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Date	November 2020	Marks available	2	Reference code	20N.3.sl.TZ0.6
Level	SL	Paper	3	Time zone	TZ0
Command term	Discuss	Question number	6	Adapted from	N/A

Question

Phospholipids are a main component of cell membranes.

Deduce the products of the hydrolysis of a non-substituted phospholipid, where $R^{1}$ and $R^{2}$ represent long alkyl chains.

[2]

A representation of a phospholipid bilayer cell membrane is shown:

Identify the components of the phospholipid labelled A and B.

[1]

b(i).

State the most significant intermolecular forces in the phospholipid in b(i).

[2]

b(ii).

Phospholipids help maintain cellular environments while fatty acid lipids have important roles in energy storage and electrical insulation. Discuss the structural properties of saturated fats needed for these roles.

[2]

Markscheme

glycerol ✔

both fatty acids AND phosphoric acid ✔

Accept either names OR structures.
Accept “long chain carboxylic acid” for “fatty acid”.

Penalize once only if an incorrect name is given for a correct structure or vice-versa.

A: phosphate/ionic group
AND
B: alkyl/hydrocarbon «chain» ✔

Accept "glycerol «fragment»" OR "glycerophosphate" OR “ester” for A.

Accept “fatty acid «tail»” for B.

Do not accept terms such as “polar head”, “non-polar tail”, “hydrophilic” OR “hydrophobic” for components alone.

b(i).

Forces occurring between components labelled A:
hydrogen/ $H$ bonding
OR
ion–dipole
OR
ionic/electrostatic «repulsion and/or attraction» ✔

Accept “dipole-dipole” for M1.
Do not accept “van der Waals/vdW” for M1.

Forces occurring between components labelled B:
dispersion/London/instantaneous dipoles/temporary dipoles ✔

Accept “van der Waals/vdW” for M2.

b(ii).

Energy storage:
not water-soluble/no hydrogen/ $H$ bonding
OR
less oxidized/more reduced
OR
high energy stored in bonds
OR
high «negative» enthalpy of combustion/oxidation ✔

Accept “potential energy” for “stored energy”.

Electrical insulator:
no delocalized electrons/conjugation ✔

Examiners report

Not as well answered as expected. However, many candidates managed to score at least one point. Quite a few lost the only mark due to wrong linkage. Some students drew fatty acid structures with aldehydes or phosphoric acid with incorrect bond linkages in the structure (OH^-).

Mostly well answered. Weaker candidates used the terms hydrophobic (non-polar) tail and/or hydrophilic (polar) head and therefore lost the mark. Students are expected to know the names of these structures.

b(i).

Many good answers. Those that failed to score often provided the inverted answer. Dipole-dipole was fairly common for M1 but VdW forces less so for M2.

b(ii).

Not well answered. In particular the first mark seemed particularly challenging for students. Even when at times wording wasn't enough to allow BOD for M2 it was evident the candidate had some idea but none for the first one. Non-polar allowed many students to score the second mark.

Syllabus sections

Options » B: Biochemistry » B.3 Lipids

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