The following points highlight the four main groups of lipids. The groups are: 1. Triglycerides 2. Phospholipids (Membrane Lipids) 3. Steroids 4. Wax.
Lipids: Group # 1. Triglycerides:
One important group of stored lipids is triglycerides, a category that includes fats and oils. Triglycerides are composed of a single molecule of glycerol bound to three fatty acids (Fig. 7.1).
Glycerol is a 3 carbon alcohol with 3 OH groups that serve as binding sites.
Fatty acids are long chain hydrocarbon molecules with a carboxyl group (COOH) at one end which is free to bind to one of the OH groups of the glycerol, thus forming a bond called ester bond.
The hydrocarbon portion of a fatty acid can vary in length from 4-24 carbons.
The fats may be saturated or unsaturated. If the carbons in the chain are single bonded, the fat is saturated; if there is at least one C = C double bond in the chain, it is unsaturated. The structure of fatty acids is responsible for the physical nature of fats and oils (liquid fats) which are greasy and insoluble.
In general, solid fats are saturated, and oils are unsaturated.
In most cells for long term, triglycerides are stored in concentrated form as droplets or globules.
Lipids: Group # 2. Phospholipids (Membrane Lipids):
A class of lipids that serve as major structural component of cell membranes is phospholipids. Although phospholipids are similar to triglycerides in containing glycerol and fatty acids, there are some significant differences. Phospholipids contain only two fatty acids attached to the glycerol, while the third glycerol binding site holds a phosphate group (Fig. 7.2).
This phosphate IS in turn bonded to an alcohol. These lipids have both hydrophilic and hydrophobic regions due to a change on the phosphoric acid/alcohol ‘head’ of the molecule and lack of a charge on the long ‘tail’ of the molecule (formed by the fatty acids).
When exposed to an aqueous solution, the charged heads are attracted to the water phase and the non-polar tails are repelled from the water phase.
The way lipids naturally assume single and double layered (bilayer) configuration makes them a valuable component of the primary framework of cell membranes.
When two single layers of polar lipids come together to form a double layer, the outer hydrophilic face of each single layer will orient itself towards, the solution and the hydrophobic portion will become immersed in the core of the bilayer.
The structure of lipid bilayer helps the membrane in functions such as selective permeability and fluid nature.
Lipids: Group # 3. Steroids:
These are complex compounds commonly found in cell membranes and animal hormones. The best known of these is the sterol called cholesterol (fig. 7.3) which reinforces the structure of the cell membrane in animal cells and in an unusual group of cell-wall deficient bacteria called mycoplasmas. The cell membrane of fungi also contains a sterol called ergosterol.
Lipids: Group # 4. Wax:
Waxes are esters formed between a long chain alcohol and saturated fatty acids (fig. 7.4). This material is typically pliable and soft when warm but hard and water resistant when cold (e.g., paraffin).
Fur, feathers, fruits, leaves, human skin and insect exoskeleton are naturally waterproofed with a coating of wax. Bacteria that cause tuberculosis and leprosy produce a wax (wax-D) that contributes to their pathogenicity. The formula of beewax is as follows (fig. 7.3a).
Fats in Plants:
Fats are commonly found in higher plants. They are especially formed in reproductive tissues (e.g.. seeds and fruits) of higher plants where they form reserve food material, such as in cotyledons of sunflower (Helianthus annuus), rape (Brassica napus), peanuts (Arachis hypogaea), almonds (Prunus amygdalus), endosperm of castor beans, coconuts, etc., and mesocarp of avocado fruit.
Some cereals, such as wheat and barley possess rich fat content in cells of aleurone layer. Generally the fats are stored in cell-organelles called spherosomes, which are distributed in cytoplasm of cells.