A-LevelBiologyCell membranes and transport9700/21 · Oct/Nov 2025 · Q19 Marks

Fig. 1.1 shows a diagram of the fluid mosaic model of the structure of the cell surface membrane.

(a)Name the structures labelled A, B and C in Fig. 1.1.[3]
(b)Draw an arrow on Fig. 1.1 to show the movement of a molecule across the membrane into the cell by facilitated diffusion.[2]
(c)Explain how the fluidity of the membrane would change if the proportion of unsaturated fatty acids in the phospholipid bilayer increased.[3]
(d)The R-groups of amino acids give them different properties. Suggest how the properties of the R-groups of the amino acids at position 1 of P may differ from the R-groups of the amino acids at position 2.[1]

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Mark scheme

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

1(a) A glycolipid;
B cholesterol ;
C glycoprotein ;
1(b) 1 arrow through protein ;
2 direction of arrow correct ;
1(c) any three from: fluidity (of membrane) increases ;
unsaturated fatty acids have double bonds ;
(double bonds) cause the fatty acid chain to, kink / bend / AW ;
unsaturated fatty acids cannot lie as close together as saturated fatty acids A ora
or ref. to increased distance between phospholipids / AW ;
ref. to weakening the hydrophobic interactions between, fatty acid chains / cholesterol ;
1(d) any one from: position 1 has amino acids with, hydrophobic / non polar / AW, R-groups ;
position 2 has amino acids with, hydrophilic / polar / ionic / AW, R-groups;
A ref. to amine and carboxyl groups as appropriate

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