IB DP Chemistry B.8 Nucleic acids (HL only) HL Paper 3

Question

Identify three structural differences between DNA and RNA. Use section 34 of the data booklet.

 

Answer/Explanation

Answer:

Question

There are many biological pigments.
Anthocyanins are pigments that can be used as pH indicators.

(a) Explain which form of the anthocyanin, X or Y, predominates at a pH of 12.

(b) State one factor, other than pH, that can affect the stability of anthocyanins.

(c) Outline one structural similarity and one structural difference between hemoglobin and myoglobin.

Similarity:
Difference:

Answer/Explanation

Answer:

(a) X AND «high pH moves» equilibrium left
OR
X AND loss of proton/\(H^+\) «at high pH/due to excess \(OH^–\)
(aq)»
OR
X AND positive ion unlikely «at high pH»

(b) temperature OR light

(c) Similarity:
heme group
OR
Fe cation/\(Fe^{2+}\)
Difference:
myoglobin has one polypeptide chain AND hemoglobin has four polypeptide
chains
OR
myoglobin has one heme «group» AND hemoglobin has four heme «groups»
OR
myoglobin has no quaternary structure AND hemoglobin has quaternary structure

Question 

a. The diverse functions of biological molecules depend on their structure and shape.
Classify vitamins $\mathrm{A}, \mathrm{C}$ and $\mathrm{D}$ as either mainly fat- or water-soluble, using section 35 of the data booklet.

b(i)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.
$b$ (ii)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.

c. 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.

▶️Answer/Explanation

Markscheme

a.

\text { all three correct } \checkmark

b(i).

– $\mathrm{CH}_2$ – must be placed next to $\mathrm{CHO}$ AND $2 \mathrm{OHs}$ on central carbons must be on same side (LHS or RHS) Accept crosses in place of $\mathrm{C}$ on three middle carbons.

b(ii).

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 $M 2$ if $M 1$ correct.
c. 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 $A N D$ is converted back to cis-retinal
OR
trans-retinal is converted back to cis-retinal through enzyme activity

 
 
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