. Cholesterol is a major precursor for fat soluble vitamins.

Details

The incorrect statement concerning the significance of cholesterol is E- Cholesterol is a major precursor  for fat soluble vitamins.

Cholesterol is an amphipathic lipid and as such is an essential structural component of membranes and of the outer layer of plasma lipoproteins. It is synthesized in many tissues from acetyl-CoA and is the precursor of all other steroids in the body, including corticosteroids, sex hormones, bile acids, and vitamin D.

Cholesterol is a precursor for vitamin D, but not for other fat soluble vitamins which are Vitamin A, E and K. Vitamin A, E and K are mainly obtained through diet (vitamin K can be synthesized in the gut by bacterial action), cholesterol has no role to play in their bioavailabilities.

As regards other option

A. Elevated levels of cholesterol are associated with atherosclerosis.

Atherosclerosis is characterized by the deposition of cholesterol and cholesteryl ester from the plasma lipoproteins into the arterial wall (figure). Diseases in which prolonged elevated levels of VLDL, IDL, chylomicron remnants, or LDL occur in the blood are often accompanied by premature or more severe atherosclerosis.

Atherosclerosis

Figure- Atherosclerosis leads to narrowing of the lumen of the arteries, eventually resulting in blockage.

B. The serum total cholesterol content should be less than 200 mg/dl.

Hypercholesterolemia is a clinical state of high serum total cholesterol content (above the normal range, which is 150- 220 mg/dl). The levels of serum total cholesterol above 200 mg/dl but less than 220mg/dl are considered borderline high, > 220 mg/dl are considered high and carry the risk of Ischemic heart disease.

High cholesterol concentrations are found in-Diabetes mellitus, nephrotic syndrome, obstructive jaundice, familial hypercholesterolemia, biliary cirrhosis and hypothyroidism.

Hypocholesterolemia- Low serum cholesterol concentration is observed in- Hyperthyroidism, malnutrition, malabsorption, anemia, persons on cholesterol lowering drugs  and physiologically lower levels are found in children.

C. Cholesterol is a major component of gall stones. Gall stones can be cholesterol, pigment or mixed stones. The cholesterol stones are the commonest of all and are formed in conditions causing reduced availability of bile salts or phospholipids. Bile salts and phospholipids keep cholesterol in the soluble form which is otherwise insoluble and has a tendency to form aggregates.

D. Excess cholesterol is excreted from the liver in the bile as cholesterol or bile salts.

This is a true statement, cholesterol is never completely oxidized to provide energy, it is excreted either as such or it is structurally modified to form bile acids which are conjugated further with glycine and taurine to form bile salts and are secreted through bile in to the duodenum.

Thus the only incorrect statement about significance of cholesterol is E.

 

 

 

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