A-LevelBiologyControl and coordination9700/42 · May/June 2025 · Q711 Marks

An axon membrane is described as being at its resting potential when an action potential is not occurring.

(a)Describe and explain how a resting potential of an axon membrane is maintained.[6]
(b)(i)Hypokalaemia is a condition in which there is a low concentration of potassium ions (K+) in the body. This can affect nervous coordination. Fig. 7.1 shows a normal action potential and Fig. 7.2 shows an action potential of a person with hypokalaemia. [Figure 7.1] [Figure 7.2] With reference to Fig. 7.1 and Fig. 7.2, describe the differences between a normal action potential and an action potential of a person with hypokalaemia.[3]
(b)(ii)Suggest how hypokalaemia may affect nervous coordination.[2]

This question uses a diagram from the original paper, which isn't shown here. Open the full paper from the past papers archive to see it.

Mark scheme

Marking points from the official mark scheme (May/June 2025). The mark or mark type is shown in brackets.

[6] (a) any six from: 1 sodium-potassium pumps (in axon membrane) ; 2 transport, sodium ions / Na+, out and, potassium ions / K+, in ; 3 by active transport / uses ATP ; 4 three, sodium ions / Na+, and two, potassium ions / K+ ; 5 sets up an electrochemical gradient; 6 potassium ions / K+, diffuse out through channels ; 7 membrane more permeable to K+/membrane has more K+ channels ; 8 (so) more, potassium ions / K+, move out of axon than, sodium ions / Na⁺, move in ; 9 ref. to anions / negatively charged ions / negatively charged proteins, inside ; 10 inside negative (relative to outside); 11 -60 mV to -70 mV ;
[3] (b)(i) any three from: hypokalaemia ora for normal 1 longer, action potential / depolarisation / repolarisation or larger depolarisation ; 2 longer hyperpolarisation or more negative / smaller, hyperpolarisation ; 3 longer refractory period; 4 more negative / lower, resting potential; 5 data quote ; (e.g. mp1: normal AP 2.6 ms vs hypo AP 3.25 ms or normal depolarisation 1.0ms vs hypo depolarisation 1.25ms or normal repolarisation 1.0ms vs hypo repolarisation 1.3ms or normal depolarisation 110mV vs hypo depolarisation 140mV; mp2: normal hyperpolarisation 0.7ms vs hypo hyperpolarisation 1.0ms or normal -92mV vs hypo -112mV; mp4: normal resting potential -70mV vs hypo resting potential -100mV)
[2] (b)(ii) any two from: 1 harder stimulus / larger stimulus / larger depolarisation, needed, to reach threshold required / to generate an action potential; 2 fewer, impulses / action potentials, transmitted or lower frequency of impulses ; 3 slower, nervous coordination / reaction times / responses / cognition or weaker, sensation / muscle contraction ; allow named examples