Question [Maximum marks: 13]
Haemoglobinopathies are inherited conditions linked to the structure and function of
haemoglobin. Sickle cell anaemia is one of these conditions in which the transport and
delivery of oxygen to tissues is less than normal.
An investigation was carried out to discover the effect of sickle cell anaemia on the ability of
blood to carry oxygen. Blood samples were taken from two people:
• person L without sickle cell anaemia
• person M with sickle cell anaemia.
The percentage saturation of haemoglobin with oxygen was determined over a range of
partial pressures of oxygen.
Fig. 3.1 shows oxygen haemoglobin dissociation curves for the two blood samples.
(a) P50 is the partial pressure of oxygen at which haemoglobin is 50% saturated with
oxygen. It is taken as a measurement of the affinity of haemoglobin for oxygen.
(i) State the P50 for the two blood samples, L and M.
(ii) With reference to Fig. 3.1, describe how the dissociation curve for person M differs from the dissociation curve for person L.
(b) Explain the advantage of the position of the dissociation curve for people with sickle cell
anaemia.
(c) The partial pressure of oxygen in the lungs at sea level is about 13.5 kPa. At an altitude
of 3000 metres the partial pressure of oxygen in the lungs is about 7.5 kPa.
When people move from sea level to high altitude they become adapted to the low
partial pressure of oxygen.
Describe and explain how humans become adapted to the low partial pressure of
oxygen at high altitude.
(d)Vaccination is used to control the spread of diseases, such as measles.
Explain why vaccination cannot be used to prevent sickle cell anaemia.
Answer/Explanation
Answer: 3(a) (i) no mark if no units used at all
L – 3.6 kPa ; award the mark if units only used once
M – 4.5 kPa ; A in range 4.45 to 4.55
(ii) ignore any similarities
1 to the right / lower (affinity) / qualified ; e.g. lower percentage saturation
2 at, higher / lower, partial pressures, small(er) difference in percentage saturation (than others) ; A ora 3 comparative data quote ; must refer to L and M
allow ecf from (i)
3(b) 1 at partial pressures in the tissues ; where oxygen is unloaded from Hb
2 haemoglobin is less saturated (than L) ;
3 because, haemoglobin / Hb, dissociates more readily ;
A idea of unloading oxygen more readily even if Hb not mentioned
4 to compensate for, fewer / less effective, red blood cells / Hb ;
3(c) 1 haemoglobin less well saturated (in lungs at high altitude) ;
2 data quote from Fig. 3.1 ; A 80–90% saturated at ‘about 7.5 kPa’
3 produce more red blood cells / increase in number of RBCs ;
4 more haemoglobin ;
5 idea of compensates for, smaller volume of oxygen absorbed / lower saturation (of haemoglobin) ;
also accept the following adaptations
6 increase in haematocrit / AW / decrease in plasma volume ;
A increase in RBCs per unit volume
R decrease in blood volume
7 increase in, breathing rate / tidal volume / heart rate / stroke volume ;
8 increase in, capillary density / number of mitochondria / myoglobin / respiratory enzymes, in muscle ; 9 ref. to (increased) secretion of, erythropoietin / EPO ;
10 increase in (2,3), BPG / DPG, in red blood cells ; A rightward shift in curve [max 4]
3(d) 1 not caused by (named type of) pathogen / non-infectious / non-transmissible / non-
communicable / AW ; 2 genetic / inherited / AW, disease ; A caused by a mutation / AW
A ‘passed down from parent(s)’
R idea of congenital diseases
R ‘you get it from your mother’
3 ref. to, no immune response / no antigen(s) ;
4 affects all red blood cells so vaccine would lead to their destruction ; [max 2]