Compare the structures and polarities of fats and phospholipids, giving one similarity and one difference in structure and one difference in polarity.

Similarity in structure:

Difference in structure:

Difference in polarity:

Vitamin D is produced from cholesterol. The structures of both molecules are given in table 21 of the data booklet. Outline one structural difference between the molecules.

Distinguish between HDL and LDL cholesterol in terms of their composition and their effect on health.

Composition:

One effect on health:

Similarity in structure:

both are (tri)esters / both made from glycerol/propane-1,2,3-triol/HOCH₂CH(OH)CH₂OH;

Difference in structure:

phospholipids have phosphate group/phosphorus and fats are triglycerides/made from three fatty/carboxylic acids / one fatty/carboxylic acid (in fat) replaced by phosphate in phospholipid;

Difference in polarity:

phospholipids are more polar / phospholipids have hydrophilic (heads/section/part/end) / fats are less polar/non-polar / fats are hydrophobic ;

(two) more carbon−carbon double bonds/alkenyl groups in vitamin D;

Accept alkene for alkenyl.

extra hexagon/6-membered ring in cholesterol / more fused rings in cholesterol / four fused rings in cholesterol and two fused rings in vitamin D;

Accept “(some) conjugation in vitamin D / (some) alternating C=C and C−C bonds in vitamin D”.

Accept “cholesterol has a steroid backbone/structure but vitamin D does not”.

Composition:

HDL has more protein and less cholesterol/fat/lipid (and vice-versa);

Accept “HDL has more protein and LDL has more cholesterol (and vice-versa)”.

Accept “HDL has higher phospholipid content compared to LDL (and vice-versa)”.

Accept “HDL particles are smaller than LDL particles (and vice-versa)” but do not penalize if “molecules” are used instead of “particles”.

One effect on health:

cardiovascular problems/increased risk of heart disease/obesity/atherosclerosis/blocked arteries from high ratio of LDL to HDL;

Accept “from (high) LDL” instead of “from high ratio of LDL to HDL”.

Accept “can result in a heart attack/stroke from high ratio of LDL to HDL”.

Accept “large amounts of HDL in blood correlate with good health / OWTTE”.

Reference must be made to LDL or HDL.

About half the candidates seemed familiar with the structures of fats and phospholipids, but only a few gave the detailed answers expected by the markscheme. Some of the answers were far too general such as stating “both contain C, H and O” for the similarity in structure.

About a third of the candidates scored the mark by stating an accurate structural difference between vitamin D and cholesterol. Some answers were not specific enough about the numbers and types of rings.

Only a few candidates were able to distinguish between the composition of HDL and LDL cholesterol, but the majority of candidates understood their effects on health well and gained the second mark. There was a comment on a G2 form that this question went beyond the syllabus, however, the question is covered in part B.4.2 (“outline the difference between HDL and LDL cholesterol”) and has also appeared in a past paper.