All lipids are composed of carbon, hydrogen, and a little oxygen. They are insoluble in water but readily soluble in organic solvents such as benzene, acetone, and ether. Like we discussed before, lipids aren’t polymeric substances but are assembled from smaller molecules by dehydration.
Lipids could be simple fatty acids or glycerol. Several lipids are composed of both fatty acids and glycerol, while some of them also contain phosphorus and a phosphorylated organic compound in their structure. Certain lipids possess more complex structures as well.
Classification of lipids
Scientists have classified lipids into subgroups as follows:
These are esters of fatty acids with various alcohols. They may be of two types – neutral/true fats or waxes.
Neutral or true fats
They are also known as glycerides. Every fat molecule is an ester of one molecule of glycerol and one to three molecules of the same or different long-chain fatty acids. Glycerol is chemically trihydroxypropane, whereas a fatty acid molecule is an unbranched chain of carbon atoms that has a carboxylic group attached to an R group.
The R group could be a methyl (-CH3) group, ethyl (-C2H5) group, or have a higher number of –CH2 groups (one carbon to 19 carbons). For example, palmitic acid (C16H32O2) has 16 carbon atoms including the carboxyl carbon. Arachidonic acid possesses 20 carbon atoms including the carboxyl carbon. Fatty acids are of two types:
- Saturated fatty acids like palmitic acid and stearic acid don’t contain double bonds.
- Unsaturated fatty acids such as oleic acid, linoleic acid, linolenic acid, and arachidonic acid possess one or more double bonds in their structure.
Oils containing polyunsaturated fatty acids (PUFA) or fatty acids with more than one double bond are recommended by doctors to people having hypertension, high blood cholesterol, and other cardiovascular diseases because they have been found to lower blood cholesterol. Sunflower oil and safflower oil are rich in polyunsaturated fatty acids.
Depending on the number of fatty acid molecules attached to a molecule of glycerol, neutral or true fats may be monoglycerides (one molecule), diglycerides (two molecules), or triglycerides (three molecules).
Based on their melting point, triglycerides can be known as fats or oils. Fats (such as butter and ghee) have high melting points and remain in the solid-state at room temperature. On the other hand, oils (such as sunflower oil and groundnut oil) have lower melting points and exist in the liquid state at room temperature.
Waxes are esters of fatty acids with alcohols of higher molecular weight other than glycerol. They play an important protective role in living organisms. They form water-insoluble coatings on the hair in skin in animals and stems, fruits, and leaves in plants.
Bee wax is chemically hexacosyl palmitate and is formed from palmitic acid and myricyl alcohol. It is secreted from the abdominal glands of worker bees for building their hives. Lanolin or wool fat is found in the form of a waterproof coating around animal furs. Mycobacterium tuberculosis and Mycobacterium leprae produce a harmful wax known as wax-D that contributes to their pathogenicity.
Compound or conjugated lipids
These are esters of fatty acids with alcohol that also contain some other chemical groups. They are classified into the following subgroups:
They are composed of a molecule of glycerol or another alcohol having:
- A phosphate group joined to one of its outer –OH groups
- Two fatty acid molecules linked to the other two –OH groups
- A nitrogen-containing chlorine molecule bound to the phosphate group
Phospholipids such as lecithin are found in cell membranes.
They contain fatty acids, the alcohol sphingosine, and galactose. One of the fatty acid molecules is replaced by the galactose residue. Glycolipids are found in the myelin sheath of nerve fibers and in the outer membrane of the cell membrane of chloroplasts.
As their name suggests, lipoproteins are found to have lipids (mostly phospholipids) and proteins in their molecular structures. They are the main constituents of membranes.
They contain colored pigments such as carotenoids (carotene, vitamin A, and more).
Although steroids don’t have fatty acid residues, they are considered lipids because of their fat-like properties. Instead of a straight-chain, they possess four fused carbon rings. Steroids differ from each other in terms of the number and position of double bonds between carbon atoms and in the side groups linked to the ring.
Sterols are the most common steroids found in nature, with the most well-known example being cholesterol. It is the most abundant steroid in animal tissues and is found abundantly in foods containing animal fats. It is synthesized in the liver and is an essential constituent of the animal cell membrane
Prostaglandins are a category of hormone-like unsaturated fatty acids that act as messenger molecules between cells. They are derived from arachidonic acid and other similar C20 fatty acids.
Diosgenin is a steroidal compound found in yams that is used to manufacture anti-fertility pills.