The two building blocks of a lipid

1. A lipid is built from two different kinds of subunit, which makes it unlike a carbohydrate or a protein.
2. Glycerol is the small three-carbon molecule that acts as the backbone, and there is exactly one of it in every lipid molecule.
3. A fatty acid is a long chain of carbon and hydrogen atoms with an acid group at one end, and three of them are attached to each glycerol.
4. The ratio of subunits is therefore $1:3$, one glycerol to three fatty acids, and a lipid built this way is called a triglyceride.
5. Because there are two kinds of subunit rather than one repeating monomer, a lipid is not a polymer in the way that starch and a protein are.
6. A lipid molecule is also a fixed size, since it always contains four subunits, whereas a polymer chain can be any length.
7. Lipids contain carbon, hydrogen and oxygen like carbohydrates, but the long fatty acid chains hold a far smaller proportion of oxygen.
8. Less oxygen means more carbon to hydrogen bonds per molecule, and that is exactly why a lipid releases so much more energy per gram than a carbohydrate.
9. The three fatty acids attached to one glycerol do not have to be identical, which is one reason different lipids have different properties.

Building and breaking lipids

1. Each fatty acid is attached to the glycerol by a condensation reaction, and each of those reactions releases one molecule of water.
2. Since three fatty acids are attached to each glycerol, building one lipid molecule releases three molecules of water.
3. The reactions are controlled by enzymes and need energy from ATP, in the same way that building any large molecule does.
4. Hydrolysis reverses the process, adding one molecule of water at each of the three bonds to release the glycerol and the three fatty acids.
5. Lipase is the enzyme that hydrolyses lipids, and it does this in the small intestine so that the products are small enough to be absorbed.
6. Once absorbed, the fatty acids and glycerol are rejoined to build the particular lipids that body needs, including the lipids of its cell membranes.
7. A cell also hydrolyses its own stored lipid whenever it needs the energy, releasing fatty acids that can be respired.
8. This is why a body that has used up its stored carbohydrate begins to lose stored fat instead.

What cells use lipids for

1. A lipid is a concentrated energy store, releasing roughly twice as much energy per gram as a carbohydrate does.
2. That matters for an animal that has to carry its store around, because the same amount of energy can be stored in half the mass.
3. Lipids are also insoluble in water, so a large store can sit in a cell without dragging water in by osmosis.
4. A layer of fat under the skin gives thermal insulation, because fat conducts heat poorly and so slows the rate at which heat is lost.
5. Fat packed around organs such as the kidneys and the heart acts as protection, cushioning them against knocks.
6. Lipids form part of every cell membrane, which is why even a cell that stores no fat at all still has to make them.
7. Plant seeds store lipid rather than starch when they need a compact energy reserve, which is why sunflower and rapeseed are pressed for oil.

Saturated and unsaturated fatty acids

1. The three fatty acids attached to the glycerol can be saturated or unsaturated, and this changes the properties of the whole lipid.
2. A saturated fatty acid has no carbon to carbon double bonds in its chain, so every carbon holds as many hydrogen atoms as it can.
3. With no double bonds the chain stays straight, so neighbouring molecules pack closely together and attract one another strongly.
4. Close packing means more energy is needed to separate the molecules, so lipids rich in saturated fatty acids are usually solid fats at room temperature.
5. An unsaturated fatty acid has one or more carbon to carbon double bonds, written as $C=C$.
6. Each double bond puts a kink in the chain, which stops the molecules packing neatly and weakens the attraction between them.
7. Loose packing means less energy is needed to separate them, so lipids rich in unsaturated fatty acids are usually liquid oils at room temperature.
8. Saturated fatty acids come mainly from animal sources such as butter, cheese, lard and fatty meat.
9. Unsaturated fatty acids come mainly from plant and fish sources such as olive oil, sunflower oil, nuts and oily fish.
10. Both kinds are still lipids, so both release about the same amount of energy per gram when they are respired.