Biology

1. 1.

   Primary producers capture 20000 kJ m^-2 year^-1. Herbivores gain 1800 kJ m^-2 year^-1 in biomass. Calculate energy-transfer efficiency and give two reasons why it is below 100%.

   [3 marks] · no calculator

   Answer explanation

   Draft walkthroughs are based on marking guidance, not independently verified derivations.

   1. The useful output is new herbivore biomass, so divide its energy by the producer energy input.
   2. 0.09 is a fraction; multiplying by 100 expresses it as a percentage.
   3. Energy is conserved overall, but not all incoming energy becomes biomass at the next trophic level.

   Marking points

   • Efficiency = (1800/20000) x 100 = 9%.
   • Some plant material is not eaten or is not digested and assimilated.
   • Respiration transfers energy to the surroundings as heat rather than new herbivore biomass.

   Examiner tip: Say energy is transferred or dissipated, not destroyed.

2. 2.

   Design an investigation into the effect of light intensity on photosynthesis in an aquatic plant using oxygen volume produced per minute. Explain how you would improve validity and reliability.

   [4 marks] · no calculator

   Answer explanation

   Draft walkthroughs are based on marking guidance, not independently verified derivations.

   1. Only light should change systematically. A lamp may heat the water, so use a controlled-temperature bath or heat shield.
   2. Collect gas volume rather than just counting bubbles: different bubble sizes make bubble counts less comparable.
   3. Acclimatisation reduces transient effects. Replication lets you assess variability; repeated results do not by themselves remove systematic bias.

   Marking points

   • Vary measured light intensity and measure oxygen volume over equal time intervals.
   • Control temperature, dissolved carbon dioxide and plant size/species.
   • Allow acclimatisation at each intensity before taking measurements.
   • Repeat measurements and calculate a mean, inspecting anomalous results.

   Examiner tip: Name the controlled variable and explain how it is controlled, not simply 'keep everything the same'.

3. 3.

   A monohybrid cross predicts a 3:1 phenotype ratio. Observed counts are 72 dominant and 28 recessive. Calculate chi-squared. Using a 5% critical value of 3.84 for one degree of freedom, interpret the result.

   [5 marks]

   Answer explanation

   Draft walkthroughs are based on marking guidance, not independently verified derivations.

   1. Compute expected frequencies from the proposed ratio, not from the observations being tested.
   2. Both deviations have magnitude 3. Square each and divide by its own expected count before adding.
   3. Assuming independent observations and suitable expected counts, the discrepancy is insufficient to reject this ratio at the stated threshold.

   Marking points

   • Expected counts are 75 and 25 out of 100.
   • Chi-squared = (72 - 75)^2/75 + (28 - 25)^2/25.
   • Chi-squared = 0.12 + 0.36 = 0.48.
   • 0.48 < 3.84, so do not reject the null hypothesis at 5%.
   • The data are consistent with 3:1; this does not prove the model is true.

   Examiner tip: 'Fail to reject' is not 'prove'. Check test assumptions before interpreting a significance test.

Original, unofficial practice questions from SubjectScout — not official past-paper questions.