DP Biology (first assessment 2025)

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Question 22N.2.HL.TZ0.b

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Date November 2022 Marks available [Maximum mark: 4] Reference code 22N.2.HL.TZ0.b
Level HL Paper 2 Time zone TZ0
Command term Describe Question number b Adapted from N/A
b.
[Maximum mark: 4]
22N.2.HL.TZ0.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.

Syllabus sections