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. (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. [Figure 5.1]

Mark scheme

Marking points from the official mark scheme (Oct/Nov 2024). The mark or mark type is shown in brackets.

1. [1] ovale / malariae ; A knowlesi must be spelled correctly if both given, must be correct spelling
2. [2] any two from: details ; pH approx. 7.4 and 37 °C A pH7 / neutral pH A 36/38 °C pH of, plasma / blood or body temperature or use conditions occurring in, body / plasma / blood / humans ; idea that oxygen uptake / haemoglobin activity / AW, affected by, temperature / pH ;
3. [3] any three from: in a person with malaria 1 affinity of haemoglobin for oxygen, decreases / AW ; context is infection so R in presence of carbon dioxide 2 lower (percentage) saturation of haemoglobin with oxygen ; allow ecf from mp1 A less oxygen binds to haemoglobin (in the lungs) A haemoglobin loads less oxygen AW 3 idea that more difficult to load oxygen in the lungs ; I slower 4 (as) higher partial pressure of oxygen needed to reach same percentage saturation of, haemoglobin ; 5 idea that oxygen released more readily in respiring tissues or in tissues, more oxygen released at any one partial pressure ; I carbon dioxide but R if stated that carbon dioxide causes oxygen to be released more readily
4. [4] any four from: 1 (diameter / width) 6–8 µm so able to pass through (lumen of) capillaries; I small size 2 small size / small diameter / just fits in a capillary / AW, so in a line / one by one / AW, which slows blood flow / maximises time to take up oxygen / reduces diffusion distance for uptake of oxygen ; A gas exchange for oxygen 3 no, nucleus / mitochondria ; A no organelles 4 so more space for haemoglobin ; AW must be linked to mp3 5 no mitochondria so oxygen is not used within cell (and can be transported) ; 6 flexible / can squeeze through / can deform / can change shape, to pass through capillary / AW ; context of biconcave or property of cell surface membrane I 'squeeze between' unless elsewhere there is ref. to rbc within a capillary 7 biconcave ; 8 (biconcave, so compared to spherical) increased, surface area / SA:V, for uptake of oxygen / gas exchange / AW ; 9 (biconcave, so) reduce distance for diffusion of oxygen to, haemoglobin / hb ; V;