A-LevelBiologyTransport in mammals9700/22 · Oct/Nov 2020 · Q68 Marks

(a) Fig. 6.1 shows an oxygen dissociation curve for adult human haemoglobin. [Figure 6.1] An increase in the partial pressure of carbon dioxide (pCO2) in respiring tissue causes the Bohr effect.

(a)(i)Sketch on Fig. 6.1 to show how the Bohr effect changes the oxygen dissociation curve.[1]
(a)(ii)Explain how an increase in pCO2 produces the Bohr effect and state the benefit of this effect for the tissue.[3]
(b)Carbon dioxide (CO2) is transported across the cell surface membrane of the red blood cell using a different mechanism to the transport of hydrogen carbonate ions (HCO3¯). Name the different mechanisms of transport used for CO2 and for HCO3¯ and explain why they are transported across the membrane by different mechanisms. CO2 HCO3¯ explanation[4]

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Mark scheme

Marking points from the official mark scheme (Oct/Nov 2020). The mark or mark type is shown in brackets.

[1] same shape and shift to the right ;
must start to rise within 2 small squares of 0 and end at or near to existing curve
[3] max 2 if concept of more not mentioned in response
any three from:
1 actively respiring tissue means more carbon dioxide;
2 increase in, formation of carbonic acid / dissociation of carbonic acid / hydrogen ions, (in the red blood cell); A from equation
3 more hydrogen ions, bind to haemoglobin / form haemoglobinic acid;
A HHb
4 (causes) more oxygen (to be), unloaded / dissociated / AW (from haemoglobin);
I ref. to faster / quicker
I incorrect ref. to affinity causing unloading, e.g. CO2
A lower (percentage) saturation of haemoglobin with oxygen
A oxygen released, more easily / readily from haemoglobin
5 haemoglobin affinity for oxygen decreases ;
6 more oxygen to meet demand for (aerobic / cellular) respiration ; AW
I more oxygen for respiring tissues
[4] CO2 = (passive / simple) diffusion ;
HCO3¯ = facilitated diffusion ;
I ref. to size
CO2 is, non-polar / not charged / not ionic and can cross, hydrophobic core / phospholipid bilayer ;
HCO3¯ is, charged / ionic / hydrophilic and (needs to) cross, via, transport/carrier, protein ; A channel protein
if explanation mps not gained, allow 1 mark for CO2 is, non-polar/not charged / not ionic and HCO3¯ is, charged / ionic / hydrophilic

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