A-LevelBiologyCell membranes and transport9700/23 · Oct/Nov 2022 · Q64 Marks

Fig. 6.1 shows some of the events that occur when a red blood cell flows through a capillary in the lungs. [Figure 6.1]

(a)State why there are transport proteins in the membranes of red blood cells to allow the movement of hydrogencarbonate ions (HCO3¯) and chloride ions (Cl¯).[1]
(b)Fig. 6.1 shows that chloride ions move out of the red blood cells. Explain why this movement is necessary when red blood cells flow through capillaries in the lungs.[1]
(c)State why carbon dioxide molecules diffuse from the red blood cells into the plasma.[1]
(d)State the name of the compound indicated by X.[1]

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Mark scheme

Marking points from the official mark scheme (Oct/Nov 2022). The mark or mark type is shown in brackets.

[1] (ions) cannot pass through, phospholipid bilayer / hydrophobic core (of membrane) ; ora
[1] (transport proteins provide a) hydrophilic pathway for ions ;
[1] for facilitated diffusion (of the two ions) ;
[1] to maintain electrical neutrality (inside cytoplasm / cell) ;
[1] concentration of carbon dioxide is lower than concentration in red blood cell ;
[1] carbon dioxide, diffuses / AW, down a concentration gradient ;
[1] so carbon dioxide can be, excreted / expelled from lungs / breathed out ;
[1] to enter the alveolus / alveolar (air) space ;
[1] as part of gas exchange, at the alveolar surface / in the lungs ;
[1] X = haemoglobinic acid ;

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