Fig. 6.1 is a ribbon model of a molecule of haemoglobin. The effect of the partial pressure of oxygen (pO2) and the effect of the partial pressure of carbon dioxide (pCO2) on the percentage saturation of haemoglobin was investigated. A sample of mammalian blood was exposed to a gas mixture that contained increasing pO2. In the experiment, the pCO2 was maintained at 2.7kPa. The percentage saturation of haemoglobin in the blood sample was determined as the pO2 increased. The experiment was repeated with further samples of blood with a pCO2 maintained at 5.3 kPa and at 10.7 kPa. The results are shown in Fig. 6.2. [Figure 6.1] [Figure 6.2]

Mark scheme

Marking points from the official mark scheme (May/June 2025). The mark or mark type is shown in brackets.

1. [1] (a)(i) haem (group); I Fe2+ I prosthetic group
2. [1] (a)(ii) (iron in haem group) combines / binds, with (one) oxygen (molecule) ; A carries / transports / attaches, oxygen A 'bonds with' if stated as iron ion / Fe2+ / ferrous ion R oxygen atom /2 or more oxygens R carbon dioxide binds
3. [1] (a)(iii) more than one polypeptide (chain); A 2 or more R more than 2 / many / multiple A haemoglobin has four polypeptides I amino acid chain
4. [1] (b)(i) accept values within the range 10–14 kPa ; A as a range or as a single figure
5. [1] (b)(ii) accept any range between 1–6 kPa ; A within range 0–6 kPa R a single figure
6. [1] (b)(ii) ref. to (steep) decrease in percentage saturation of haemoglobin as pO2 decreases ; R increasing saturation A (oxy)haemoglobin, dissociates / releases its oxygen A haemoglobin has a low affinity for oxygen
7. [1] (b)(iii) 1 (dissociation) curve shifts to the right; I curve shifts downwards / graph if given for curve
8. [1] (b)(iii) 2 (percentage) saturation (of haemoglobin) decreases (as pCO2 increases) ; A decrease affinity of Hb for O2
9. [1] (b)(iii) 3 comparative data quote giving percentage saturation at the same pO2 and at two different values of pCO2; A percentage of oxygen released
10. [1] (b)(iii) 4 any ref. to difference between position of curves, e.g. large difference in percentage saturation in middle of the range / little difference at high pO2;
11. [1] (b)(iv) Bohr, shift / effect ;
12. [1] (b)(v) supplies more oxygen (to respiring tissues); A more oxygen is released / AW I quicker / faster I haemoglobin releases oxygen more readily
13. [1] (b)(v) idea that allows oxygen to be supplied to, (named) tissues / cells, to meet demand / as demand increases ; (maintains) aerobic respiration ;