Paper 3 (all topics, practical skills and essay)

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.

Explain the changes in heart rate and stroke volume after training.

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.

Write an essay on ONE of the following topics. EITHER The importance of membranes in living organisms. OR The ways in which organisms use energy released from ATP. You should select and use information from different parts of the specification.