A-LevelBiologyCell membranes and transport9700/23 · May/June 2025 · Q48 Marks

Lysosomes are membrane-bound organelles found in mammalian cells.

(a)(i)Scientists measured the concentration of cholesterol in the membranes of lysosomes in a mammalian cell. The concentration of cholesterol in the lysosome membranes was found to be lower than the concentration in other membranes inside mammalian cells. State how a lower concentration of cholesterol would make the properties of lysosome membranes different from other membranes in mammalian cells.[1]
(a)(ii)Suggest why the lower concentration of cholesterol in lysosome membranes would help lysosomes carry out their function.[1]
(b)(i)The enzyme α-galactosidase is present in lysosomes. Students investigated the effect of substrate concentration on the rate of reaction catalysed by α-galactosidase at pH 4.5 and at pH 5.9. The results are shown in Fig. 4.1. [Figure 4.1] Determine the Michaelis–Menten constant, Km, for α-galactosidase at pH 4.5 using the data in Fig. 4.1. State the unit for the Km value in your answer. Km = .............................................. unit..............................................[2]
(b)(ii)With reference to Fig. 4.1, describe the differences in the results at pH 4.5 and pH 5.9 and suggest explanations for the differences.[4]

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Mark scheme

Marking points from the official mark scheme (May/June 2025). The mark or mark type is shown in brackets.

[1] any one from:
[1] (membrane) fluidity will increase / AW ;
[1] increase in (lateral) movement of phospholipids (in bilayer);
[1] AVP: e.g. increase in passage of, polar molecules / ions, across membrane
[1] easier to fuse with other membranes
[1] idea that (greater membrane fluidity) makes it easier for lysosomes to fuse with, vacuoles / vesicles / phagosomes / endosomes / membranes (of other organelles);
[1] allow ecf on fluidity decreases in Q4(a)(i) e.g. makes more impermeable to prevent hydrolytic enzymes exiting
[2] Km = 120;
[2] unit = µmol dm-3 ;
[4] any four from:
[4] differences
[4] 1 rate of reaction is lower at pH 4.5, throughout / at all substrate concentrations / AW ; ora
[4] 2 Vmax / plateau, is 2.8 (at pH 4.5 ) v 5.9 (at 5.9) pmol min⁻¹ ;
[4] 3 Vmax is reached at a lower substrate concentration with pH 4.5; A ora ; A data 330-400 µmol dm⁻³ compared with 700 µmol dm⁻³
[4] explanations to max 3
[4] at pH 4.5/lower pH
[4] 4 fewer enzyme-substrate complexes form ;
[4] 5 tertiary structure of enzyme / shape of active site, changes / AW ;
[4] 6 (at pH 4.5) active site shape becomes less complementary to substrate ;
[4] 7 detail; e.g. increased presence of hydrogen ions has an effect on, R-group interactions / ability to lower activation energy / ability to bind substrate
[4] 8 suggestion that pH 5.9, is / is closer to, the optimum pH ; ora

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