AQA A-Level Biology: Biological Molecules
6 exam-style questions with full mark schemes and model answers. Write your own answer and the AI examiner marks it against the mark scheme.
Proteins show enormous variety in shape and function, yet all are built from the same basic monomers.
Describe and explain how the structure of proteins is related to their function. In your answer you must refer to at least two named proteins: one fibrous protein and one globular protein. You should cover the levels of protein structure, the bonds that hold each level together, and link specific structural features to the function of each named protein.
(6 marks)
A student investigated the activity of the enzyme catalase, which breaks down hydrogen peroxide into water and oxygen. They added a fixed mass of catalase to a fixed volume of hydrogen peroxide solution at 25 °C and measured the cumulative volume of oxygen collected over time. The results are shown below.
| Time / s | Cumulative volume of oxygen / cm³ |
|---|---|
| 0 | 0 |
| 30 | 12.0 |
| 60 | 21.0 |
| 90 | 27.0 |
| 120 | 30.0 |
| 150 | 31.5 |
(a) Calculate the mean rate of reaction over the first 60 seconds. Show your working and give the units. (2 marks)
(b) Explain why the rate of reaction decreases as time goes on, even though the temperature and the amount of enzyme are unchanged. (4 marks)
A mixture of amino acids was separated using thin-layer chromatography. After the chromatogram had been sprayed with ninhydrin to reveal the spots, a student measured the distance moved by each spot and the distance moved by the solvent front from the origin.
| Spot | Distance moved by spot / mm |
|---|---|
| A | 16 |
| B | 36 |
| C | 60 |
The solvent front moved 80 mm from the origin.
Reference Rf values in this solvent system are given below.
| Amino acid | Rf value |
|---|---|
| Alanine | 0.20 |
| Serine | 0.30 |
| Glycine | 0.45 |
| Valine | 0.60 |
| Leucine | 0.75 |
(a) Calculate the Rf value of each spot (A, B and C) to 2 decimal places. Show your working. (3 marks)
(b) Using your results, identify the amino acid present in each spot. (2 marks)
Scientists studying a newly discovered soil microbe found that it stores energy as an unusual polysaccharide they named microglucan. Microglucan is a polymer of alpha-glucose that is very highly branched (far more branch points than glycogen) and the chains are tightly coiled into compact granules. It is insoluble in water.
The microbe survives long periods of starvation and, when food returns, must release glucose for respiration very rapidly.
Suggest and explain how the structural features of microglucan make it well-suited as an energy store for this microbe. (5 marks)
Two substances, X and Y, both slow down the same enzyme-catalysed reaction.
- X has a molecular shape similar to that of the substrate and binds to the enzyme's active site.
- Y binds to a region of the enzyme away from the active site, changing the shape of the molecule.
Explain how X and Y each affect the rate of reaction as the substrate concentration is increased. Use the terms active site and induced fit in your answer. (4 marks)
Two molecules of alpha-glucose can be joined together to form the disaccharide maltose.
Describe how a condensation reaction joins two monosaccharides to form a disaccharide. In your answer, name the type of bond formed and the molecule that is removed. (3 marks)