Elements: C, H, O with very little O.

Triglycerides have the highest energy content per gram. The fat molecule is composed of a glycerol molecule to which three fatty acid molecules are attached.

The OH groups of the glycerol and the C-OH and C=O groups of the fatty acids are hydrophilic. The long C-H tail of the fatty acid is hydrophobic. Triglycerides are strongly hydrophobic.

The glycerol and fatty acid molecules are joined by a condensation reaction between the two adjacent OH groups of the glycerol and the fatty acid giving an ester bond.

 

The typical triglyceride may form an oil if there are many double bonds in the fatty acid chains. In this case there is very little hydrogen on the molecule and it is said to be unsaturated. As the number of double bonds in the carbon tail decreases, the fatty acid tails get longer and the molecules tend to intertwine and clump together to form more solid materials such as lard. If the tails are very long and fully saturated the material formed is wax. The typical triglyceride may form an oil if there are many double bonds in the fatty acid chains. In this case there is very little hydrogen on the molecule and it is said to be unsaturated. As the number of double bonds in the carbon tail decreases, the fatty acid tails get longer and the molecules tend to intertwine and clump together to form more solid materials such as lard. If the tails are very long and fully saturated the material formed is wax.

It is possible to replace a fatty acid with a phosphate group to create a phospholipid in which the phosphate head is strongly hydrophilic. The fatty acid tails are strongly hydrophobic, therefore when placed in water the phosphate heads are pulled into the water and the tails repelled. The molecules stand on their heads next to each other forming a monolayer.

 

When there is water on both sides of the group of molecules, as is the case with cells, the molecules are forced to form a bilayer with their heads in the water and their tails towards the centre. This is the basis for the fluid mosaic structure of the animal and plant cell membrane.

One group of substances included in the lipid group but having no chemical similarity save the fact that they are fat soluble are a group called the steroids that include cholesterol. This has a complex ring structure and can be used as the basis for a wide range of hormones within organisms.

There are two simple tests for fats:

1) The smear test: The food is smeared over some greaseproof paper and the paper is left to dry. If a translucent spot is present on the paper after drying then fat is present.

2) The emulsion test: The food is crushed and mixed with ethanol. The mixture is thoroughly agitated. The alcohol is separated from the bits of food material and water is added to the clear alcohol. A milky white emulsion indicates that fats are present.