State one example of each of the following types of fatty acids (refer to Table 22 of the Data Booklet if necessary).

Saturated:

Mono-unsaturated:

Poly-unsaturated:

Describe, by completing the equation below, the condensation of glycerol and the three fatty acids named in (a) to make a triglyceride.

(i)     State the names of two other types of lipids present in the human body.

(ii)     Compare their composition with that of triglycerides.

Saturated:

octanoic / \({{\text{C}}_7}{{\text{H}}_{15}}{\text{COOH/C}}{{\text{H}}_3}{{\text{(C}}{{\text{H}}_2}{\text{)}}_6}{\text{COOH}}\) /

lauric / \({{\text{C}}_{11}}{{\text{H}}_{23}}{\text{COOH/C}}{{\text{H}}_3}{{\text{(C}}{{\text{H}}_2}{\text{)}}_{10}}{\text{COOH}}\) /

palmitic / \({{\text{C}}_{15}}{{\text{H}}_{31}}{\text{COOH/C}}{{\text{H}}_3}{{\text{(C}}{{\text{H}}_2}{\text{)}}_{14}}{\text{COOH}}\) /

stearic / \({{\text{C}}_{17}}{{\text{H}}_{35}}{\text{COOH/C}}{{\text{H}}_3}{{\text{(C}}{{\text{H}}_2}{\text{)}}_{16}}{\text{COOH}}\);

Mono-unsaturated:

oleic / \({{\text{C}}_{17}}{{\text{H}}_{33}}{\text{COOH/C}}{{\text{H}}_3}{{\text{(C}}{{\text{H}}_2}{\text{)}}_7}{\text{CH=CH(C}}{{\text{H}}_2}{{\text{)}}_7}{\text{COOH}}\);

Poly-unsaturated:

linoleic / \({{\text{C}}_{17}}{{\text{H}}_{31}}{\text{COOH/C}}{{\text{H}}_3}{{\text{(C}}{{\text{H}}_2}{\text{)}}_4}{{\text{(CH=CHC}}{{\text{H}}_2}{\text{)}}_2}{{\text{(C}}{{\text{H}}_2}{\text{)}}_6}{\text{COOH}}\) /

linolenic / \({{\text{C}}_{17}}{{\text{H}}_{29}}{\text{COOH/C}}{{\text{H}}_3}{\text{C}}{{\text{H}}_2}{\text{(CH}}\)=\({\text{CHC}}{{\text{H}}_2}{{\text{)}}_3}{{\text{(C}}{{\text{H}}_2}{\text{)}}_6}{\text{COOH}}\);

Accept name or formula.

Accept other correct examples of fatty acids.

Accept systematic names instead of trivial names.

This scheme is only one of many possible examples.

the release of three molecules of water;

correct structure of all three ester groups;

Accept more condensed structural formulas.

Ester group must be written correctly, glycerol–OOC–R (not glycerol–COO–R).

Do not penalize for minor mistakes in the hydrocarbon chains or the use of R.

(i)     phospholipids and steroids;

Do not accept cholesterol/other specific examples.

(ii)     all three types of lipids are (predominantly) hydrophobic/non-polar/consist mostly of hydrocarbon fragments;

triglycerides and (most) phospholipids contain (a fragment of) glycerol;

steroids are (poly)cyclic compounds/contain (several) rings;

phospholipids contain phosphate (group);

triglycerides and phospholipids are esters;

Allow phosphoric acid/phosphorus instead of phosphate in phospholipids.

Allow cholesterol is (poly)cyclic compound/contains (several) rings as ECF from (i).

Most candidates were able to transfer the required information for part (a) correctly from the Data Booklet.

In (b), the mark for three molecules of water was more often gained than the mark for the correct drawing of the ester link which was often the wrong way round (glycerol–COO–R rather than glycerol–OOC–R) – or just wrong.

In (c), cholesterol was most often given in place of steroids. On reflection, composition might not have been the best word to use in (c)(ii), but it seemed to cause candidates little difficulty and the mark-scheme allowed candidates to score well. Candidates need to consider the best way to set out answers asking for a comparison.