Lipids — AQA A-Level Biology

Lipids contain carbon, hydrogen and oxygen, but with proportionally less oxygen than carbohydrates. They are insoluble in water but soluble in organic solvents like ethanol. The specification names two groups: triglycerides and phospholipids. Triglycerides consist of one glycerol joined to three fatty acids. They are non-polar, hydrophobic molecules, making them ideal energy storage lipids in adipose tissue. Phospholipids consist of one glycerol joined to two fatty acids and a phosphate-containing group. This structure makes them amphipathic: they have a hydrophilic phosphate head and hydrophobic fatty acid tails. This property is essential for forming the bilayer of cell membranes.

A triglyceride is made from one molecule of glycerol and three molecules of fatty acid. Glycerol carries three hydroxyl groups; each fatty acid carries a carboxyl group, written RCOOH. Each hydroxyl group reacts with one carboxyl group in a condensation reaction, so three condensations occur, three ester bonds form and three molecules of water are released for every triglyceride assembled. The three fatty acids need not be the same, and it is their R groups - the hydrocarbon tails - that determine the properties of the finished molecule. Because all three tails are hydrocarbon, the completed triglyceride is non-polar and insoluble in water, so it can be stored as large droplets in a cell without lowering the water potential or drawing water in by osmosis.

An ester bond forms between the carboxyl group of a fatty acid, written -COOH, and one hydroxyl group of glycerol. The -OH from the carboxyl group and the -H from the hydroxyl group leave together as a molecule of water, and the remaining oxygen bridges the two molecules, giving the -CO-O- linkage. It is a condensation reaction, so exactly one water molecule leaves for each ester bond formed, and a triglyceride with three of them releases three. Naming matters here: a glycosidic bond joins sugars, a peptide bond joins amino acids, and an ester bond joins glycerol to a fatty acid, so the wrong name loses the mark even when the rest of the description is right. The R group of the fatty acid takes no part in the bond at all; it stays as a free hydrocarbon tail, which is why it is the R group and not the ester bond that decides whether a lipid behaves as a fat or an oil.

The R group of a fatty acid is a hydrocarbon chain, and the question is always whether it contains any carbon-carbon double bonds. A saturated fatty acid has none: every carbon carries the maximum possible number of hydrogen atoms, so the chain is straight. An unsaturated fatty acid has one or more C=C double bonds - monounsaturated with one, polyunsaturated with more than one - and each double bond puts a kink in the chain. That geometry is what matters. Straight saturated tails pack closely together, so more intermolecular forces form between neighbouring molecules and more energy is needed to separate them: the melting point is higher and the lipid is a solid fat at room temperature, typical of animals. Kinked unsaturated tails cannot pack as closely, the forces between molecules are weaker, the melting point is lower and the lipid is a liquid oil, typical of plants and fish.

A phospholipid is a triglyceride in which one of the three fatty acids has been replaced by a phosphate-containing group. That single substitution splits the molecule into two halves that behave in opposite ways. The phosphate head is charged and polar, so it is hydrophilic and interacts with water; the two remaining fatty acid tails are non-polar, so they are hydrophobic and are repelled by water. A molecule with both properties is described as amphipathic. In water, phospholipids therefore arrange themselves automatically into a bilayer, with the hydrophilic heads facing outwards into the aqueous cytoplasm and tissue fluid on each side and the hydrophobic tails pointing inwards, away from the water, creating a hydrophobic core.

Both molecules are built on glycerol, so compare them by what is attached. A triglyceride has three hydrocarbon tails and no polar group, so the whole molecule is non-polar, hydrophobic and insoluble in water. That makes it an excellent energy store: the high proportion of carbon-hydrogen bonds releases about twice as much energy per gram as carbohydrate when oxidised, it can be packed into droplets without lowering the water potential of the cell, and its oxidation also releases metabolic water. Stored under the skin, it insulates and protects organs as well. A phospholipid has only two tails and a hydrophilic phosphate-containing head, so it is amphipathic rather than simply hydrophobic. In water, phospholipids arrange themselves into a bilayer with heads facing the aqueous environment and tails shielded inside, forming the basis of cell-surface and organelle membranes.

The emulsion test uses the fact that lipids dissolve in ethanol but not in water. Take a completely dry test tube, add the sample, add about two centimetres cubed of absolute ethanol and shake thoroughly so that any lipid present dissolves. Then pour the ethanol solution into an equal volume of distilled water in a second tube. If lipid is present it comes out of solution as it meets the water, forming millions of tiny droplets suspended through the liquid; these scatter light, so the mixture turns a cloudy, milky-white emulsion. If no lipid is present the mixture stays clear. The order matters: ethanol first, water second. The obvious control is to repeat the whole procedure with distilled water in place of the sample and show that it stays clear.

This is the applied version of the lipid content, and its two halves are tested in different ways. To recognise fatty acids from a diagram, scan the hydrocarbon chain for a carbon-carbon double bond: no double bond means saturated, and the chain is drawn straight; one or more means unsaturated, and the chain is usually drawn with a kink at the double bond. Do not count carbon or hydrogen atoms unless asked, because the double bond is the feature being tested. To explain properties, run each answer from structure to consequence. Three hydrophobic tails and no polar group make a triglyceride non-polar and insoluble, so it is stored in droplets without affecting water potential and yields about twice the energy per gram of carbohydrate when respired. A phospholipid has two tails and a hydrophilic phosphate-containing head, so it is amphipathic and forms a bilayer in water.