1. Introduction
Lipids, like carbohydrates, contain carbon, hydrogen and oxygen although the proportion of oxygen is much smaller in lipids. Lipids are chemically diverse compounds grouped together because they have little or no affinity for water. They are hydrophobic. This hydrophobicity of lipids is based on their molecular structure. Although lipids have some polar bonds associated with oxygen, they consist mostly of hydrocarbon regions with non-polar bonds.
There are 2 basic types of lipids. Fats which are solid are room temperature and oils which are liquid at room temperature. There are no major biochemistry differences between fats and oils.
2. Uses of Lipids
As an energy source, lipids release 38 kJ of energy per gram of itself; twice as much as starch. Lipids are insoluble in water and its high energy content makes it the ideal energy store. Subcutaneous fats insulate the body and conducts heat slowly. As phospholipids, they are the major constituent of the cell membrane. Oils and waxes prevent evaporation of water.
3. Constituents of Lipids
Lipids are formed by joining 2 different types of molecules by a condensation reaction known as esterification. Lipids are made up of fatty acids and glycerol
Fatty acids have the general formula RCOOH. Fatty acids are carboxylic acids with long hydrocarbon side chains usually 15-17 carbon atoms in length. This long hydrocarbon tail is attached to the carboxyl group (COOH). The non-polar C-H bonds in the tails of fatty acids are the reasons fats are hydrophobic. If the fatty acids contain the max number of hydrogen atoms possible, it is said to be saturated. If some adjacent carbon atoms form a double covalent bond, the fatty acid is said to be unsaturated. Such double bond is formed by the removal of hydrogen atoms from the carbon skeleton resulting in a kink in its shape wherever a double bond occurs. Most animal fats are saturated and are solid at room temperature. Fats of plants and fish are generally unsaturated and are liquid at room temperature.
Glycerol is a form of alcohol. It’s formula is C3H8O3. It has 3 carbons each bearing a hydroxyl group. Most lipids are esters of glycerol and are therefore termed as glycerides.
4. Types of Lipids
Triglycerides are large molecules comprising of three fatty acids each being joined to a glycerol by an ester linkage which is a bond between a hydroxyl group and a carboxyl group. They are non polar and are therefore relatively insoluble in water.
4.1.1 Biological Functions of Triglycerides
The major function of fat is energy storage. Fats store more than twice the amount of energy as carbohydrates. Plants are immobile therefore they can store energy in the form of starch which is bulky. Animals, on the other hand, have to carry their energy store with them, so there is an advantage of having a more compact reservoir of fuel in the form of fats.
A given mass of lipids yields more than twice the amount of energy on oxidation than an equal mass of carbohydrates. Lipids have a higher calorific value. This is because lipids have a higher proportion of hydrogen and an almost insignificant proportion of oxygen than carbohydrates.
Subcutaneous fats, layers of fats beneath the skin, serve as heat insulators for animals living in cold climates.
Fats are stored in adipose tissues which are also used to cushion vital organs such as kidneys. This serves as a protection for these organs against damages.
When fats are oxidised, the by-product is water. Desert animals store fats as a source of water.
4.2 Phospholipids
These are lipids containing a phosphate group. The most common type is formed when one of the 3 hydroxyl groups of glycerol combines with phosphoric acid instead of a fatty acid. The other 2 OH groups combine with fatty acids as in triglycerides. The molecule consists of a phosphate head with 2 hydrocarbon tails. The phosphate head carries an electrical charge and is therefore water soluble but the tails are insoluble. This is important in the formation of membranes.
4.2.1 Biological Functions of Phospholipids
The phospholipid molecules have one end which is attracted to water and another end which is repelled by water and it is this property that enables them to form the cell membrane. The cell surface membrane is made up of a double layer of phospholipid molecules, each molecule lying with its hydrophilic head in the watery liquid and its tail pointing away. This arrangement is called a phospholipid bilayer.
When phospholipids are added to water, they self-assemble into aggregates that shield their hydrophobic tails from water forming micelles which help in the transport of fat between gut and the liver.
Phospholipids are associated with hydrophilic oligosaccharides to form glycolipids. Oligosaccharides are short polysaccharides, some of which are covalently bonded to lipids to form glycolipids which function as markers that distinguish one cell from another.
Phospholipids also act as a source of acetylcholine, a neurotransmitter.
4.3 Steroids
Structurally, steroids have little in common with other lipids. They share some properties such as they are insoluble in water and are soluble in organic solvents. They are characterized by a carbon skeleton consisting of 4 fused rings. Cholesterol is a steroid and is an important constituent of membranes and also a key intermediate in the synthesis of steroid hormones such as oestrogen.
4.3.1 Biological Functions of Steroids
They are components of steroid hormones e.g. testosterone.
Cholesterol is involved in the regulation of fluidity of the cell membrane. It is a precursor of other steroid-like bile acids and sex hormones. LDL (low density lipoprotein), which is derived from saturated fats and which take cholesterol into tissues, can increase the risk of coronary heart disease. HDL (high density lipoprotein) which is derived from unsaturated fats, removes cholesterol from tissues and arteries thereby decreasing the risk of heart disease.
4.4 Waxes
Waxes are similar to fats and oils but their long-chained fatty acids are linked to a long-chained alcohol. Waxes are formed by combining fatty acids with an alcohol which is not glycerol and this alcohol is much larger than glycerol. Waxes are totally insoluble in water and are chemically inert.
4.4.1 Biological Functions of Waxes
In plants, a waxy cuticle covers the epidermis of leaves and stems to prevent excessive water loss. In animals, insects also have a waxy cuticle to help them cut down water loss. It is also an important component in the honeycomb of bees.
