Question 19M.3.sl.TZ2.7
| Date | May 2019 | Marks available | [Maximum mark: 9] | Reference code | 19M.3.sl.TZ2.7 |
| Level | sl | Paper | 3 | Time zone | TZ2 |
| Command term | Deduce, Describe, Identify, Predict, Suggest | Question number | 7 | 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.
This was very poorly answered. Although the phosphodiester was a challenging mark it could be awarded in both the protonated and deprotonated form. The two esters should have been much more straight forward mark, and both were required to receive the second mark. Students struggled with proper structural drawings for both marks and many students simply left this question blank. The functional groups did need to be drawn out in their full structural form to receive the mark as indicated in the question.
Identify the type of reaction in (a).
[1]
condensation [✔]
Note: Accept “esterification”.
Accept “nucleophilic substitution/SN”.
This question was 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 max] 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 mark was fairly well answered but the explanation or second mark was often not correct or complete.
Predict, giving a reason, the relative energy density of a carbohydrate and a lipid of similar molar mass.
[1]
carbohydrates less energy dense AND carbohydrates higher ratio of oxygen to carbon/more oxidized/less reduced [✔]
This question was not well answered even though it has appeared on previous tests. In any cases the students did not give the relative energy density or the reason. It is important that candidates read question carefully and responds completely to each question as asked.
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 [✔]
Note: Accept “alcohol/hydroxy/OH” for “hydroxyl” but not “hydroxide”.
This question was also not answered well even though it has appeared on previous tests. Many students missed the idea of a long or large non-polar chain when describing the structure.
Phospholipids are also found in lipoprotein structures.
Describe two effects of increased levels of low-density lipoprotein (LDL) on health.
[2]
atherosclerosis/cholesterol deposition «in artery walls» [✔]
increases risk of heart attack/stroke/cardiovascular disease/CHD [✔]
Note: Accept “arteries become blocked/walls become thicker”, “increases blood pressure”, or “blood clots”.
Do not accept “high cholesterol”.
Students were required to state two effects of increased LDL. High cholesterol is not an accepted answer but still frequently seen. Many students also repeated similar answers that could not receive the same mark.