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
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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
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Answer: D [1]
Question 3
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 |
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- (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 4
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.
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- 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