A-LevelBiologyBiological molecules9700/21 · May/June 2021 · Q29 Marks

Fig. 2.1 shows three molecules of water. [Figure 2.1] The human enzyme, salivary amylase, is composed of one polypeptide. Fig. 2.2 represents the structure of a molecule of salivary amylase. [Figure 2.2]

(a)Describe the hydrogen bonding that occurs between the water molecules shown in Fig. 2.1.[3]
(b)(i)Explain the role of hydrogen bonding in maintaining the secondary structure of proteins, such as salivary amylase.[1]
(b)(ii)Explain the role of hydrogen bonding in maintaining the tertiary structure of proteins such as salivary amylase.[2]
(c)Outline the importance of water as a solvent in plants.[3]

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 (May/June 2021). The mark or mark type is shown in brackets.

[3] any three from
1 (diagram shows) hydrogen bond is a weak bond;
2 each oxygen (atom) forms two hydrogen bonds / each hydrogen (atom) forms one hydrogen bond;
3 (attraction) between oxygen (atom) of one water molecule and hydrogen (atom) of another (forms a hydrogen bond) ;
R cohesion / adhesion – if used for attraction
4 water is dipolar;
5 detail;
e.g. electrons not shared equally between oxygen and hydrogen
A oxygen is more electronegative (than hydrogen)
A ref. to (two) lone pair(s) (of electrons) on oxygen
A uneven distribution of, electrons / charge
e.g. oxygen has, small / slight, negative charge / δ-, and, hydrogen has, small / slight, positive charge / δ+
only needs to state 'small' once
[1] idea that (H-bonds) maintain / AW, (shape / structure, form of)
α-helices / β-pleated sheets ;
A allows formation of, α-helices / β-pleated sheets
R if bonds are between R groups
[2] any two from
1 idea that hydrogen bonds help to, stabilise / AW, further folding of, amylase / polypeptide / protein ;
2 between, R groups with amine and carboxyl groups ;
A between R groups with -NH and, -CO / -OH
3 idea that may be between amino acids far apart in primary structure ;
4 either helps to maintain / form / AW, globular shape / 3D shape / structure (of amylase / polypeptide / protein)
or
maintains / forms / AW, (specific) shape / structure, of, active site / binding site ;
[3] any three from
1 dissolves / AW, ions / minerals / salts, and (named) polar molecules ;
A 'assimilates' as polar I substances / nutrients
2 transports, solute(s) / named solutes / dissolved substance, in, xylem / phloem / xylem and phloem ;
3 storage of, solutes / named solutes, in vacuoles ;
4 metabolic / chemical / cellular, reactions occur in water ;
5 dissolves, carbon dioxide / oxygen, with ref to, respiration / photosynthesis;