Question 19M.3.hl.TZ2.11
| Date | May 2019 | Marks available | [Maximum mark: 7] | Reference code | 19M.3.hl.TZ2.11 |
| Level | hl | Paper | 3 | Time zone | TZ2 |
| Command term | Deduce, Describe, Identify, Suggest | Question number | 11 | Adapted from | N/A |
Phosphatidylcholine is an example of a phospholipid found in lecithin.
Phosphatidylcholine may be formed from propane-1,2,3-triol, two lauric acid molecules, phosphoric acid and the choline cation.
Deduce the structural formula of phosphatidylcholine.
[2]
phosphodiester correctly drawn [✔]
both ester groups correctly drawn [✔]
Note: Accept protonated phosphate.
Accept phosphodiester in centre position.
Almost all the candidates struggled with this part. Although the phosphodiester was a challenging mark it could be awarded in both the protonated and deprotonated form. The two ester groups were required for the second mark. Candidates were not able to draw correctly for both marks, and many left this question blank.
Identify the type of reaction in (a).
[1]
condensation [✔]
Note: Accept “esterification”.
Accept “nucleophilic substitution/SN”.
This part was very well answered.
Lecithin is a major component of cell membranes. Describe the structure of a cell membrane.
[2]
phospholipid bilayer/double layer
OR
two layers of phospholipids [✔]
polar/hydrophilic heads facing aqueous environment AND non-polar/hydrophobic tails facing away from aqueous environment [✔]
Note: Award [2] for a suitably labelled diagram.
Award [1] for a correct but unlabelled diagram.
Accept “polar/hydrophilic heads on outside AND non-polar/hydrophobic tails on inside” for M2.
This question was another one where the first part was fairly well answered, but the explanation or second mark was often not correct or incomplete.
Lecithin aids the body’s absorption of vitamin E.
Suggest why vitamin E is fat-soluble.
[1]
long non-polar/hydrocarbon chain «and only one hydroxyl group»
OR
forms London/dispersion/van der Waals/vdW interactions with fat [✔]
Many candidates missed the idea of a long or large non-polar chain when describing the structure of vitamin E. Simply stating non-polar chain was not sufficient for the mark.
Phospholipids are also found in lipoprotein structures.
Describe one effect of increased levels of low-density lipoprotein (LDL) on health.
[1]
Any one of:
atherosclerosis/cholesterol deposition «in artery walls» [✔]
increases risk of heart attack/stroke/cardiovascular/heart disease/CHD [✔]
Note: Accept “arteries become blocked/walls become thicker”, “increases blood pressure”, or “blood clots”.
Do not accept “high cholesterol”.
Candidates were required to state one effect of increased LDL. Majority of the candidates scored well on this part. High cholesterol is not an acceptable answer but was frequently seen.