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Cholesterol is essential for all living organisms. It is synthesized from simpler substances within the body. Cholesterol can also be obtained from food.
Biological Significance of cholesterol
Cell membrane synthesis – Cholesterol helps to regulate membrane fluidity over the range of physiological temperatures. It has a hydroxyl group that interacts with the polar head groups of the membrane phospholipids and sphingolipids. These exist along with nonpolar fatty acid chain of the other lipids. Cholesterol also prevents the passage of protons (positive hydrogen ions) and sodium ions across the plasma membranes.
Cell transporters and signalling molecules – The cholesterol molecules exist as transporters and signalling molecules along the membrane. Cholesterol also helps in nerve conduction. It forms the invaginated caveolae and clathrin-coated pits, including caveola-dependent and clathrin-dependent endocytosis. Endocytosis means engulfing of foreign molecules by the cell. Cholesterols help in cell signalling by assisting in the formation of lipid rafts in the plasma membrane.
Cholesterol in the myelin sheaths – The nerve cells are covered with a protective layer or myelin sheath. The myelin sheath is rich in cholesterol. This is because it is derived from compacted layers of Schwann cell membrane. It helps in providing protection, insulation and allows more efficient conduction of nerve impulses.
Role inside the cells – Within the cells, cholesterol is the precursor molecule in several biochemical pathways. For example, in the liver, cholesterol is converted to bile, which is then stored in the gallbladder. Bile is made up of bile salts. This helps in making the fats more soluble and helps in their absorption. Bile salts also aid in absorption of fat soluble vitamins like Vitamins A, D, E and K.
Hormones and Vitamin D - Cholesterol is an important precursor molecule for the synthesis of Vitamin D and the steroid hormones like Corticosteroids, Sex-steroids (Sex hormones like Estrogen, Progesterone and Testosterone etc.)
