Malaria is an infectious disease caused by the protoctist, Plasmodium.
As part of its lifecycle, Plasmodium infects human red blood cells. Researchers can compare haemoglobin from the red blood cells of a healthy person with haemoglobin from a person with malaria.
(a) Throughout the world, most deaths from malaria are caused by P. vivax and P. falciparum.
Name one other species of Plasmodium that causes malaria.
Plasmodium ……
(b) In the laboratory, oxygen at different partial pressures can be bubbled through a solution of haemoglobin to determine the percentage saturation of haemoglobin at each partial pressure. A graph constructed from the results is known as an oxygen dissociation curve.
Fig. 5.1 is an oxygen dissociation curve for normal adult haemoglobin in humans.
(i) In the experiment used to obtain the results shown in Fig. 5.1, the temperature and pH were standardised.
Explain what the researchers would consider when deciding which temperature and pH to use in the experiment.
(ii) Using a different, more rapid technique, researchers compared the haemoglobin contained in red blood cells of a healthy person with the haemoglobin of a person with malaria who had been infected with P. vivax.
By analysing the results, the researchers concluded that the oxygen dissociation curve of a person with malaria would be shifted to the right.
With reference to Fig. 5.1, explain how a shift to the right of the oxygen dissociation curve would affect oxygen loading in the lungs, and unloading in respiring tissues, in a person with malaria.
(c) A red blood cell that is infected with Plasmodium cannot carry out its function as effectively as a normal red blood cell.
Describe how the size and structure of a red blood cell is related to its function, other than the fact that it contains a very large number of haemoglobin molecules.
▶️ Answer/Explanation
(a) ovale / malariae / knowlesi
Explanation: Besides P. vivax and P. falciparum, other Plasmodium species that cause malaria include P. ovale, P. malariae, and P. knowlesi. These species are less common but still significant causes of malaria infections worldwide.
(b)(i)
Answer: They would use pH approximately 7.4 (neutral/body pH) and temperature 37°C (body temperature).
Explanation: Researchers would choose conditions that mimic the human body’s internal environment because:
- Hemoglobin’s oxygen binding properties are sensitive to pH and temperature changes
- Using physiological conditions (pH 7.4, 37°C) ensures the results are biologically relevant to how hemoglobin functions in the human body
- These standard conditions allow for meaningful comparison with other studies
(b)(ii)
Answer: In a person with malaria:
- Hemoglobin has decreased affinity for oxygen
- Less oxygen is loaded in the lungs (lower saturation at same pO₂)
- Oxygen is released more readily in tissues (unloading occurs at higher pO₂)
Explanation: A rightward shift in the oxygen dissociation curve indicates:
- In the lungs (where pO₂ is high), hemoglobin doesn’t bind oxygen as efficiently, resulting in less oxygen being loaded onto hemoglobin molecules.
- In the tissues (where pO₂ is lower), oxygen is released more readily from hemoglobin due to its decreased affinity, which might seem beneficial but actually reflects impaired oxygen transport.
- This shift means higher partial pressures of oxygen are needed to achieve the same percentage saturation as normal hemoglobin, indicating overall reduced oxygen transport efficiency.
(c)
Answer:
- Small size (6-8μm diameter) allows passage through narrow capillaries
- Biconcave shape increases surface area for gas exchange
- Flexible membrane enables squeezing through capillaries
- Lack of nucleus and organelles maximizes space for hemoglobin
Explanation: The structure of red blood cells is perfectly adapted for their oxygen transport function:
- The small diameter (6-8μm) is ideal for moving through the narrowest capillaries, ensuring oxygen delivery to all tissues. Their single-file movement slows blood flow slightly, allowing more time for oxygen exchange.
- The biconcave disc shape provides several advantages: it increases surface area for gas exchange, reduces diffusion distance to hemoglobin molecules, and creates flexibility.
- The flexible cell membrane allows red blood cells to deform and squeeze through capillaries smaller than their resting diameter, then return to their original shape.
- The absence of a nucleus and most organelles (including mitochondria) serves two purposes: it maximizes space for hemoglobin, and prevents the cell from using the oxygen it’s transporting for its own metabolism.