Question 1
Paper 1A style
Between which groups does a peptide bond form when two amino acids are joined?
- The R-group of one amino acid and the R-group of the other
- The carboxyl group of one amino acid and the amine group of the other
- The amine groups of both amino acids
- The alpha carbon of one amino acid and the hydrogen of the other
Show mark scheme for question 1
Answer: B [1]
Question 2
Paper 1A style
How many different tripeptides could be made from the 20 amino acids coded for in the genetic code?
- 60
- 400
- 1140
- 8000
Show mark scheme for question 2
Answer: D [1]
Question 3
Paper 1A style
HL only (what this means)
HL only: additional Higher Level content, only for HL students. SL students can skip it. What the labels meanWhich bond holds an alpha helix in shape?
- Disulfide bonds between pairs of cysteines
- Ionic bonds between charged R-groups
- Hydrogen bonds between C=O and N–H groups of the polypeptide backbone
- Hydrophobic interactions between non-polar R-groups
Show mark scheme for question 3
Answer: C [1]
Question 4
Paper 1A style
HL only (what this means)
HL only: additional Higher Level content, only for HL students. SL students can skip it. What the labels meanWhich protein is conjugated?
- Collagen
- Insulin
- Haemoglobin
- A single alpha helix
Show mark scheme for question 4
Answer: C [1]
Question 5
Paper 1B style
Samples of an enzyme from a mammal were kept at different temperatures for 10 minutes, then cooled to 37 °C. The activity of each sample was then measured at 37 °C and given as a percentage of the activity of an unheated sample. The data are for practice. (a) Describe the effect of pre-incubation temperature on enzyme activity. [2] (b) Calculate the percentage decrease in activity between 50 °C and 60 °C. [1] (c) Explain why the activity of the samples kept at 70 °C and 80 °C did not recover when cooled. [2] (d) Suggest why activity was measured at 37 °C for every sample. [1]
| Pre-incubation temperature / °C | Activity / % of unheated sample |
|---|---|
| 20 | 100 |
| 40 | 98 |
| 50 | 80 |
| 60 | 35 |
| 70 | 6 |
| 80 | 0 |
Show mark scheme for question 5
- (a) little / no loss of activity up to 40 °C [1]
- (a) above 40 °C activity falls, steeply between 50 °C and 70 °C / no activity at 80 °C [1]
- (b) (80 − 35) ÷ 80 × 100 = 56 % [1] Accept 56.25 %
- (c) the enzyme was denatured / its three-dimensional shape / active site changed [1]
- (c) hydrogen bonds / weak bonds broken by increased vibration; denaturation is (usually) permanent so cooling does not restore the shape [1]
- (d) so that any difference in activity is due only to the effect of the pre-incubation / irreversible change / to control temperature during the assay [1]
Question 6
Paper 2A style
Distinguish between essential and non-essential amino acids and outline why people following a vegan diet need to pay attention to protein sources.
Show mark scheme for question 6
- essential amino acids cannot be synthesised by the body / must be obtained from food [1]
- non-essential amino acids can be made from other amino acids [1]
- some plant proteins contain little of certain essential amino acids [1]
- so vegans need a variety of plant protein sources / combine e.g. legumes and cereals [1]
- max 3
Question 7
Paper 2A style
HL only (what this means)
HL only: additional Higher Level content, only for HL students. SL students can skip it. What the labels meanExplain how interactions between R-groups stabilise the tertiary structure of a water-soluble globular protein.
Show mark scheme for question 7
- hydrogen bonds form between polar R-groups [1]
- ionic bonds form between positively charged (–NH₃⁺) and negatively charged (–COO⁻) R-groups [1]
- R-groups become charged by gaining / losing hydrogen ions [1]
- disulfide (covalent) bonds form between pairs of cysteines [1]
- hydrophobic / non-polar R-groups cluster together in the core, away from water [1]
- polar / charged R-groups on the surface keep the protein soluble [1]
- max 4
Question 8
Paper 2B style
HL only (what this means)
HL only: additional Higher Level content, only for HL students. SL students can skip it. What the labels meanExplain how the structures of insulin and collagen are related to their functions.
Show mark scheme for question 8
- primary structure / amino acid sequence determines how each protein folds [1]
- insulin is a globular protein / compact rounded shape [1]
- insulin has two polypeptide chains linked by disulfide bonds / non-conjugated quaternary structure [1]
- hydrophilic R-groups on the surface make insulin soluble, so it is transported in blood plasma [1]
- precise shape binds a specific insulin receptor on target cells [1]
- binding triggers uptake of glucose by target cells / lowers blood glucose [1]
- collagen is a fibrous protein / long, narrow and insoluble [1]
- three polypeptides wound into a triple helix / non-conjugated quaternary structure [1]
- hydrogen bonds between the chains / every third amino acid is glycine, allowing tight packing [1]
- molecules cross-linked side by side into fibrils [1]
- high tensile strength, so it resists pulling forces in tendons / ligaments / skin [1]
- max 7