(a)[3]
Outline how oxygen gets into the blood stream from an alveolus.
(b(i))[1]
Use Fig. 4.1 to calculate the difference in percentage saturation of haemoglobin at the lower partial pressure of oxygen of $2.7\,\text{kPa}$ compared with the higher partial pressure of $13.0\,\text{kPa}$. Show your working.
(b(ii))[3]
Explain why having a difference in percentage saturation of haemoglobin at lower and higher partial pressures of oxygen is advantageous.
(c)[2]
Define the term disease.
(d)[5]
Outline the differences between the $Hb^{A}$ (normal) and $Hb^{S}$ (sickle cell) alleles of the gene coding for the $\beta$-globin polypeptide and explain how those differences produce a change in the haemoglobin molecule formed.