Lipids are a chemically diverse group that share the same characteristic of being sparingly soluble in water and highly soluble in non-polar organic solvents like acetone. The group has two major roles; in structure, particularly in the membranous organelles and in energy storage as lipids have a higher calorific value than carbohydrates.
The lipid group includes:
Fatty acids
Most fatty acids in cells are linked to other molecules. They are either saturated or unsaturated fatty acids. In both cases, the hydrocarbon chain is unbranched.
Saturated fatty acids have long hydrocarbon chains terminating in a carboxyl group. The general formula of a saturated fatty acid is CH3-(CH2)n-COOH. In naturally occurring fatty acids, the n referred to in the general formula is normally an even number and is somewhere between 2 and 22.
In unsaturated fatty acids, at least two and no more than six carbon atoms in the hydrocarbon chain are linked together by double bonds.
Triglycerides
A triglyceride consists of a glycerol and three fatty acids. It is the major type of lipid used for energy storage and it is found in droplets within the cytoplasm. It is non-polar and relatively insoluble. Triglycerides are divided into two groups dependent on their state at 20 oC; if they are solid at this temperature, they are called fats and if they are liquid, they are called oils. The higher the proportion of unsaturated fatty acids in the triglyceride the lower its melting point. Triglycerides are less dense than water, therefore they float.
Phospholipids
Phospholipids are lipids containing a phosphate group, they are found in cellular membranes.
Glycolipids
Glycolipids are lipids with short chains of sugar attached to them. They are commonly found in cellular membranes. They are involved in the immune system and cell-to-cell recognition.
Waxes
Waxes are esters of fatty acids combined with long chain alcohols. Waxes are mainly used as a waterproofing material by plants and animals, e.g. on leaves of plants and on the feathers of animals.
Lipoproteins
Lipoproteins are combinations of lipids and proteins, they are found in membranes. When lipids are transported through the blood and lymphatic systems, they are in the form of lipoproteins.
Steroids
Steroids do not contain fatty acids. Steroids have widely different physiological properties. Hormones such as oestrogen are steroids, as are vitamins such as D2. Steroids are constituents of the plasma membrane, influencing the membrane’s structure and its role in transport and permeability. The most well known plasma membrane steroid is cholesterol.
Sources
Biological Science 1&2 by Green, Stout and Taylor, 2nd ed. Cambridge
Cell and Molecular Biology by Sheeler and Bianchi, 3rd ed. Wiley
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