Questions on lipids: triglycerides, phospholipids and ester bonds

Question 1

Describe how you would test a sample of food for the presence of a lipid, including a suitable control.

Accepted Answer

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

Question 2

A triglyceride has a relative molecular mass of 890. Given that glycerol has a relative molecular mass of 92 and that three molecules of water, each of relative molecular mass 18, are released when it forms, calculate the total relative molecular mass of the three fatty acids used to build it.

Accepted Answer

M1 total mass of reactants, glycerol plus three fatty acids, equals the mass of the triglyceride plus the mass of water released, since condensation removes mass as water M1 3 × 18 = 54, so total mass of reactants = 890 + 54 = 944 M1 mass of the three fatty acids = total mass of reactants minus the mass of glycerol, 944 − 92 A1 852, so the mean relative molecular mass of one fatty acid is about 284

Question 3

Describe how a triglyceride is formed from its components.

Accepted Answer

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

Question 4

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.

Accepted Answer

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

Question 5

Before migrating, a bird lays down a store of triglyceride rather than an equivalent store of glycogen. Explain the advantage of storing triglyceride.

Accepted Answer

B1 a triglyceride releases about twice as much energy per gram as a carbohydrate when respired, because its fatty acid tails are hydrocarbon and contain little oxygen B1 so the same energy is carried for less mass, which reduces the energy needed for flight B1 triglyceride is insoluble, so it has no effect on the water potential of the cell storing it, and respiring it releases water that the bird can use

Question 6

Compare the behaviour of a triglyceride and a phospholipid when each is added to water.

Accepted Answer

B1 both are insoluble in water, because their fatty acid tails are non-polar and hydrophobic B1 a triglyceride is hydrophobic at every point, so it forms droplets, whereas a phospholipid has a hydrophilic phosphate head that interacts with water B1 so phospholipids arrange themselves with their heads in the water and their tails away from it, forming a bilayer, which triglycerides do not

Question 7

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.

Accepted Answer

B1 the side chains on the outside of the protein are largely non-polar, so they are hydrophobic and are not attracted to water B1 such a surface is compatible with the hydrocarbon tails of the phospholipids rather than with water B1 so the protein sits within the hydrophobic core of the bilayer rather than on the surface of the membrane

Question 8

A triglyceride is a large molecule, but a student is marked wrong for calling it a polymer. Explain why a triglyceride is not a polymer, even though it is built by condensation reactions in the same way a polysaccharide is.

Accepted Answer

B1 a polymer is a large molecule made of many similar, repeating monomer units joined together, such as glucose units in a polysaccharide B1 a triglyceride is built from one glycerol molecule and three fatty acid molecules, which are not the same unit repeated, so there is no single repeating monomer B1 both being assembled by condensation, releasing water, describes the type of reaction rather than the structure of the product, so it does not by itself make a triglyceride a polymer

Question 9

Explain what is meant by describing a fatty acid as monounsaturated, and explain how this differs from a polyunsaturated fatty acid.

Accepted Answer

B1 a monounsaturated fatty acid has exactly one carbon to carbon double bond in its hydrocarbon tail B1 a polyunsaturated fatty acid has more than one carbon to carbon double bond in its tail B1 each additional double bond adds a further permanent kink to the tail, so a polyunsaturated fatty acid packs even less closely against its neighbours, giving it a lower melting point

Question 10

A double bond in a fatty acid tail can be cis, which puts a permanent bend in the tail, or trans, which leaves the tail almost straight. Partial hydrogenation of vegetable oil, once widely used to make margarine, turns some cis double bonds into trans double bonds. Explain why a fat rich in trans fatty acids has a higher melting point than an oil with the same number of cis double bonds.

Accepted Answer

B1 the bend at each cis double bond holds neighbouring tails apart, so the molecules cannot pack closely B1 a trans tail is almost straight, so its molecules pack closely together, as saturated tails do B1 closer packing gives more attraction between the tails, so more energy is needed to separate them and the melting point is higher; the fat is solid or semi-solid at room temperature

Question 11

Describe what is meant by describing a phospholipid as amphipathic, and describe the two ends of the molecule the term refers to.

Accepted Answer

B1 amphipathic means having both a hydrophilic and a hydrophobic part in the same molecule B1 the phosphate-containing head group is hydrophilic, carrying a charge that interacts with water B1 the two hydrocarbon tails are hydrophobic, non-polar and excluded from water

Question 12

In a cell-surface membrane, individual phospholipid molecules are free to move sideways past their neighbours. Suggest what holds the phospholipid bilayer together, given that no covalent bond joins one phospholipid to the next, and suggest the consequence for the membrane's physical properties.

Accepted Answer

B1 the bilayer is held together by the hydrophobic effect, since any other arrangement would put more hydrophobic tails in contact with water, together with weak van der Waals attractions between tails packed side by side B1 because these are non-covalent, collective interactions rather than covalent bonds joining specific molecules, individual phospholipids are free to move past one another within the layer B1 the consequence is that the membrane behaves as a fluid rather than a rigid, fixed structure, which is why it is described using the fluid mosaic model

Question 13

Name the two kinds of molecule from which a triglyceride is formed, and name the bond that joins them.

Accepted Answer

A1 glycerol and fatty acids A1 an ester bond

Question 14

State one structural difference between a phospholipid and a triglyceride, and state one property of a phospholipid that results from it.

Accepted Answer

B1 a phospholipid has two fatty acids and a phosphate group, whereas a triglyceride has three fatty acids B1 the phosphate group is charged, so the head of a phospholipid is hydrophilic while the tails are hydrophobic

Question 15

A cell hydrolyses 180 molecules of triglyceride completely. Calculate the number of molecules of water used.

Accepted Answer

M1 each triglyceride contains three ester bonds, and hydrolysing one bond uses one molecule of water A1 180 × 3 = 540 molecules of water