LipidsEdexcel A-Level Biology B: Revision notes
Section 1
Triglycerides: structure and synthesis
A triglyceride is made of one glycerol molecule and three fatty acids. Each fatty acid has a carboxyl group (–COOH) and a long hydrocarbon tail.
Each fatty acid joins glycerol in a condensation reaction between the OH of glycerol and the COOH of the fatty acid. A molecule of water is removed and an ester bond forms. Three fatty acids means three water molecules released and three ester bonds formed.
glycerol + 3 fatty acids → triglyceride + 3H₂O
A triglyceride is not a polymer. It is made from three fatty acids and glycerol, which are different molecules, not repeated monomers.
Section 2
Saturated and unsaturated lipids
Saturated fatty acids have no C=C double bonds in the hydrocarbon tail: every carbon holds the maximum number of hydrogen atoms. The chains are straight and pack closely, so saturated fats (e.g. butter) are usually solid at room temperature.
Unsaturated fatty acids have at least one C=C double bond, which makes the chain bend (kink). The molecules cannot pack as closely, so there are fewer intermolecular forces and unsaturated lipids (e.g. olive oil) are usually liquid at room temperature. Monounsaturated means one double bond; polyunsaturated means more than one.
Section 3
Triglyceride structure and function
The long hydrocarbon tails give triglycerides several roles:
- Energy storage: a high proportion of C–H bonds, so more energy is released per gram on oxidation than from carbohydrate. They are also insoluble, so they do not affect water potential.
- Waterproofing: the non-polar, hydrophobic tails repel water, e.g. oils on bird feathers and waxy plant cuticles.
- Insulation: fat is a poor conductor of heat, so a layer under the skin (e.g. blubber) reduces heat loss.
Fat is also less dense than water, so it helps buoyancy in aquatic animals.
For 'energy storage' always give the reason: many C–H bonds, so more energy per gram than carbohydrate.
Section 4
Phospholipid structure
A phospholipid is like a triglyceride in which one fatty acid is replaced by a phosphate group. It has a glycerol backbone, two fatty acid tails and a phosphate head.
The phosphate head is hydrophilic (polar, attracted to water). The fatty acid tails are hydrophobic (non-polar, repelled by water). This makes a phospholipid amphipathic.
Section 5
Phospholipids in cell membranes
In water, phospholipids arrange into a bilayer: heads face outwards to the water on both sides and tails face inwards, shielded from water.
This suits membranes because:
- the hydrophobic core is a barrier to ions and polar molecules, so it separates the inside of the cell from the outside
- small non-polar molecules (oxygen, carbon dioxide) can pass through the core
- phospholipids move sideways in each layer, so the membrane is fluid and flexible and can fuse with other membranes
Worked example: a membrane contains 1 × 10⁶ phospholipid molecules per cell in one layer. A complete bilayer therefore has 2 × 10⁶ molecules.
Do not say the tails 'attract' each other as the reason for the bilayer. They are shielded because they are hydrophobic and repelled by water.
That's the notes covered.
Carry on to the next subtopic.
Exam questions on Lipids
- A food technologist is comparing two cooking fats. Butter is solid at room temperature and contains mostly saturated fatty acids, whereas olive oil is liquid at room temperature and contains mostly unsaturated fatty acids.Explain why olive oil is liquid at room temperature but butter is solid.2 marks
- A camel stores fat in its hump, which it can use as an energy reserve in the desert. A seal living in cold Arctic water has a thick layer of fat, called blubber, under its skin.Explain how the blubber helps the seal to survive in cold water.2 marks
- A student uses a model kit to build a triglyceride. The model contains one glycerol molecule and three fatty acid molecules. Two of the fatty acids each contain one C=C double bond in the hydrocarbon chain and the third has no C=C double bonds.Describe how the glycerol and fatty acids in the model are joined to form a triglyceride.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).