A triglyceride is a lipid formed from glycerol and three fatty acids that are linked together by ester bonds. Each fatty acid molecule consists of a carboxyl group –COOH. Triglycerides, which have an unsaturated fatty acid, tend to be a liquid at room temperature and are called oils. Those, which contain saturated fatty acid, tend to be a solid at room temperature and are known as fats or waxes. Triglycerides are a useful energy store in animals and plants as they are insoluble in water as they are non -polar and can be stored inside cells in the form of droplets. Lipids droplets are also found in mitochondria as they have a relatively low density allowing them to release twice as much energy per gram as carbohydrates.
Whales and other mammals, which live in water, have cells that contain a lot of fat beneath their skin. These cells make up adipose tissue whose function is not only to act as an energy store but also provides the animal with buoyancy; therefore, it is important that lipids are insoluble in water. Lipids also provide the animal with insulation. This ensures that heat loss from the skin into the water is kept to a minimal. Humans also have adipose tissue beneath their skin, which also prevents heat loss, therefore it is important that lipids are good heat insulators otherwise a lot of heat would be lost. Lipids help to protect internal organs within the human body, such as the kidneys, by providing a cushioning layer. This protects against mechanical damage.
Waxes are esters of fatty acids and long chains of alcohols. The main function of waxes and oils is to provide excellent waterproof layers. For example, in plants the leaves are covered by waxy cuticles, which prevent excess water loss or entry. In animals, such as ducks, oil is found on their feathers to ensure excess water is not absorbed. In vertebrates, a waxy layer is found on their skin and fur, which is deposited on the outside of the cuticle in insects. Some bees are able to synthesise their own wax, which is used to construct a honeycomb.
Phospholipids are lipids formed from glycerol, two fatty acids, and a phosphate group. Phospholipids are found in the cell membrane. This molecule has a hydrophobic and hydrophilic part. The hydrophobic fatty acids part is not attracted to water and is therefore found on the outside part of the cell membrane. The hydrophilic phosphate part is attracted to water and is therefore found in the inner part. This forms a phospholipid bilayer.
Glycolipids are made up of carbohydrates and lipids. The carbohydrate component allows substances such as antibodies and hormones to recognise it. As carbohydrates are soluble in water, they are able to form hydrogen bonds with water, which increases the stability of the membrane. The lipid component allows the glycolipid to become less soluble this is found in the form of cholesterol. Excess cholesterol is responsible for cardiovascular diseases such as heart attacks as they cause a blocking in the coronary artery. Some hormones are also made up of lipids and are known as steroids.
The ester bonds in lipids smell sweet this helps to attract bees to plants, this helps pollination. Lipids are also found in hair to make it waterproof it is able to do this, as lipids are insoluble in water.
It can be seen that lipids have many functions, which depend enormously on its structure. It is important that they are both saturated and unsaturated to ensure it carries out it function effectively.
