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Lipids unlike carbohydrates, proteins and nucleic acids, do not form polymers. This means they have low melting points, and in living organisms are often in liquid form. They are composed of the elements carbon, hydrogen and oxygen. Lipids are mainly used in living organisms for storage, insulation and protection. They can also form a part of more complex molecules with a wider range of functions. The test for the presence of lipids is the emulsion test.

The term lipids covers both fats and oils. Chemically these molecules are similar, the difference is that fats are solid at room temperature, whereas oils are liquid at room temperature. All lipids are composed of glycerol and fatty acid subunits.

Figure 1

As can be seen glycerol is a three carbon compound with three hydroxyl groups

Figure 2
Fatty acid

Figure 2 introduces the R group. R is used in chemical diagrams to indicate a hydrocarbon chain of variable length. Typically fatty acids will have between 16-18 carbons in their R group. The carboxylic group (COOH) is a weak organic acid, which gives the molecule it's acidic properties. Fatty acids are polar molecules, the COOH group is attracted to water (hydrophilic) and the hydrocarbon chain is repelled by water (hydrophobic).

The most common form of lipid molecule is the triglyceride. Triglycerides are composed of one glycerol molecule with three attached fatty acid molecules. Each fatty acid joins by a condensation reaction which forms an ester bond and releases a molecule of water. The process is shown in figure 3 below.

Figure 3

If only one fatty acid is joined, a monoglyceride is formed (shown below in figure 4). If two fatty acids are joined a diglyceride is formed.

Figure 4

These diagrams are not a good representation of the shape of the lipid molecule because of the R abbreviation. The figure 5 shows the shape more clearly.

Figure 5

If there are no C=C double bonds in the hydrocarbon chain of a fatty acid, then the fatty acid is saturated (i.e. saturated with hydrogen). These fatty acids form straight chains which pack together fairly well, so make lipids with a reasonably high melting point.

If there are C=C double bonds in the hydrocarbon chain of a fatty acid, then the fatty acid is unsaturated (i.e. unsaturated with hydrogen). These fatty acids form bent chains which do not pack together as well, so make lipids with low melting points. Fatty acids with more than one double bond are called poly-unsaturated fatty acids.

This is explained by in figure 6 below.

Figure 6

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