A-LevelBiologyGas exchange9700/21 · May/June 2023 · Q511 Marks

Fig. 5.1 is a longitudinal section of a capillary in muscle tissue as viewed with a transmission electron microscope.

(a)State the evidence, visible in Fig. 5.1, that identifies the cells inside the capillary as red blood cells.[2]
(b)Explain how the structure of the capillary wall is related to its functions.[3]
(c)(i)An increase in respiration results in an increase in the carbon dioxide concentration in the blood and the release of more oxygen from red blood cells to tissues. Explain how an increase in carbon dioxide in the blood leads to the release of more oxygen from red blood cells.[4]
(c)(ii)Chloride ions are a constituent of blood plasma. The concentration of chloride ions in the plasma of deoxygenated blood is between 2–4 mmoldm⁻³ lower than in the plasma of oxygenated blood. Explain why the concentration of chloride ions in the blood plasma of deoxygenated blood is lower than in the plasma of oxygenated blood.[2]

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Mark scheme

Marking points from the official mark scheme (May/June 2023). The mark or mark type is shown in brackets.

[2] any two from: no nucleus ; no organelles / uniform appearance / homogenous cytoplasm ; (some are) biconcave shape / described ; A cells have different shapes same, width / size, as the lumen of the capillary; A same width as capillary / diameter of cells is 6 - 7 µm R colour / shading / AW
[3] any three from: 1 (wall is) thin / one cell thick / 1–2 µm in thickness ; R 'thin cell wall' I tunica intima 2 ref. to, endothelial cells / endothelium ; I squamous / epithelium 3 short distance for diffusion ; 4 endothelial pores / fenestrations or pores / gaps, between / within, (endothelial) cells of the wall ; 5 for passage of, (named) small molecules / AW, from / to, plasma / blood / tissue fluid ; A phagocytes A ultrafiltration 6 AVP; ref. to pinocytosis across endothelial cells
[4] mark to max 3 if no ref. to 'more' at least once in the response any four from: 1 more carbon dioxide diffuses into red blood cells ; 2 more carbonic acid is formed by carbonic anhydrase ; 3 formation of more hydrogen ions; A H+ 4 haemoglobin has a high affinity for hydrogen ions; A H+ 5 haemoglobin binds more hydrogen ions to form, haemoglobinic acid (HHb) ; A H+ 6 (formation of HHb) decreases affinity of haemoglobin for oxygen ; 7 haemoglobin releases more oxygen; 8 carbon dioxide binds to -NH2/N terminal, of, globin / polypeptides / a chains and β chains ; 9 forms carbaminohaemoglobin ; R carboxyhaemoglobin also lowers affinity 10 allosteric effect / change in tertiary structure / AW, in (oxy)haemoglobin (causes, release / AW, more oxygen) ;
[2] any two from: 1 hydrogencarbonate ions / HCO₃⁻, pass out of red blood cells (into the plasma to increase concentration in deoxygenated blood) ; 2 chloride ions pass into red blood cells ; 3 to replace the, negatively-charged ions / anions / HCO₃⁻; A to achieve electroneutrality / balance the electrical charge 4 chloride shift; 5 AVP; chloride ions pass through, channel proteins / anion exchangers / transport proteins / by facilitated diffusion