AS Papers - Experimental design

• The scientist used units of µmol g−1 h−1 for the rate of oxygen uptake. Suggest why he used µmol in these units. [1 mark]

  • (Measures) small uptake / amount / quantity / volume / concentration / rate of oxygen uptake
  • OR Avoids use of powers of ten / standard form / many decimal places
• The scientist decided to use the ratio of surface area to mass, rather than the ratio of surface area to volume. He made this decision for practical reasons.

  Suggest one practical advantage of measuring the masses of frog eggs, tadpoles and adults, compared with measuring their volumes. [1 mark]

  • More accurate / less error (in measuring mass)
  • OR Causes less distress / damage to animal (to measure mass)
  • Easier / quicker to find mass BECAUSE irregular shapes;
  • Fewer measurements / calculations;
• Explain why oxygen uptake is a measure of metabolic rate in organisms. [1 mark]

  • Oxygen used in respiration, which provides energy / ATP;
  • Oxygen is used in respiration, which is a metabolic process / chemical reaction;
• A student who looked at these results said that they could not make a conclusion about the relationship between stage of development and metabolic rate. Use information in Table 2 to explain reasons why they were unable to make a conclusion. [3 marks]

  1. No information about egg
  2. So cannot compare ALL STAGES in Table 2
  3. No statistical information / test / t-test / comparison of standard deviations OR No measure of significant differences;

  
• Explain why the student: [2 marks]

  • used only the first 5 mm from the tip of an onion root
  • Where dividing cells are found / mitosis occurs
  • OR No dividing cells / mitosis in tissue further away / more than 5mm from tip
  • OR To get (soft) tissue that will squash
  • OR Length that will fit under cover slip;
• A student prepared a stained squash of cells from the tip of an onion root and box observed it using an optical microscope.

  Explain why student pressed down firmly on the cover slip [1 mark]

  • 2. Single / thin layer of cells / spread out cells SO
  • light passes through making cells / nuclei visible
• Describe and explain what the student should have done when counting cells to make sure that the mitotic index he obtained for this root tip was accurate. [2 marks]

  1. Examine large number of fields of view / many cells
  2. To ensure representative sample
  3. Repeat count
  4. To ensure figures are correct
  5. Method to deal with part cells shown at edge /count only whole cells
  6. To standardise counting;