A-LevelBiologyBiological molecules9700/21 · Oct/Nov 2024 · Q413 Marks

Fig. 4.1 shows the structure of sucrose, a disaccharide produced by plant cells. [Figure 4.1]

(a)(i)Name the covalent bond that joins the two monomers in sucrose.[1]
(a)(ii)Sucrose is hydrolysed by the enzyme sucrase in the human digestive system. The products of this hydrolysis reaction are the monosaccharides a-glucose and fructose. Complete the diagram to show the hydrolysis of sucrose to form a-glucose and fructose. [Figure of sucrose hydrolysis][3]
(b)(i)Name the structure labelled X in Fig. 4.2.[1]
(b)(ii)Explain the results shown in Table 4.1.[4]
(c)Suggest and explain why a person with this deletion mutation cannot digest sucrose.[4]

This question uses a diagram from the original paper, which isn't shown here. Open the full paper from the past papers archive to see it.

Mark scheme

Marking points from the official mark scheme (Oct/Nov 2024). The mark or mark type is shown in brackets.

4(a)(i) glycosidic;
4(a)(ii) any three from: label one monomer correctly;
α-glucose with a hydroxyl group on C1 ;
fructose drawn with a hydroxyl group on C2 ;
water used in the reaction ;
4(b)(i) sieve plate;
4(b)(ii) any four from: 1 carbohydrate is transported in (phloem) sieve tubes ; A sucrose transported A sieve tube elements
2 as the percentage of outer stem tissue removed increases, the quantity of phloem (sieve tubes) remaining decreases ;
3 less, mass flow / translocation ;
4 0 mg at 100% removal because all phloem removed / AW ;
5 carbohydrate is not transported in xylem tissue, as there is no transport when xylem tissue is left intact;
6 ref. to movement to lower part of stem from source to sink;
7 AVP; e.g. suggestion that most of the (active) phloem is in the inner part of the outer stem idea that phloem sieve tubes are arranged in layers
4(c) any four from: max 3
1 example of structural change to sucrase ; e.g. translation stops prematurely / truncated polypeptide / AW change in the primary structure / different amino acid(s) polypeptide does not, fold into / form, (correct) tertiary structure
2 suggestion of why sucrase is non-functional; e.g. active site, not complementary / changed shape binding site changed shape, sucrose does not bind to active site catalytic site changed shape, glycosidic bond cannot be broken
3 polypeptide not produced as mRNA does not attach to ribosome;
4 polypeptide recognised as abnormal and degraded ; max 3
5 deletion mutation is loss of one or more nucleotides (in the gene coding for sucrase);
6 (causes a) change in the sequence of, nucleotides / bases / base pairs, (in the, gene coding for sucrase / DNA molecule);
7 (causes a) change in the sequence of, nucleotides / bases, in, mRNA / an mRNA molecule ;
8 consequence to codons ; (if not a deletion of, three/multiples of three) all the codons after the deletion mutation are altered causes a frameshift (deletion, of three / multiples of three) one or more codons absent, then same sequence
9 mutation may, form a premature stop codon / introduce a stop codon;