Paper 3 (all topics, practical skills and essay)

Investigation (written for this paper): the effect of hydrogen peroxide concentration on the rate of oxygen production by catalase in potato extract was measured.

H₂O₂ / mol dm⁻³	0.5	1.0	1.5	2.0	2.5
O₂ collected in 60 s / cm³	6	11	15	17	17
SD	0.8	1.1	1.4	1.2	2.6

Calculate the rate of oxygen production at 1.5 mol dm⁻³ in cm³ min⁻¹ and in cm³ s⁻¹.

Explain the results between 1.5 and 2.5 mol dm⁻³.

Give two variables that should be controlled.

What does the standard deviation at 2.5 mol dm⁻³ suggest about the data?

Suggest two improvements to the method.

Describe how to prepare a root tip squash to observe mitosis.

In a squash, 48 of 400 cells were in mitosis. Calculate the mitotic index.

6 of 200 cells were in anaphase. The cell cycle takes 24 hours. Estimate the duration of anaphase in minutes.

During a heart dissection, describe how you would show that the left ventricle wall is thicker than the right, and explain why.

Give two safety precautions when using a scalpel in a dissection.

Describe how a belt transect could be used to investigate how the distribution of a plant changes up a sand dune.

Spearman's rank correlation coefficient between distance and plant cover was 0.82. The critical value at P = 0.05 is 0.64. Interpret this result.

Explain why a correlation does not show that distance causes the change in plant cover.

Leaf lengths of a plant from sunny and shaded sites (n = 30 each) (data are illustrative, written for this paper): sunny mean 42 mm, SD 4.1; shaded mean 58 mm, SD 6.3. A t-test gave t = 11.6 (critical value 2.00 at P = 0.05). (a) State a null hypothesis. (b) Interpret the t-test result. (c) Suggest an explanation for the difference. (d) Explain why the samples should be collected at random.

Critical analysis (written for this paper): wheat plants were grown with and without mycorrhizal fungi at two levels of phosphate fertiliser for 8 weeks (n = 15 per group).

Group	Mean dry mass / g	Mean phosphate in leaves / mg g⁻¹
Low P, no fungi	2.1	1.2
Low P, with fungi	3.4	2.3
High P, no fungi	3.8	2.9
High P, with fungi	3.9	3.0

A fertiliser company claims that 'adding mycorrhizal fungi is as good as adding phosphate fertiliser'. Calculate the percentage increase in mean dry mass caused by fungi at low phosphate.

Explain why fungi increase growth at low phosphate but not at high phosphate.

Evaluate the company's claim.

Explain why dry mass rather than fresh mass was measured.

Suggest how the plants should have been treated to make this a fair test.

Write an essay on ONE of the following topics. EITHER The importance of proteins in living organisms. OR How organisms respond to changes in their environment. You should select and use information from different parts of the specification.