Paper 3 (all topics, practical skills and essay)

Investigation: temperature and respiration in yeast

A student added yeast suspension to glucose solution in a water bath and collected the carbon dioxide produced over 5 minutes. Each temperature was repeated five times.

Temperature / °C	20	30	40	50
Mean volume of CO₂ / cm³	1.2	2.6	4.9	1.1
Standard deviation / cm³	0.2	0.3	0.9	0.4

1.11 mark

Calculate the mean rate of CO₂ production at 40 °C in cm³ min⁻¹.

1.22 marks

Explain the increase in the volume of CO₂ produced between 20 °C and 40 °C.

1.32 marks

Explain the result at 50 °C.

1.43 marks

The student concluded that the mean volumes at 30 °C and 40 °C are significantly different. Use the standard deviations to evaluate this conclusion.

1.52 marks

Give two variables the student should have controlled, other than temperature.

1.62 marks

Suggest how the student could find the optimum temperature more precisely.

1.72 marks

Suggest why the standard deviation at 40 °C was larger than at the other temperatures, and one way to improve the precision of the method.

1.82 marks

Yeast can respire anaerobically. Explain how anaerobic respiration allows glycolysis to continue.

2.13 marks

A student has a 10 mmol dm⁻³ glucose solution and distilled water. Describe how to make 10 cm³ of each of the solutions 5, 2.5 and 1.25 mmol dm⁻³ by serial dilution.

2.22 marks

Using Figure 2, find the glucose concentration of a sample with absorbance 0.55.

Figure 2

2.32 marks

Explain why absorbance decreases as glucose concentration increases when a colorimeter is used with Benedict's reagent and the precipitate is removed.

2.43 marks

Give three steps the student should take to make the colorimeter readings valid.

2.52 marks

A urine sample gave an absorbance of 0.08. Explain why this value cannot be read reliably from Figure 2 and suggest what the student should do.

Passage: Phage therapy (written for this paper)

(1) Bacteriophages, or phages, are viruses that infect bacteria. Each type of phage recognises and binds to specific proteins on the surface of particular bacterial cells. (2) Once attached, the phage injects its genetic material, which takes over the bacterial cell's enzymes and ribosomes to make new phages. (3) Within an hour the cell bursts, releasing hundreds of phages that can infect neighbouring bacteria. (4) Phages were used to treat infections in the early twentieth century, but in most countries they were abandoned after antibiotics became widely available. (5) The rise of antibiotic-resistant bacteria has renewed interest. (6) In several recent cases, patients with resistant infections that had not responded to any antibiotic were treated with a mixture, or 'cocktail', of phages chosen to match their particular strain of bacteria. (7) Critics point out that bacteria can also become resistant to phages, for example by changing the proteins on their surface. (8) Supporters reply that phages evolve too, and that a resistant bacterium is often less able to cause disease. (9) Because each cocktail must be matched to the patient's bacteria, phage therapy is difficult to test in the large, standardised clinical trials that regulators usually require.

3.13 marks

Use information in the passage and your knowledge to explain why each type of phage infects only particular bacteria (sentence 1).

3.23 marks

Sentence 2 says the phage uses the bacterial cell's enzymes and ribosomes. Explain why phages need to do this.

3.33 marks

Explain how a population of bacteria could become resistant to a phage (sentence 7).

3.43 marks

Suggest why a bacterium that is resistant to a phage is 'often less able to cause disease' (sentence 8).

3.53 marks

Evaluate the use of phage therapy as a treatment for antibiotic-resistant infections, using information from the passage.

410 marks

A student investigated whether there is a correlation between the light intensity at six sites and the number of plant species found there. She ranked both variables:

Site	A	B	C	D	E	F
Rank of light intensity	1	2	3	4	5	6
Rank of number of species	2	1	4	3	6	5

(a) State a suitable null hypothesis. (b) Calculate Spearman's rank correlation coefficient, r_s = 1 - (6Σ d²)/(n(n² - 1)). (c) The critical value for n = 6 at p = 0.05 (two-tailed) is 0.886. What should the student conclude? (d) Explain why Spearman's rank was used rather than a t-test. (e) Suggest one improvement to the investigation.

525 marks

Write an essay on ONE of the following topics. EITHER The importance of hydrogen bonds in living organisms. OR The ways in which cells communicate with each other. You should select and use information from different parts of the specification.

Finished?

Answer every question to finish (20 left), or finish when the time’s up.

Score 0 / 78 · 0 of 20 marked

This paper is written by Plasmo in the style of the AQA A-Level Biology exam. It is not a past paper and is not produced or endorsed by the exam board.