2.3. Lipids

Lipids are a large and varied group of organic compounds. Like carbohydrates, they contain carbon, hydrogen and oxygen, although the proportion of oxygen is much smaller in lipids. They are insoluble in water but dissolve readily in organic solvents such as acetone, alcohols and others. They are of two types: fats and oils. There is no basic difference between these two; fats are simply solid at room temperatures (10-20°C) whereas oils are liquid. The chemistry of lipids is very varied but they are all esters of fatty acids and an alcohol, of which glycerol is by far the most abundant.

Functions of lipids

1. An energy source.

Upon breakdown they yield 38 kJg^-1 of energy. This compares favourably with carbohydrates which yield 17 kJg^-1 and means lipids are excellent energy stores. This makes them especially useful for animals where locomotion requires mass to be kept to a minimum. In plants they are useful in seeds where dispersal by wind or insects makes small mass a necessity. This explains the abundance of oils extracted from seeds and fruits, e.g. olive, peanut, coconut and sunflower. Their insolubility is another advantage, as they are not easily dissolved out of cells.

2. Insulation.

Fats conduct heat only slowly and so are useful insulators. In endothermic animals, such as mammals, fat is stored beneath the skin (subcutaneous fat) where it helps to retain body heat. In aquatic mammals, such as whales, seals and manatees, hair is ineffective as an insulator because it cannot trap water in the same way as it can air. These animals therefore have extremely thick subcutaneous fat, called blubber, which forms an effective insulator.

3. Protection.

Another secondary use to which stored fat is put is as a packing material around delicate organs. Fat surrounding the kidneys, for instance, helps to protect them from physical damage.

4. Buoyancy.

Being less dense than water, lipids aid buoyancy of aquatic vertebrates such as sharks, seals and whales. Sharks have extremely fatty livers which make up to 25% of their body volume and contain a lipid, squaline, with a specific gravity of only 0.86. Oils on bird feathers are especially important in keeping aquatic varieties afloat.

5. Waterproofing.

Terrestrial plants and animals have a need to conserve water. Animal skins produce oil secretions, e.g. from the sebaceous glands in mammals, which waterproof the body. Oils also coat the fur, helping to repel water which would otherwise wet it and reduce its effectiveness as an insulator. Birds spread oil over their feathers for the same purpose. Insects have a waxy cuticle to prevent evaporative loss in the same way that plant leaves have one to reduce transpiration.

6. Cell membranes.

Phospholipids are major components of the cell membrane and contribute to many of its properties.

7. Other functions.

Lipids perform a host of miscellaneous functions in different organisms. For example, plant scents are fatty acids (or their derivatives) and so aid the attraction of insects for pollination. Bees use wax in constructing their honeycombs.