The diverse functions of biological molecules depend on their structure and shape.

Classify vitamins A, C and D as either mainly fat- or water-soluble, using section 35 of the data booklet.

The diverse functions of biological molecules depend on their structure and shape.

Deduce the straight chain structure of deoxyribose from its ring structure drawn in section 34 of the data booklet.

The diverse functions of biological molecules depend on their structure and shape.

Draw the nitrogenous base that is paired with guanine in DNA, showing the hydrogen bonds between the bases. Use section 34 of the data booklet.

The diverse functions of biological molecules depend on their structure and shape.

Retinal is the key molecule involved in vision. Explain the roles of cis- and trans-retinal in vision and how the isomers are formed in the visual cycle.

all three correct ✔

–CH₂– must be placed next to $\mathrm{CHO}$ AND 2OHs on central carbons must be on same side (LHS or RHS) ✔

Accept crosses in place of $C$ on three middle carbons.

cytosine drawn ✔

appropriate representation of three hydrogen bonds AND between correct atoms ✔

Structure of cytosine must be given for M1.

Ignore missing hydrogens on carbon atoms in cytosine.

Dashed lines (horizontal or vertical) OR dots can be used to represent hydrogen
bonds.

Only award M2 if M1 correct.

Any three of:
cis-retinal binds to «the protein» opsin
OR
cis-retinal «binds to opsin and» forms rhodopsin ✔

opsin extends conjugation in retinal
OR
conjugation in rhodopsin is larger/more extended/involves more atoms than that in retinal
OR
rhodopsin allows absorption of visible/blue/green light ✔

when visible light is absorbed cis-retinal changes to trans-retinal ✔

change «to trans-retinal» triggers an electrical/nerve signal ✔

trans-retinal detaches from opsin AND is converted back to cis-retinal
OR
trans-retinal is converted back to cis-retinal through enzyme activity ✔

Classification of the three vitamins A, C and D as fat-soluble, water-soluble and fat-soluble was very well answered and nearly all scored the single mark here.

The straight chain structure of deoxyribose was poorly represented. All sorts of errors were seen, such as not having –CHO at one end, not having the methylene group adjacent to –CHO etc.

This question proved to be very challenging and although many were able to identify and draw cytosine correctly, few were able to score M2 which required the representation of the multiple hydrogen bonds between guanine and cytosine. It was very disappointing that such a high percentage of candidates did not understand the nature of a hydrogen bond and equally tried to represent a hydrogen bond using a full line notation (typical of a covalent bond), instead of the more customary dashed lines (either horizontal or vertical) or dots type representation, used to represent hydrogen bonds.

This question which centred on retinal as the key molecule in vision was well answered and many scored all three marks. The most common incomplete answer was "cis-retinal changing to trans-retinal" without stating that this occurs when visible light is absorbed. Some also were unclear of the difference between opsin and rhodopsin.