Lipids can be broken up into triglycerides (fats and oil) and waxes, steroids, terpenes and phospholipid.
Properties of Lipid
- insoluble in water
- soluble in non-polar solvent such as benzene
- they contain carbon, hydrogen and oxygen but less oxygen but more hydrogen than carbohydrates
- they generally give twice as much energy when they respire than carbohydrates
- less dense than water
The most common types are triglycerides usually known as fats and oils. Fats are solid at room temperature and oils are liquid. Triglycerides are made by the combination of three fatty acids with one glycerol. The higher the proportion of unsaturated fatty acids the more likely they will be liquid at room temperature. Each of the fatty acids joins to glycerol by condensation reaction.
Triglycerides are n on-polar. This means there is no uneven distribution of charge within the molecule so they cannot form hydrogen bonds with water and therefore do not dissolve in water. They are soluble in organic solvents such as ether and chloroform. They are hydrophobic and are less dense than water.
Structure of Glycerol
Fatty acids contain an acidic group –COOH. The general formula is R.COOH where R is hydrogen or a group such as –CH2, C2H5 increasing by CH2 for each subsequent member. The long chain of carbon and hydrogen atoms forms a hydrocarbon tail. The tail is hydrophobic and non-polar making the triglyceride insoluble in water.
Fatty acid sometimes contains one or more double bonds. If it does, the fatty acid, as well as the lipid containing them are said to be unsaturated. Fatty acids and lipids lacking double bonds are described as saturated. Double bonds make fatty acids and lipids melt more easily, so most oils are unsaturated.
Formation of a Triglyceride
The Role of Triglycerides
- Energy stores – a given mass of lipid will yield more energy on oxidation than an equal mass of carbohydrate. This is because lipids have a higher proportion of hydrogen atoms and an almost insignificant proportion of oxygen as compared with carbohydrates
- Animal store extra fat when hibernating and fat is also found below the dermis of the skin of vertebrates where it serves as an insulator. It is extensive in mammals in cold climates, particularly in the form of blubber in aquatic mammals such as whales and also contributes to its buoyancy.
- It is a metabolic source of water. When oxidized in respiration, it forms carbon dioxide and water. This is important in some desert animals such as kangaroo rat, which never drinks water and survives on metabolic water from its fat intake.
Waxes are lipid molecules whose alcohol is not glycerol and they usually have 1 hydrogen chain e.g. chitin
Steroids are ring compounds e.g. Vitamin D, sex hormones such as testosterone,
Terpenes are similar to steroids but smaller e.g vitamin A, C, K
In a phospholipid, one of the three fatty molecules is replaced by a phosphate group, which carries an electrical charge and can therefore dissolve in water. the phosphate group is hydrophillic. These have a polar phosphate head and two non-fatty acid tails. Polar groups or molecules are charged and have an affinity for water (hydrophillic). Non- polar groups or fatty acids do not mix with water (hydrophobic). The phospholipids arrange themselves as a bilayer with the hydrophillic head facing the aqueous external and the internal environment and the hydrophobic tails shielded forming a micelle. Some of the phospholipod tails are saturated and some are unsaturated. The more unsaturated they are , the more fluid the membrane. This is because the unsaturated tails are bent or have kinks which prevent close packing so they fit together loosely. The shorter the length of the fatty acid tails, the more fluid the membrane
Membranes phospholipids move within the membrane by oscillation of the fatty acid tails (oscillation being greatest away from the head), flip-flop (less often) and lateral diffusion.
These are phospholipids with short branching carbohydrate chains attached to the external surface. They also polar heads and non-polar tail.
is completely and thus found in the hydrophobic region of the phospholipid bilayer. Cholesterol regulates membrane fluidity preventing it from becoming too fluid or to rigid. Fluidity is advantageous as it allows the membrane to seal itself. Cholesterol provides mechanical stability as without membranes quickly break and cells burst open. Cholesterol also reduces uncontrolled leakage, by diffusion, of polar molecules, ions and water through the membrane ensuring that they must pass through special channels where they are controlled.