1Why lipids are hydrophobic

A fatty acid's long tail is a chain of carbon atoms with hydrogens attached. Carbon and hydrogen share electrons almost equally, so the C–H and C–C bonds are nonpolar, and the tail has no partial charges for water to hydrogen-bond to. In water, the water molecules hydrogen-bond to each other and leave lipid molecules clumped together: oil floating on water in a droplet is the everyday picture.

A fat (also called a triglyceride) is built by dehydration synthesis from one molecule of glycerol and three fatty acids. Each fatty acid joins to the glycerol by a covalent bond, releasing one water molecule, so making one fat releases three. Because a fat is three nonpolar tails on a small glycerol, it is almost entirely hydrophobic.

You do not need to draw any lipid in full; the CED leaves the molecular structure of particular lipids out of the exam. You do need to recognise a fatty acid chain and count its double bonds.

2Saturated and unsaturated fatty acids

Figure 1 shows three fatty acids, each 18 carbons long, drawn as zigzag chains (every corner is a carbon atom). The top one has only single bonds between carbons. Every carbon in the tail carries as many hydrogen atoms as it can, so the chain is called saturated (with hydrogen). The middle one has one carbon–carbon double bond and the bottom one has two: both are unsaturated. Look at what each double bond does to the shape: the chain bends at it, a kink.

Why the kink matters: straight, saturated chains can lie close together, touching along their length, so they attract each other strongly and pack into a solid. Kinked, unsaturated chains cannot pack closely; they hold each other apart, attract each other less, and stay liquid at lower temperatures. One double bond makes a monounsaturated fatty acid; two or more, a polyunsaturated one. The more double bonds, the more kinks, and the more liquid the lipid.

That is why animal fats such as butter and lard, which are rich in saturated fatty acids, are solid at room temperature, while plant oils such as olive or sunflower oil, rich in unsaturated fatty acids, are liquid.

More double bonds mean more kinks, looser packing, and a lipid that is more liquid at room temperature.

3Predicting from data: a worked example

The suggested skill for this topic is to predict the effect of a change. Here is an invented data set for four fats, each with fatty acids of the same length but a different mean number of carbon–carbon double bonds per fatty acid.

Read the pattern first: as the number of double bonds rises, the melting point falls. Then use it. Room temperature is about 20–25 °C, so fat A (melting point 62 °C) is solid at room temperature, and B, C and D (all melting below 20 °C) are liquid. The biggest drop, 44 °C, comes with the first double bond: the first kink matters most.

A prediction question might then ask about a fat with a mean of 1.5 double bonds. It should melt between B and C, between 18 and −4 °C, so it would be liquid at room temperature. Write it as claim, evidence and reasoning: the fat will be liquid (claim); fats with one and two double bonds per fatty acid melted at 18 and −4 °C (evidence); its double bonds kink its chains so they cannot pack closely, so less energy is needed to separate them (reasoning).

4Fats: energy store and insulation

Fats are the long-term energy store of animals and of many plant seeds. A gram of fat releases about twice as much energy in respiration as a gram of carbohydrate, and because fat is hydrophobic it is stored without water around it, so it packs a lot of energy into little mass. That suits animals, which carry their stores around. Fat also supports cell function more broadly: fatty acids released from stored fat are an important fuel for many cells.

In mammals, layers of fat under the skin also give insulation. Fat conducts heat poorly, so a thick layer, like a seal's blubber, slows heat loss to cold surroundings and helps keep body temperature steady.

5Steroids and cholesterol

Steroids are lipids built on a skeleton of four linked carbon rings, rather than on fatty acid chains. They are still mostly nonpolar. Many steroids are hormones, chemical signals carried in the blood: sex hormones such as testosterone and oestrogen control growth and development; others, such as cortisol, help regulate energy metabolism and keep internal conditions stable (homeostasis). Because they are nonpolar, steroid hormones can pass through the lipid membranes of the cells they act on, which you will use in Unit 4.

Cholesterol is a steroid with a structural job. Its molecules sit among the phospholipids of animal cell membranes and give the membrane structural stability, keeping it from becoming too fluid or too rigid as temperature changes. Animal cells also use cholesterol as the starting material for steroid hormones.

6Phospholipids: the stuff of membranes

A phospholipid looks like a fat with one fatty acid replaced by a phosphate-containing group. The result has two different ends:

• a hydrophilic head, the charged phosphate group, which is attracted to water;
• two hydrophobic tails, fatty acid chains, which are not.

A molecule with a water-loving part and a water-avoiding part is called amphipathic. In water, phospholipids arrange themselves with no help: heads facing the water on both sides, tails tucked inside away from it. This double sheet is the phospholipid bilayer, the basis of the plasma membrane and of the membranes around organelles. Unit 2 builds on this: the bilayer's hydrophobic core is why ions and large polar molecules cannot cross a membrane on their own.

The tails decide how fluid a membrane is, by the same logic as section 3. More unsaturated (kinked) tails pack loosely and keep a membrane fluid in the cold; organisms living in cold places tend to have a higher proportion of unsaturated fatty acids in their membranes than relatives in warm places.