The formulas of some fatty acids are shown in Table 22 of the Data Booklet. State the equation for the reaction between glycerol and stearic acid to form a triglyceride.

Compare the structures of the two fatty acids: linoleic and linolenic acids.

State why these two fatty acids are so important in the human diet.

Distinguish between HDL and LDL cholesterol.

Compare the composition of cholesterol with a phospholipid such as lecithin.

Determine whether cholesterol or lecithin is more soluble in water.

correct structure of glycerol and correct formula of stearic acid;

correct structure of triglyceride;

\({\text{3}}{{\text{H}}_{\text{2}}}{\text{O}}\) and coefficient of 3 on stearic acid;

Accept displayed or condensed formulas for molecules.

both have first double bond on C9 with carbon / linoleic has an \(\omega  - 6{\text{ C=C}}\) double bond and linolenic acid has an \(\omega  - 3{\text{ C=C}}\) double bond;

linoleic acid has 2 double carbon bonds and linolenic acid 3 double carbon bonds;

fatty acids are essential / body cannot synthesize them / OWTTE;

LDL is (a) larger (molecule) than HDL;

LDL transports cholesterol to arteries and HDL removes cholesterol from arteries;

LDL produced from saturated fats/trans fatty acids;

LDL increases the risk of heart disease/problems;

Accept converse statements for HDL.

Do not accept LDL is bad cholesterol and HDL is good cholesterol.

cholesterol is composed of C, H and O only and phospholipid contains C, H, O, P and N;

lecithin;

Candidates could not write an equation for the reaction between glycerol and stearic acid to form a triglyceride. Where candidates did write the correct equation they often did not balance the equation correctly.

In part (b) many candidates did not correctly recognize the difference in the number of carbon – carbon double bonds in the two fatty acids, nor the location of the double bonds and hence the significance of the omega-3 and omega-6 terminology.

Some candidates correctly identified that these fatty acids cannot be synthesised by the body and hence are essential.

In part (c) candidates could not distinguish between HDL and LDL, often referring simply and inadequately to ‘good’ and ‘bad’ cholesterol. Candidates had great difficulty comparing the composition of cholesterol with lecithin. An elemental comparison was required.

In part (c) candidates could not distinguish between HDL and LDL, often referring simply and inadequately to ‘good’ and ‘bad’ cholesterol. Candidates had great difficulty comparing the composition of cholesterol with lecithin. An elemental comparison was required.

In part (c) candidates could not distinguish between HDL and LDL, often referring simply and inadequately to ‘good’ and ‘bad’ cholesterol. Candidates had great difficulty comparing the composition of cholesterol with lecithin. An elemental comparison was required.