Exam mistakes PAPER 2 from Examiners' report

• • The role of the sympathetic nervous system in increasing the heart rate was the most frequently credited mark point.
  • However, the increase in frequency of impulses from the medulla was only described in the best answers.
• Introducing Sterile insects (Sterile insect technique) reducing transmission of dengue.

  • The idea of reducing the success of reproduction of the mosquito was well understood by the vast majority of students.
  • Any reference to COMPETITION was far less often included, resulting in only approximately 10% of students gaining both marks.
• Describe how the mark-release-recapture method could be used to determine the population of A. aegypti at the start of the investigation. [3 marks]

  • The most common omission was not mentioning leaving sufficient time for the mosquitoes to disperse before second sampling.
  • A significant minority also failed to refer to releasing the mosquitoes after first sampling.
• • The most common correct response referred to the greater range of diameters of muscle fibres in adult mice. Better answers often provided a numerical comparison of the ranges, to score two marks
  • Common error was referring to Modal value as mean values (Read the graph carefully)

  
• Explain why students set up Tube 1 [2 marks]

  • Many answers simply referred to Tube 1 as being a control, or that it was set up to allow comparison with other tubes.
  • The idea that Tube 1 shows that light does not affect DCPIP was less often mentioned.

  
• • Slightly more than 40% of students obtained this mark, often by stating that different chemicals or concentrations of chemicals could be compared as weed killers.
  • Fewer students referred to using IC50 as a standard or benchmark.
  • Responses which did not gain credit often simply stated ‘for comparison’, WITHOUT QUALIFICATION, or simply repeated the information provided in the stem of the question.
• • Many students provided responses which showed understanding that the light-dependent reaction would not take place, and hence neither would the Calvin cycle. - this was then linked to glucose for growth
  • However, many of these answers failed to mention the products of the light-dependent reaction.
  • Better answers did refer to ATP and/or reduced NADP not being produced for use in the light-independent reaction.
  • Relatively few students specified that the reduction of GP to form TP would not take place.
• • Most students did not refer to biomass as representing the DRY MASS or mass of carbon in a plant.
  • Alternatives such as the chemical energy store were also rarely credited.
  • Weaker responses also included loss due to faeces as part of this equation
• • The lack of responsiveness to insulin was not always linked to CELLS or RECEPTORS, or was described as a ‘resistance’ or ‘immunity’ to insulin.
  • Although students often understood that diet and lack of exercise may be involved in causing type II diabetes
  • they did not always suggest that exercise and diet may be used in treating type II diabetes.
• • The definition of phenotype was generally well known with over 90% of students obtaining at least one mark.
  • Students scoring zero usually provided a GCSE standard answer in terms of the phenotype representing an observable feature.
  • Responses awarded a single mark usually described the genetic influence on the phenotype.
  • Better answers also included the effect of the environment, to gain both marks.
• Describing the role of DNA polymerase in Reverse transcriptase-PCR. [1]

  Many other students omitted any reference to either DNA or nucleotides; these incomplete descriptions were not credited.
• Any DNA in the sample is hydrolysed by enzymes before the sample is added to the reaction mixture. Explain why. [2 marks]

  • Some responses did suggest that the hydrolysis of DNA would ensure that only the RNA would then lead to amplification of DNA
  • The mark scheme required students to clearly state that DNA was removed or ‘destroyed’.
• • Almost 60% of students obtained at least one mark, often for stating that the primers are ‘COMPLEMENTARY’.
  • However, when referring to the RNA viruses, many students often omitted either the term ‘base’ or ‘sequence’, so that only one in five students gained both marks.
• Gene pool - All the ALLELES in a population
• • When students did describe sympatric speciation,
  • relatively few mentioned that it occurs in the same area/place/population
  • Fewer still referred to disruptive selection
• • The most frequently credited mark points related to reproductive isolation, described in a variety of ways
  • Providing a definition of a species, i.e. the inability to breed to produce fertile offspring
  • Better students did appreciate that a mutation would have led to a difference in flowering times, and that different alleles would be passed on in the reproductively isolated subpopulations.
• Suggest and explain two reasons why there is a high frequency of the E280A mutation in Yaramul (isolated place)

  • The most frequently credited response was that isolation had resulted in a small gene pool or low genetic diversity / inbreeding
  • Invariably, students referred to the gene or mutation, rather than the ALLELE being inherited through generations from a (common) ancestor.
• The age at which the E280A mutation is expressed to cause AD can vary (lines 11–12). Suggest an explanation [2 marks]

  • Over a third of students obtained one mark, usually by naming an environmental factor such as diet, smoking or stress.
  • Answers specifically referring to epigenetics for at least one mark were infrequent.
  • These responses often gained a second mark, 10% of students, for mentioning methylation or acetylation.
  • Students who described these processes generally provided correct details
• • Explaining why there were only 74 potential AD cases when 75 mutations had been detected proved very challenging, with only 2% of students gaining this second mark.
  • A common misconception was that one individual was heterozygous for the condition.
  • It is actually one person that is homozygous dominant