Practical skills and required practicals

In a root tip squash, 24 of 160 cells were in a stage of mitosis. Calculate the mitotic index as a percentage.

A student placed potato cylinders in sucrose solutions and recorded the percentage change in mass (Figure 2). Use Figure 2 to estimate the water potential of the potato tissue as a sucrose concentration, and explain how you used the graph.

Explain why the student calculated percentage change in mass rather than change in mass.

In a chromatogram of leaf pigments, a pigment travelled 3.2 cm and the solvent front travelled 8.0 cm. Calculate the Rf value and explain how it could identify the pigment.

In a respirometer containing germinating seeds and potassium hydroxide solution, a coloured liquid moved 25 mm along a capillary tube of radius 0.5 mm in 10 minutes. (a) Explain why the liquid moved towards the seeds. (b) Calculate the rate of oxygen uptake in mm³ min⁻¹. Volume = πr²l.

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

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

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

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

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.

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.

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

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

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

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.

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.

Describe the emulsion test for lipids.

In a potometer, an air bubble moved 24 mm in 5 minutes along a capillary tube of radius 0.5 mm. Calculate the rate of water uptake in mm³ min⁻¹. (Volume = πr²l)

Isolated chloroplasts were added to blue DCPIP solution. The DCPIP decolourised in the light but not in the dark. Explain these results.

In a mark–release–recapture study, 40 beetles were marked. Later 50 were caught, of which 8 were marked. Estimate the population size.

Investigation (written for this paper): beetroot discs of equal size were washed and placed in water at different temperatures for 30 minutes. The absorbance of the water was measured with a colorimeter.

Temperature / °C	20	40	60	80
Mean absorbance	0.08	0.12	0.54	0.91

Explain why the discs were cut to the same size and washed before the experiment.

Calculate the percentage increase in mean absorbance between 40 °C and 60 °C.

Explain the results between 40 °C and 80 °C.

Give two advantages of using a colorimeter instead of comparing colours by eye.

Give two variables, other than temperature and size of disc, that should be controlled.

Describe the aseptic techniques used when spreading bacteria onto an agar plate.

An antibiotic disc produced a clear zone with a diameter of 18 mm. Calculate the area of the clear zone.

Explain why a disc soaked in sterile water was also placed on the plate.

Describe how you would cut a transverse section of a plant stem and prepare it to observe xylem.

Give one safety and one ethical consideration when carrying out a dissection of an animal organ.

Describe how a student could estimate the percentage cover of a plant species in a field using random sampling.

Percentage cover in 6 quadrats was 12, 0, 25, 8, 30 and 15. Calculate the mean percentage cover.

Explain why a belt transect would be more suitable than random sampling to study how the plant's distribution changes away from a hedge.

Woodlice were placed in a choice chamber with damp and dry sides. Describe how the student should set up the choice chamber.

Of 30 woodlice, 22 moved to the damp side and 8 to the dry side. Calculate the chi-squared value and state whether the result is significant. (Critical value at P = 0.05 for 1 degree of freedom = 3.84.)

Give two ethical precautions when using woodlice in this investigation.

Critical analysis (written for this paper): 20 untrained adults completed 12 weeks of endurance training. Measurements at rest before and after:

	Before	After
Heart rate / beats min⁻¹	74	62
Stroke volume / cm³	70	85
Standard deviation of heart rate	6.1	5.8

The researchers concluded that 'endurance training makes the heart more efficient'. Calculate the resting cardiac output before and after training, in dm³ min⁻¹.

Evaluate the researchers' conclusion.

Name a suitable statistical test to find whether the change in heart rate is significant, and give a null hypothesis.

Suggest four improvements to the design of this study.

Describe the biuret test for proteins.

Product formed by an enzyme (data are illustrative, written for this paper):

Time / s	0	10	20	30
Product / µmol	0	18	30	36

Calculate the initial rate of reaction and explain why the rate decreases over time.

40 eyepiece graticule divisions match 100 µm on a stage micrometer. A cell is 18 divisions long. Calculate its length.

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.

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.