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Lipids: triglycerides, phospholipids and the ester bond questions
Glycerol and fatty acids, and the ester bond made by condensation. Saturated against unsaturated fatty acids and what a double bond does to packing. Triglycerides as an energy store, phospholipids and why the head is hydrophilic, and the emulsion test for a lipid.
9 original questions · 25 marks · the lipids: triglycerides, phospholipids and the ester bond notes · Biological molecules, water and inorganic ions
Every question here is written for this library rather than taken from a past paper. Write your answer out before opening a mark scheme: the schemes award marks point by point, and the points are much easier to see when you have something of your own to compare against.
Describe how you would test a sample of food for the presence of a lipid, including a suitable control.
Mark scheme
- B1 shake the sample with absolute ethanol so that any lipid dissolves
- B1 pour the ethanol into an equal volume of water in a clean tube
- B1 a white, cloudy emulsion shows that a lipid is present, because the lipid comes out of solution as droplets that scatter light
- B1 as a control, repeat the procedure using distilled water in place of the sample; the mixture should remain clear
Describe how a triglyceride is formed from its components.
Mark scheme
- B1 one molecule of glycerol reacts with three fatty acid molecules
- B1 a hydroxyl group of the glycerol reacts with the carboxyl group of each fatty acid in a condensation reaction
- B1 three ester bonds are formed and three molecules of water are released
Sunflower oil is liquid at room temperature and beef fat is solid at the same temperature. Explain this difference in terms of the fatty acids each contains.
Mark scheme
- B1 sunflower oil contains a high proportion of unsaturated fatty acids, which have one or more carbon to carbon double bonds, whereas beef fat is largely saturated with none
- B1 a double bond puts a permanent bend in the hydrocarbon tail, so the molecules cannot pack closely together
- B1 fewer attractions form between neighbouring molecules, so less energy is needed to separate them and the melting point is lower
Before migrating, a bird lays down a store of triglyceride rather than an equivalent store of glycogen. Explain the advantage of storing triglyceride.
Compare the behaviour of a triglyceride and a phospholipid when each is added to water.
A membrane protein is removed from a cell surface membrane and placed in pure water, where it does not dissolve. Suggest what this indicates about the amino acids on the outside of that protein, and suggest where in the membrane the protein sits.
Name the two kinds of molecule from which a triglyceride is formed, and name the bond that joins them.
State one structural difference between a phospholipid and a triglyceride, and state one property of a phospholipid that results from it.
A cell hydrolyses 180 molecules of triglyceride completely. Calculate the number of molecules of water used.
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