Question 19M.2.SL.TZ1.b
| Date | May 2019 | Marks available | [Maximum mark: 3] | Reference code | 19M.2.SL.TZ1.b |
| Level | SL | Paper | 2 | Time zone | TZ1 |
| Command term | Explain | Question number | b | Adapted from | N/A |
Explain how gases are exchanged between the air in the alveolus and the blood in the capillaries.
[3]
a. O2 concentration in alveolar air greater than in capillary/blood «prior to gas exchange»
OR
hemoglobin in blood binds oxygen maintaining the concentration gradient ✔
b. O2 gas dissolves in water lining the alveolus ✔
c. O2 diffuses through wall of alveolus and capillary into blood ✔
d. CO2 concentration in blood greater than in alveolar air «prior to gas exchange» ✔
e. CO2 diffuses through wall of capillary and alveolus into alveolar airspace ✔
The command term was “explain”. Only a few candidates actually explained the process. Candidates gave very general responses without reference to diffusion or the correct concentration gradients. Very often diffusion was referred to in general for gas exchange and it was not linked to concentration gradients. Some students got max 2 for correct references to diffusion with reference to both gases. Students getting max 3 were able to include references to correct concentration gradients as well.
