A-LevelBiologyBiological molecules9700/23 · Oct/Nov 2018 · Q211 Marks

Proteins are macromolecules composed of many amino acids.

(a)(i)Two amino acids are represented in the diagram in Fig. 2.1. Complete the diagram to show how the two amino acids react together to form a dipeptide. [Figure 2.1][3]
(a)(ii)State what is represented by R₁ and R₂ in Fig. 2.1.[2]
(b)Amylose and cellulose are polysaccharides. Fig. 2.2 shows the structure of part of a cellulose molecule. With reference to Fig. 2.2, state how the structure of a cellulose molecule differs from the structure of an amylose molecule. [Figure 2.2][2]
(c)Cellulose is the main component of plant cell walls. Explain why cellulose is suitable as a component of plant cell walls.[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 2018). The mark or mark type is shown in brackets.

[(a)(i)] water shown formed from –OH and –H ; A H2O formed without indicating where from
[(a)(i)] peptide bond shown correctly between C of carboxylic acid and N of amino group ;
[(a)(i)] complete dipeptide drawn ; A hydrogen 'up or down'
[(a)(ii)] R group / side chain / variable group / residual group / functional group ;
[(a)(ii)] indicate, different amino acids / type of amino acid; A specific to each amino acid A the two amino acids are different A two examples of R groups, e.g. -H and -CH3
[(a)(ii)] both amino acids have different R groups = 2 marks
[(b) any two from]
straight chain / linear, v helix / helical ; I coil(ed) R branched / branching
β-glucose, not α-glucose ; A β-1:4 glycosidic bond v α-1:4 glycosidic bond
(β-)glucose / monomers / residues, are arranged, rotated 180° to each other / AW ;
AVP; e.g. more hydrogen bonds
[(c) any four from]
[1] (molecules) form fibrils and fibres ;
[2] hydrogen bonding between (cellulose) molecules ;
[3] (cellulose molecule) is straight / linear;
[4] (straight chain allows) molecules lie parallel to each other ; mp4 dependent on mp3
[5] gives strength (to cell wall) to, prevent cell bursting / withstand (turgor) pressure / AW ; R if only in context of a cellulose molecule
[6] ref. to fibres at angles / criss-cross / AW ;
[7] idea of many gaps, in wall / between fibres, allowing passage of water / named) substances / making cell wall permeable ; I plasmodesmata R partially / AW, permeable
[8] AVP; e.g. cellulose is insoluble many –OH groups (for hydrogen bonding)