DP Biology (first assessment 2025)

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Question 22N.2.HL.TZ0.6

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Date November 2022 Marks available [Maximum mark: 15] Reference code 22N.2.HL.TZ0.6
Level HL Paper 2 Time zone TZ0
Command term Describe, Explain, Outline Question number 6 Adapted from N/A
6.
[Maximum mark: 15]
22N.2.HL.TZ0.6

Multicellular organisms benefit from cell specialization and division of labour.

(a)

Outline the processes occurring during interphase in the cell cycle.

[4]

Markscheme
  1. growth/increase in cell size;
  2. division of mitochondria/chloroplasts/production of more organelles/number of organelles doubled;
  3. replication of DNA/amount of DNA is doubled;
  4. transcription of genes/production of mRNA;
  5. protein synthesis;
  6. cell respiration/production of ATP;

Allow organelles ‘replicated’ for mpb.

Examiners report

Many candidates included growth and DNA replication in their answers. Other processes such as protein synthesis and cell respiration were mentioned less often.

(b)

Describe what occurs in a neuron when an action potential is propagated along the axon.

[4]

Markscheme
  1. sodium ions/Na+ enter/diffuse in;
  2. depolarization/membrane potential/voltage changes from negative to positive;
  3. potassium channels open AND potassium ions/K+ exit/diffuse out;
  4. repolarization/membrane potential/voltage changes back from positive to negative;
  5. local current due to diffusion of sodium ions along the neuron;
  6. (local currents) cause next sodium channels to open/next part of axon to depolarize;
  7. opening of sodium channels triggered when threshold potential/-50mV reached;

Do not award mpa for sodium being pumped in (rather than diffusing).
Allow mpc with ECF if direction of both sodium and potassium is wrong, or if both movements are described as pumping.

Examiners report

Answers were very varied and some were excellent. A few missed the point and described synaptic transmission. In some weaker answers there was confusion about the role of the sodium-potassium pump and what makes sodium and potassium ions move across the membrane during depolarisation and repolarisation. The mechanism of propagation by the formation of local currents was sometime attempted but rarely expressed clearly.

(c)

Explain how cells in the bloodstream cause a specific immune response.

[7]

Markscheme
  1. (specific immune response is) production of antibodies in response to a particular pathogen;
  2. antibody is specific to/binds to a specific antigen;
  3. macrophages/phagocytes engulf/present antigens from pathogens/viruses/bacteria;
  4. T lymphocytes activated by antigens/antigen presentation/antigens presented by macrophage;
  5. (activated) T lymphocytes activate B lymphocytes;
  6. only B lymphocytes that produce antibodies against the antigen/pathogen are activated;
  7. (activated) B lymphocytes clone/divide by mitosis to form plasma cells;
  8. plasma cells then secrete (large quantity) of an antibody/secrete antibodies of same type;
  9. some B lymphocytes/plasma cells form memory cells;
  10. memory cells give long-lasting immunity/faster response to a disease/pathogen;

Accept B and T cells instead of B and T lymphocytes.

Examiners report

Here again there was a wide range of answers. The best focussed throughout on how specificity in immune responses is achieved. The weakest were very muddled, with confusion between antigens and pathogens and between antigens and antibodies. Nearly all candidates mentioned memory cells and their role in long term immunity.

Syllabus sections