A-LevelBiologyCell membranes and transport9700/23 · Oct/Nov 2019 · Q66 Marks

Fig. 6.1 is a diagram of the cell surface membrane of a squamous epithelial cell lining an alveolus.

(a)(i)A student measured the line A–B and calculated the actual width of the membrane in Fig. 6.1. State the unit that the student should use for the actual width of the membrane.[1]
(a)(ii)With reference to Fig. 6.1, state how to identify the external surface of the cell surface membrane.[1]
(b)Name R and S in Fig. 6.1 and describe their roles in the membrane. component R name .......................................................................................................................................... role .......................................................................................................................................... component S name .......................................................................................................................................... role ..........................................................................................................................................[4]

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Mark scheme

Marking points from the official mark scheme (Oct/Nov 2019). The mark or mark type is shown in brackets.

6(a)(i) nanometres / nm ; A ora throughout
6(a)(ii) (presence of) carbohydrate / sugar, chains / residues, on, (glyco)proteins / (glyco)lipids ; A there are no sugar chains on the inner surface A (presence of) glycoproteins / glycolipids (on external surface) A (presence of) cell surface antigens / receptors (on external surface) A (presence of) glycocalyx
6(b) R 1 cholesterol ; one from 2 maintains / regulates, fluidity of, membrane / phospholipid bilayer or at low temperatures, maintain / increase fluidity / prevents close packing A prevents hydrophobic 'tails' interacting at low temperatures or at high temperatures, stabilises the membrane / decreases fluidity ; 3 prevents passage (across membrane) of, hydrophilic / polar, substances ; S 4 phospholipid (monolayer) ; R phospholipid bilayer one from 5 forms a bilayer ; 6 (bilayer is a) barrier to, water soluble molecules / polar molecules / ions ; ora not a barrier to lipid soluble molecules / allows lipid soluble molecules to cross membranes A non-polar / AW 7 (bilayer is a) non-polar barrier between cytoplasm and (aqueous) surroundings ; 8 hydrophilic 'head' forms hydrogen bonds with water ;

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