Lipids

Fats and oils belong to a group of molecules called lipids. Lipids, do not dissolve in water, but are soluble in non-polar solvents. 

The molecules that form a lipid are glycerol and fatty acids. Chemists call glycerol 'propane 1,2,3-triol'- C3H8O3.

The zigzag represents a long hydrocarbon chain

The molecules that form a fat are glycerol and fatty acids. A condensation reaction occurs producing a fat and water. 

• a monoglyceride is formed 
• a diglyceride is formed 
• Finally, a triglyceride is formed

Most natural fats are triglycerides. Three fatty acids have joined with glycerol. 

Fats are solid at room temperature, oils are liquid at room temperature. The reason why fats have a higher melting point is that they are more saturated. Each carbon atom is joined to other carbons by a single bond.

When two hydrogen atoms from adjacent carbon atons are removed, a double bond is formed between the two carbon atoms to form an unsaturated hydrocarbon chain. 

This hydrocarbon is unsaturated

This fatty acid is monounsaturated, because it has a single double bond. Unsaturated trigycerides no longer pack together so neatly, so their melting point is lower.

Saturated fats are mainly of animals origin, for example stearic acid, which is present in fatty tissues. Ch3 (CH2)16 COOH

Unsaturated fats are found mainly in plants and have a lower melting point than saturated fats. Oleic acid is a fatty acid with one double bond in the hydrocarbon chain. CH3(CH2)7CHCH(CH2)7COOH

Another unsaturated fat, linoleic acid, is a fatty acid with two double bonds in the hydrocarbon chain

CH3(CH2)4CHCHCH2CHCH(CH2)7COOH

This is the general structure of a phosolipid, where X is a variable group attached to the phosphate group. 

A phospholipid consists of a head and a tail. The phosphate 'head' is polar and hydrophilic. The 'tail' is non-polar and formed from hydrocarbon chains. Phospholipids will arrange themselves in water so that the polar heads are in contact with the water, and the tails are away from the water, in contact with each other. 

Cholesterol is not a trigylceride it is a different type of lipid called a steroid. Cholesterol is obtained in the diet but can also be synthesised in body cells. Cholesterol is an important component of cell membranes. The steroid sex hormones (e.g. progesterone and testesterone) and some growth hormones are produced from cholesterol. Bile salts, involved in lipid digestion and assimilation, are formed from cholesterol, which is essential for metabolism in a healthy body. Like all lipids, cholesterol is not soluble in water. In order to be transported in the bloodstream, insoluble cholesterol is combined with proteins to form lipoproteins.