Question 21M.2.SL.TZ1.7b
| Date | May 2021 | Marks available | [Maximum mark: 7] | Reference code | 21M.2.SL.TZ1.7b |
| Level | SL | Paper | 2 | Time zone | TZ1 |
| Command term | Explain | Question number | b | Adapted from | N/A |
Explain the process of gas exchange taking place in the alveoli.
[7]
a. O2 diffuses into blood and CO2 diffuses out from blood ✔
b. blood entering the alveoli is high in CO2/low in O2 ✔
OR
air in alveolus is high in O2/low in CO2 ✔
c. diffusion (in either direction) take place due to concentration gradients ✔
d. concentration gradients maintained by ventilation/blood flow ✔
e. large surface area created by many alveoli/spherical shape of alveoli for more efficient diffusion ✔
f. rich supply of capillaries (around alveoli) allows efficient exchange ✔
g. type I pneumocytes are thin to allow easy diffusion/short distances ✔
h. gases must dissolve in liquid lining of alveolus in order to be exchanged ✔
i. type II pneumocytes secrete surfactants to reduce surface tension/prevent lungs sticking together ✔
j. type II pneumocytes create moist conditions in alveoli ✔
This question was attempted by approximately 56 % of candidates.
Some coherent, clear answers were given earning the maximum marks. However, commonly, valid ideas were given but were jumbled. Coherent writing was usually missing. The term diffusion was missing in many answers. The idea that alveoli are surrounded by capillaries to enable exchange of gases between the air in alveolus and the blood did not seem to be a mental picture for many. Concentration gradients as a reason for diffusion of oxygen or carbon dioxide between the alveoli and the blood was rarely explained. However, it seemed that everyone knew about surfactants from type II pneumocytes.
