Functions of lipids
At room temperature a solid lipid is called a fat, and a liquid lipid is called an oil. The functions of lipids are:
- A source of energy - lipids can be respired to release energy to generate ATP. Respiration of lipids first requires hydrolysis and then can be broken down into water and carbon dioxide. The respiration of one gram of lipid gives out about twice as much energy as 1g of carbohydrate. It also gives out more water.
- Energy storage - stored in adipose cells in 'fat stores'
- Biological membranes
- Insulation e.g. blubber in whales and electrical insulation around long nerve cells
- Protection e.g. cuticle
- Steroid-based hormones
Glycerol and fatty acids
The glycerol molecule is always the same. It consists of a 3 carbon molecule with three OH groups.
Fatty acids can differ. They always have an acid group at one end. The rest of the molecule is a hydrocarbon chain which can be 2-20 carbons long. There are essential fatty acids that animals can't make.
A saturated fatty acid is when all possible bonds are made with hydrogen. Unsaturated fatty acids have C=C bonds. One double bond gives a mono-unsaturated fatty acid. Two or more C=C bonds makes a polyunsaturated fatty acid.
Introducing carbon double bonds changes the shape of the hydrocarbon chain by making the molecules in a lipid push apart and so are more fluid.
Animals have more saturated fatty acids which are solid and therefore fats e.g. lard. Plants have more unsaturated fatty acids which are oils e.g. olive oil.
A triglyceride consists of one glycerol molecule bonded to three fatty acid molecules. A condensation reaction happens between the acid group of the fatty acids and one of OH (hydroxyl) groups of the glycerol. A water molecule is eliminated.
The new bond is called an ester bond. They are insoluble in water so they are called hydrophobic. This is because the charge is evenly distributed so hydrogen bonds cannot form.
Hydrolysis of a triglyceride makes one glycerol molecule and three fatty acids.
The main role of a triglyceride is as a compact energy store as it doesn't affect the water potential of a cell. In animals it is stored as fat so has insulation and protection properties.
In phospholipids, 2 fatty acids are bonded by condensation reactions to the glycerol molecule. The third OH group is covalently bonded to a phosphate group by a condensation reaction as well.
The phosphate head is hydrophilic while the rest of the molecule is hydrophobic and insoluble. This is the ideal basis for cell membranes. Phosphate groups may have carbohydrate parts attached to them - glycolipids.
The fatty acids that make up the phospholipid can be saturated or unsaturated which means organisms can control the fluidity of the membrane. Organisms in colder climates are more likely to have unsaturated fatty acids in the cell membranes so that they remain fluid despite low temperatures.
Cholesterol is a type of lipid. It is a small molecule made from four carbon-based ring structures joined together. It is hydrophobic and so can sit between the hydrocarbon chain of the fatty acids in a phospholipid membrane, to give the membrane stability.
The steroid hormones testosterone, oestogen and vitamin D are made from cholesterol. They are lipids and so can pass though the phospholipid bilayer and reach their target receptor. They can also pass through the nuclear envelope.
Excess cholesterol is a problem as:
- In bile, it forms lumps called gallstones
- In blood, it can be deposited in blood vessels causing atherosclerosis
A genetic condition known as familial hypercholesterolaemia causes excess manufacture and secretion of cholesterol due to lack of a receptor. This may cause a myocardial infarction (heart attack) and strokes.