The electron micrograph shows a section through an alveolus showing blood in a capillary.

[Source: © International Baccalaureate Organization 2019]

Identify the cells found at X and Y.

X: 

Y:

Explain, with the aid of an annotated diagram, how physical exercise affects the affinity of hemoglobin for oxygen. 

a. X: «type I» pneumocyte ✔ 

b. Y: endothelial cells ✔

Accept endothelium/capillary wall

a. diagram showing normal oxygen dissociation curve ✔

b. diagram showing curve with increased CO₂ to the right ✔

c. both axes correctly labelled ✔

d. where tissues are respiring there is a higher concentration of CO₂ ✔

e. exercise increases the amount of CO₂ in the blood ✔

f. an increase in CO₂ lowers the pH of the blood ✔

g. a lower pH causes hemoglobin to release oxygen ✔

h. lower pH decreases hemoglobin affinity for O₂/changes hemoglobin conformation ✔

i. oxygen is released in tissue where it is required for respiration ✔

j. this is known as the Bohr effect/shift ✔

k. at the lungs the low concentration of CO₂ means oxygen attaches to hemoglobin ✔

l. «Bohr» effect particularly important during exercise ✔

Apply [4 max] if no diagram.

Many students did correctly identify that X was a pneumocyte I, but very few correctly identified cell Y as an endothelial cell. Many assumed that the second cell would be a pneumocyte II.

Students who did well on this question tended to achieve all marking points, including correct information on the graphs. These students also composed detailed explanations, including the role of reduced pH and its effect on affinity. Some students remain unsure about how, where and why CO₂ is produced and how this changes hemoglobin affinity for oxygen. Weaker students did not realise that the annotated diagram referred to in the question would be the dissociation curve; they often included caricatures of people exercising or attempts at drawing hemoglobin.