Lipids - 2

Functions of lipids

1. Storage - lipids are non-polar and so are insoluble in water.
2. High-energy store - they have a high proportion of H atoms relative to O atoms and so yield more energy than the same mass of carbohydrate.
3. Production of metabolic water - some water is produced as a final result of respiration.
4. Thermal insulation - fat conducts heat very slowly so having a layer under the skin keeps metabolic heat in.
5. Electrical insulation - the myelin sheath around axons prevents ion leakage.
6. Waterproofing - waxy cuticles are useful, for example, to prevent excess evaporation from the surface of a leaf.
7. Hormone production - steroid hormones. Oestrogen requires lipids for its formation, as do other substances such as plant growth hormones.
8. Buoyancy - as lipids float on water, they can have a role in maintaining buoyancy in organisms.

Phospholipids

A phosphate-base group replaces one fatty acid chain.

Phospholipids have a hydrophilic (water loving) head, and hydrophobic (water repelling) tails. This results in the formation of a phospholipid bilayer (double layer), which forms the basis for the plasma membrane.

• These membranes are what separate cells in the body, and enable the transfer of different chemicals between cells and with their environment.
• Most chemicals in cells are water soluble so lipid membranes act as good barriers that can control the movement of different substances between cells.
• Only certain small molecules would be able to cross the membrane freely.
• Other compounds would require carriers or special channels to enable them to overcome the repelling effect between the membrane and their particular chemical state e.g. chemical charge, size, etc.

Lipids in health

Lipids are central to cell function and health. However, a diet high in certain types of fat has been previously associated with cardiovascular disease.

Saturated fats were thought to contribute to disease by increasing low-density lipoproteins (LDL) in the blood, while unsaturated fats could counteract this effect by increasing the high-density lipoproteins (HDL) in the blood.