Question

Long answer question.

Describe the types of lipids and mention their biological significance.

Answer 1

Lipids are classified into three main types:

• Simple lipids:

1. These are esters of fatty acids with various alcohols. Fats and waxes are simple lipids.
2. Fats are esters of fatty acids with glycerol (CH₂OH-CHOH-CH₂OH).
3. Triglycerides are three molecules of fatty acids and one molecule of glycerol.
4. Unsaturated fats are liquid at room temperature and are called oils. Unsaturated fatty acids are hydrogenated to produce fats e.g. Vanaspati ghee.
   Biological significance:

1. Fats are a nutritional source with high calorific value and they act as reserved food materials.
2. In plants, fat is stored in seeds to nourish embryo during germination.
3. In animals, fat is stored in the adipocytes of the adipose tissue.
4. Fats deposited in subcutaneous tissue act as an insulator and minimize loss of body heat.
5. Fats deposited around the internal organs act as cushions to absorb mechanical shocks.
6. Wax is another example of a simple lipid. They are esters of long-chain fatty acids with long-chain alcohols.
7. They are found in the blood, gonads, and sebaceous glands of the skin.
8. Waxes are not as readily hydrolyzed as fats.
9. They are solid at ordinary temperatures.
10. Waxes form water-insoluble coating on hair and skin in animals, waxes form an outer coating on stems, leaves, and fruits.

• Compound lipids: 

1. These are ester of fatty acids containing other groups like phosphate (Phospholipids), sugar (glycolipids), etc.
2. They contain a molecule of glycerol, two molecules of fatty acids and a phosphate group or simple sugar.
3. Some phospholipids such as lecithin also have a nitrogenous compound attached to the phosphate group.
4. Phospholipids have both hydrophilic polar groups (phosphate and nitrogenous group) and hydrophobic non-polar groups (hydrocarbon chains of fatty acids).
5. Glycolipids contain glycerol, fatty acids, simple sugars such as galactose. They are also called cerebrosides.
   Biological significance:

1. Phospholipids contribute in the formation of the cell membranes.
2. Large amounts of glycolipids are found in the brain white matter and myelin sheath.

• Derived Lipids: 

1. They are composed of fused hydrocarbon rings (steroid nucleus) and a long hydrocarbon side chain.
2. One of the most common sterols is cholesterol.
   Biological significance:

1. It is widely distributed in all cells of the animal body, but particularly in nervous tissue.
2. Cholesterol exists either free or as a cholesterol ester.
3. Adrenocorticoids, sex hormones (progesterone, testosterone) and vitamin D are synthesized from cholesterol.
4. Cholesterol is not found in plants.
5. Sterols exist as phytosterols in plants.
6. Yam Plant (Dioscorea) produces a steroid compound called diosgenin. It is used in the manufacture of antifertility pills. i.e. birth control pills.