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Date May 2021 Marks available 1 Reference code 21M.2.SL.TZ1.4
Level Standard level Paper Paper 2 Time zone Time zone 1
Command term Estimate Question number 4 Adapted from N/A

Question

Squid have been used for research on nerve impulses. The mechanism of nerve transmission in these animals is the same as that in humans. The image shows an oscilloscope trace from a nerve impulse in a squid’s giant axon.

[squid] Pixabay.

Estimate the resting potential for this axon.

[1]
a.

Outline the role of the sodium–potassium pump in maintaining the resting potential.

[2]
b.

Markscheme

−65 mV

Unit required; minus sign required. Accept answers from −62mV to −65mV.

a.

a. sodium/potassium pump pumps sodium ions Na+ out of axon and pumps potassium ions/K+ in ✔

b. requires energy/ATP/against concentration gradients/active transport ✔

c. three sodium ions pumped out for every two potassium ions pumped in ✔

d. results in charge difference between inside and outside where outside is positive relative to inside ✔

b.

Examiners report

[N/A]
a.

Strong students answered this question showing clear understanding of the process. However, most students showed poor understanding with errors: correct direction of ions being pumped not known; the direction of ions during repolarization was mixed up with resting state. Few wrote about the charge difference between inside and outside of the axon during the resting potential.

b.

Syllabus sections

Core » Topic 6: Human physiology » 6.5 Neurons and synapses
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