DP Biology (last assessment 2024)

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Question 22M.2.SL.TZ1.5

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Date May 2022 Marks available [Maximum mark: 7] Reference code 22M.2.SL.TZ1.5
Level SL Paper 2 Time zone TZ1
Command term Describe, Explain, State Question number 5 Adapted from N/A
5.
[Maximum mark: 7]
22M.2.SL.TZ1.5

The graph shows the change in the membrane potential of an axon during an action potential.

(a)

State the approximate value of the membrane potential at X.

[1]

Markscheme

0 mV; (accept answers in the range of – 10 mV to +10 mV) (Units required)

Examiners report

There were some truly brilliant answers to Question 5. However, the weakest answers in the examination were also found here. Of the questions, these were the most commonly left blank.

Candidates were often unable to approximate X. Missing units resulted in no marks awarded.

(b)

Y is the threshold potential. State what happens when the threshold potential is reached.

[1]

Markscheme

sodium channels (start to) open
OR
depolarization/axon begins to depolarize
OR
action potential occurs;

Reject pumping of ions.

Examiners report

Fundamental terms such as action potential or depolarization were not used in Q5(b) and (c). Some candidates failed to distinguish between elements and ions.

(c)

Describe the movements in ions that occur during time t.

[2]

Markscheme

Na+/sodium ions diffuse into the axon (in the first part/half of t);
K+ /potassium ions diffuse out of the axon (in the second half/part of t)

Do not accept the name of the element without indication that it is an ion.

Examiners report

Some candidates talked about ion movement but with no reference to a specific ion; some described the Na+/K+ pump here and received no marks.

(d)

Explain how a nerve impulse is passed on to other neurons.

[3]

Markscheme
  1. impulses pass to another neuron at a synapse/across synaptic gap/cleft;
  2. (depolarization causes) Ca2+/calcium ions to diffuse into the (presynaptic) neuron/axon;
  3. depolarization (of presynaptic neuron) causes release of a neurotransmitter
    OR
    neurotransmitters diffuse across the synapse;
  4. (neurotransmitters) bind to receptors on postsynaptic neuron/membrane;
  5. (if the threshold potential is reached) an action potential occurs/sodium gates open (in the postsynaptic neuron);
Examiners report

Use of term "synapse" was often missing; involvement of the brain was sometimes built into the passage of a nerve impulse to another neuron.

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