LipidsEdexcel International A Level Biology: Revision notes
Section 1
What are lipids?
Lipids are a group of biological molecules made of carbon, hydrogen and oxygen. In this topic you need the triglycerides, the fats and oils found in animals and plants, which are used mainly to store energy.
A triglyceride is built from two kinds of smaller molecule: glycerol and fatty acids.
Section 2
Glycerol and fatty acids
Glycerol (C₃H₈O₃) is a small molecule with three hydroxyl (–OH) groups.
A fatty acid has a carboxyl group (–COOH) at one end and a long hydrocarbon chain (usually 14–20 carbon atoms) attached to it. Stearic acid, for example, is C₁₈H₃₆O₂.
All the fatty acids in a triglyceride can be the same or different.
Section 3
Triglyceride synthesis
A triglyceride forms by condensation reactions. Each –OH group of glycerol reacts with the –COOH group of a fatty acid. A molecule of water is removed each time and an ester bond forms.
glycerol + 3 fatty acids → triglyceride + 3 H₂O
So one triglyceride has three ester bonds, and three water molecules are released. The reverse reaction is hydrolysis, catalysed by lipase, which uses three water molecules and releases glycerol and three fatty acids.
Worked example: glycerol C₃H₈O₃ + 3 stearic acid (3 × C₁₈H₃₆O₂) = C₅₇H₁₁₆O₉. Subtract 3 H₂O: the triglyceride is C₅₇H₁₁₀O₆.
Do not call the bond in a triglyceride a glycosidic or peptide bond. It is an ester bond.
Section 4
Saturated and unsaturated fatty acids
A saturated fatty acid has no C=C double bonds in its hydrocarbon chain. Every carbon holds the maximum number of hydrogen atoms, and the chain is straight.
An unsaturated fatty acid has one or more C=C double bonds. If there is one double bond it is monounsaturated; two or more, polyunsaturated. Each double bond makes a kink in the chain, and the fatty acid has two fewer hydrogen atoms for each double bond. Oleic acid, C₁₈H₃₄O₂, has one C=C double bond.
Section 5
Structure and physical state
Straight saturated chains pack closely, so the forces between molecules are stronger and more energy is needed to melt them. Fats rich in saturated fatty acids, such as butter and lard, have higher melting points and are solid at room temperature.
Kinked unsaturated chains cannot pack closely, so the forces between molecules are weaker. Oils rich in unsaturated fatty acids, such as olive oil, have lower melting points and are liquid at room temperature.
Link double bonds to kinks, kinks to poor packing, and poor packing to a lower melting point. Do not say the double bonds are weaker bonds that break more easily.
Must Know
- Triglyceride = one glycerol + three fatty acids
- Condensation forms an ester bond and releases a water molecule for each fatty acid
- Saturated fatty acids have no C=C; unsaturated have one or more
- Double bonds cause kinks, so unsaturated fats have lower melting points
That's the notes covered.
Carry on to the next subtopic.
Exam questions on Lipids
- A food scientist compares butter with olive oil. Butter is solid at room temperature and olive oil is liquid. Both are made mostly of triglycerides.Olive oil contains a high proportion of unsaturated fatty acids. Explain why olive oil is liquid at room temperature whereas butter is solid.2 marks
- Stearic acid and oleic acid are two fatty acids found in animal fats. Both have 18 carbon atoms in each molecule. Stearic acid has no carbon–carbon double bonds. Oleic acid has one carbon–carbon double bond.The formula of stearic acid is C₁₈H₃₆O₂ and the formula of glycerol is C₃H₈O₃. Deduce the molecular formula of the triglyceride formed from one glycerol molecule and three stearic acid molecules. Show your working.2 marks
- Cheese made from cow's milk is rich in triglycerides. These are digested in the small intestine by the enzyme lipase. Stearic acid, C₁₈H₃₆O₂, is one of the fatty acids in the triglycerides and has no C=C double bonds.Oleic acid, another fatty acid in cheese, also has 18 carbon atoms but has one C=C double bond. Deduce the number of hydrogen atoms in a molecule of oleic acid and explain your answer.3 marks
Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).