Question 21N.2.HL.TZ0.3
| Date | November 2021 | Marks available | [Maximum mark: 5] | Reference code | 21N.2.HL.TZ0.3 |
| Level | HL | Paper | 2 | Time zone | TZ0 |
| Command term | Explain, Outline, State | Question number | 3 | Adapted from | N/A |
The image shows human red blood cells.
[Source: someoneice/123rf.com.]
Outline what will happen to human red blood cells if transferred to distilled water.
[1]
cells absorb water by osmosis and swell/increase in volume
OR
cells burst/lyse;
This was mostly well answered with candidates realising that water entry due to osmosis would cause the cells to burst.
Stem cells can be used to treat Stargardt’s disease. State one other condition treated using stem cells.
[1]
leukemia/other diseases of the hematopoietic system / skin burns;
A range of conditions were suggested, most of which are not currently treatable using stem cells. Diabetes and neurological diseases were not accepted. 60 % of candidates gave an acceptable answer, with leukaemia much the commonest. The correlation between the mark on this question and overall scores was fairly low, which may suggest that inappropriate examples have been learned in some schools. It is important not to raise hopes of stem cell treatment for specific diseases when research is still continuing and regulatory approval is unlikely for many years.
Explain the propagation of nerve impulses along the membrane of a neuron.
[3]
a. depolarization of part of axon/membrane triggers/causes depolarization of next part;
b. local currents;
c. diffusion of sodium ions between depolarized part and next/polarized part (of axon);
d. resting potential reduced/polarization of membrane becomes less /change from -70 to -50mV;
e. sodium channels open when -50mV/threshold potential reached;
f. entry of sodium ions causes depolarization;
g. saltatory conduction in myelinated neurons/axons;
Allow answers in an annotated diagram
This was a more difficult question with a much higher correlation coefficient. Many candidates did not understand what is meant by propagation of an impulse along the membrane of a neuron and answers tended to include accounts of the whole sequence of events in an action potential or in synaptic transmission. The mark most frequently awarded was for the mechanism of saltatory conduction.
