ll lipids are polymers which are long chains of repeating units joined together in a condensation reaction during which water is released. The commonest lipids are triglycerides – three fatty acids and glycerol linked by an ester bond.

How do animals obtain their lipids?

Since all cells contain lipids, any animal that eats a plant or animal cell will take in some lipid. Triglycerides and phospholipids are hydrolysed by lipase into glycerol and fatty acids. Since lipids do not dissolve in water, they are not easy to digest and their digestion is accelerated in vertebrates by the secretion of bile salts, which emulsify them into smaller particles, greatly increasing the surface area on which lipase can act.
Fats are hydrolysed by pancreatic lipases into fatty acids, monoglycerides and cholesterol. Single triglycerides, phospholipids and cholesterol then diffuse across the brush border of gut epithelial cells.
The three fatty acids may be identical but a mixture of fatty acids is also possible. Thus, many different triglyceride structures occur and this in turn means that they have a range of functions. Saturated fats are those which contain single bonds between the carbons in the hydrocarbon chain of fatty acids and these are usually solids at room temperature. Unsaturated fats, e.g. vegetable oils, are liquid at room temperature and have some/ many double bonds between the carbons.

Functions:

There are eight major functions of lipids.
1.Energy storage
Lipids form excellent energy storage molecules, e.g. as lipid deposits in the stroma of the chloroplast and as fats in seeds and adipose tissue of vertebrates. Oxidation of fats to release energy occurs in the mitochondria. As in the oxidation of glucose, acetyl coenzyme A is produced in the first stage, but so are many molecules of NAD and
FAD. These are reoxidised in the electron transport chain and hence oxidation of fat yields more ATP than oxidation of carbohydrates.
They are therefore high-energy molecules, which is important in fruits or seeds that need to be dispersed.

2.Structural components
Phospholipids usually make up 40% of cell membranes where their amphipathic nature (having a polar and a non-polar end) enable them to contribute to the spontaneous formation of the bilayer. Cholesterol is also a major component of animal cell membranes.

3.Thermal insulation
Fat conducts heat slowly and therefore the triglycerides, which are stored as subcutaneous fat in vertebrates, are important for maintaining optimum temperature for metabolism.

4.Mechanical protection
e.g. of delicate organs, such as kidneys.

5.Electrical insulation
e.g. Sphingomyelin is a specialised phospholipid in the myelin sheet of nerve axons.

6.Waterproofing
e.g. The waxy cuticle on the leaf epidermis or as oils on birds’ feathers.
The presence of waxy suberin in the Casparian strip of endodermal cells in plant roots forces water into the symplast pathway which is under the control of the nucleus. This strip therefore gives the plant control over substances entering the xylem.

7.Buoyancy
Since fat is less dense than water, fat reserves provide buoyancy for aquatic animals.



The role of lipids in living organisms
Lipids are organic compounds that are insoluble in water but soluble in organic solvents such as ethanol. They are composed mainly of carbon, hydrogen and oxygen, and a number of important biological molecules can be classified as lipids. This essay will describe the general structure of lipids and review the wide variety of roles that they play in living organisms.

The simplest lipids are triglycerides, composed of glycerol joined to three fatty acid molecules. The fatty acids are joined to the glycerol molecule by condensation reactions and the resulting bonds are known as ester bonds. The fatty acids within a particular triglyceride may be identical or different. In general, fatty acids