Question

Unloading of oxygen from haemoglobin is enhanced under the following conditions.
A. Increase in pH, increase in $C{O_2}$, decrease in temperature
B. Decrease in pH, increase in $C{O_2}$, increase in temperature
C. Decrease in pH, decrease in $C{O_2}$, increase in temperature
D. Increase in pH, decrease in $C{O_2}$, decrease in temperature

Answer 1

Haemoglobin binds oxygen in a cooperative manner. Loading of oxygen occurs when haemoglobin has a high affinity for ${O_2}$ and unloading occurs when affinity becomes low.

Complete answer: The question can be solved easily if we understand the haemoglobin-oxygen dissociation curve. It is a curve that plots the oxy-haemoglobin on the vertical axis against the partial pressure of oxygen $\left( {P{O_2}} \right)$ on the horizontal axis. When the values are plotted, it gives a sigmoid curve. The sigmoid form results from the allosteric interactions of the globin proteins as they bind oxygen molecules. The affinity of haemoglobin to oxygen can be influenced by many factors. The factors cause the curve to shift towards the left or towards the right. Shifting to the left indicates increased affinity to oxygen while shifting to the right indicates decreased affinity to oxygen.
Factors that affect the curve’s position are as follows:
a) Temperature: When it is increased, the curve shifts to the right. If decreased, the curve shifts to left.
b) pH: If pH is increased, the curve shifts to left and in low pH it moves to the right.
c) $C{O_2}$: It leads to the formation of bicarbonate ions (this decreases pH). So, if $C{O_2}$ is high, the curve shifts to the left and when low, it shifts to the right.
Unloading occurs only when the affinity for oxygen remains low i.e. when the curve moves to the right.
So, the correct answer is option C.

Note: Up to four oxygen molecules bind to any haemoglobin molecule. It takes place by cooperative binding. ${O_2}$ binding enhances haemoglobin's affinity for more ${O_2}$ molecules. A conformational transition in the haemoglobin molecule induces increased affinity.