A-LevelBiologyBiological molecules9700/22 · Feb/Mar 2025 · Q29 Marks

Fig. 2.1 is an incomplete diagram of the structure of an α-glucose molecule. Complete Fig. 2.1 to show the structure of an α-glucose molecule.

(a)Fig. 2.1 is an incomplete diagram of the structure of an α-glucose molecule. Complete Fig. 2.1 to show the structure of an α-glucose molecule.[2]
(b)(i)Fig. 2.2 shows part of a glycoprotein molecule. [Figure 2.2] State the types of covalent bond labelled C and D in Fig. 2.2. C D[2]
(b)(ii)Many glycoproteins in the cell surface membrane are involved in cell signalling. State the role in cell signalling of glycoproteins in cell surface membranes.[1]
(c)One of the proteins found in milk is β-casein. A molecule of β-casein consists of a single polypeptide of approximately 200 amino acids. Molecules of β-casein have a high proportion of the amino acids proline and leucine. These amino acids have hydrophobic R-groups. Fig. 2.3 compares the structure of proline (Pro) with the general structure of an amino acid. This shows that proline is an unusual amino acid because it has a cyclic R-group and the nitrogen atom is attached to only one hydrogen atom. [Figure 2.3] When a molecule of proline becomes part of a polypeptide, the hydrogen atom is lost from the nitrogen atom and is therefore not available for the formation of hydrogen bonds. Molecules of β-casein have a relatively low proportion of the amino acids cysteine and serine. Table 2.1 shows the R-groups of cysteine and serine. Table 2.1 amino acid R-group feature of R-group cysteine (Cys) −CH₂SH can form a disulfide bond serine (Ser) −CH₂OH hydroxyl group present Compared to other protein molecules with a similar number of amino acids, β-casein molecules have: • a much less organised secondary structure • relatively little tertiary structure. With reference to the four named amino acids, proline, leucine, serine and cysteine, suggest possible explanations for these two observations. less organised secondary structure relatively little tertiary structure[4]

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Mark scheme

Marking points from the official mark scheme (Feb/Mar 2025). The mark or mark type is shown in brackets.

[2] H and OH correct on C1 ; all other additions complete and correct;
[2] C - peptide (bond) ; D – glycosidic (bond) ;
[1] act as receptors / bind to ligands, qualified ;
[4] any four from: less organised secondary structure (max three): 1 high proportion of proline so less hydrogen bonding; 2 so less, alpha helix / beta-pleated sheet, formation ; 3 less opportunity to form hydrogen bonds between –NH of one amino acid and –C=O of another amino acid (close by) ; 4 AVP; e.g. greater proportion would have random, coiling / structure ; relatively little tertiary structure (max three): 5 fewer amino acids will be forming ionic bonds between R-groups ; 6 little cysteine so, less / no, disulfide bonding (for tertiary structure); 7 (relatively little serine) so less hydrogen, bonding (for tertiary structure) ; 8 AVP; e.g. high proportion of hydrophobic amino acids (proline and leucine) so tertiary structure will be more dependent on hydrophobic interactions