Question 20N.3.HL.TZ0.23
| Date | November 2020 | Marks available | [Maximum mark: 9] | Reference code | 20N.3.HL.TZ0.23 |
| Level | HL | Paper | 3 | Time zone | TZ0 |
| Command term | Describe, Explain, State | Question number | 23 | Adapted from | N/A |
The graph shows the Bohr shift.
[Source: From The New England Journal of Medicine, Connie C.W. Hsia, Respiratory Function of
Hemoglobin, Volume 338(4):239–47. Copyright © 1998, Massachusetts Medical Society. Reprinted with permission
from Massachusetts Medical Society.]
Using the graph, explain the Bohr shift.
[3]
a. respiring tissues produce CO2;
b. CO2 leads to an increase in H+/decrease in blood pH;
c. increased acidity/decreased pH shifts the oxygen dissociation curve to the right;
d. affinity of the hemoglobin for oxygen is reduced;
e. greater release of oxygen from hemoglobin (at the same partial pressure of oxygen) in tissues;
Most candidates did well in this question, but strongest candidates distinguished themselves with complete and very detailed answers. Although candidates did well, there were many inaccuracies in some answers. Bohr shift is the shift to the right of the oxygen dissociation curve in respiring tissues; it does not refer to the reverse process at lung level.
State two modes of transport of carbon dioxide in blood.
1.
2.
[2]
a. dissolved/carried in plasma;
b. forms carbonic acid/H2CO3 (in plasma)
OR
as hydrogencarbonate (HCO3-) ions (in plasma);
c. binds to hemoglobin in red blood cells;
The majority of candidates did well also for this question, although some answers lacked accuracy.
State the effect of carbon dioxide in blood on the rate of ventilation.
[1]
increased carbon dioxide in blood increases the rate of ventilation
OR
positive correlation/relationship;
Most candidates knew that CO2 affects the rate of ventilation, but some did not specify that an increase in CO2 caused an increase in ventilation rate, or vice versa.
Describe what happens to the hemoglobin from old or damaged red blood cells that are broken down.
[3]
a. hemoglobin (from broken red blood cells) taken up by Kupffer cells (in the liver);
b. hemoglobin broken down into heme and globin;
c. globin hydrolyzed/broken down to amino acids;
d. iron removed from heme group
OR
heme broken down to form bilirubin/bile pigment;
Do not accept red blood cells broken down into heme and globin.
The majority of candidates described the process correctly; some stated incorrectly that red blood cells, instead of hemoglobin, are broken down into heme and globin.