Bacteria may be classified according to differences in cell wall structure. The differences are shown by using the Gram stain. • A Gram-positive bacterium has a cell wall mainly composed of a thick layer of peptidoglycan (murein). • A Gram-negative bacterium has a more complex cell wall. This wall is composed of a much thinner layer of peptidoglycan and an outer layer known as the outer membrane. Escherichia coli is a Gram-negative bacterium. Fig. 3.1 is a diagram through the cell surface membrane and the cell wall of E. coli. [Figure 3.1]

Mark scheme

Marking points from the official mark scheme (May/June 2018). The mark or mark type is shown in brackets.

1. [3] A antibiotic for penicillin throughout
2. three from:
3. 1 (penicillin) weakens / AW, the cell wall;
4. I punches holes / holes made
5. 2 (penicillin) acts, on growing cells / when cell wall being synthesised (during growth) ;
6. 3 inhibits / binds to / AW, enzymes / transpeptidases (for cross linkage formation);
7. I ref. to synthesis of peptidoglycan
8. 4 prevents formation of cross, links / linkages (between, peptidoglycan / murein, molecules) / AW;
9. suggestions why antibiotic is less effective on Gram negative
10. 5 outer membrane, prevents / interferes with / protects from / AW, entry (of penicillin);
11. A idea of, more difficult / further, to reach peptidoglycan layer
12. 6 proteins in outer membrane may pump out antibiotic ;
13. A presence of efflux pumps
14. 7 enzymes may be present (in periplasm) to degrade antibiotic / AW ;
15. 8 suggestion that antibiotic cannot cross hydrophobic region of (outer) membrane ;
16. 9 AVP; e.g. proportionately, less / lower concentration of, penicillin reaches murein for, enzyme / transpeptidase, inhibition
17. [1] ATP not required; A (metabolic) energy not required
18. [4] four from:
19. 1 channel protein; A pore protein / has a pore / has a channel
20. 2 channel / pore, can form from polypeptides
21. or
22. (protein) has quaternary structure ;
23. 3 hydrophilic R-groups, on amino acids lining channel / face inwards (towards channel) ;
24. A hydrophilic, lining / channel
25. A water-filled, channel/ AW
26. A idea that passage is through hydrophilic region of protein
27. 4 allows facilitated diffusion ; A diffusion alone if in context of through the protein but R if via phospholipid bilayer
28. 5 increases permeability for, movement of water / osmosis;
29. I faster
30. 6 no (specific) binding sites / (channel) not specific / not selective / allows more than one type of substance through / AW ;
31. 7 globular;
32. 8 AVP; e.g. ref. to hydrophobic part of protein, faces / interacts with, hydrophobic, region / core / fatty acid tails
33. ref. to hydrophilic parts of, protein / polypeptide, extend into, external environment / periplasm / aqueous regions / AW
34. [2] two from:
35. translation, cannot/does not, begin / occur
36. or
37. polypeptide / protein, synthesis, cannot occur / decreases ;
38. I ompF not made
39. A chain of amino acids
40. R transcription does not occur so translation does not occur
41. R if in context of mutation or enzyme inhibition
42. mRNA cannot attach to, ribosome / small subunit / ribosomal subunit ;
43. (first) tRNA (with UAC anticodon) / tRNA carrying met, cannot bind (to START codon);
44. [2] (composed of) many / chain of / polymer of / AW, monosaccharides / sugar monomers / sugar units ;
45. A glucose, molecules / residues
46. A more than two / many sugars
47. further detail; e.g. carbohydrate
48. may be, branched / unbranched in context of polysaccharide
49. macromolecule in context of polysaccharide
50. glycosidic bonds in context of between sugar monomers
51. (sugar monomers) joined by condensation reactions
52. [3] three from:
53. 1 specificity / specific (in correct context);
54. 2 (B/T, -) lymphocytes have receptors complementary to antigen;
55. A immunoglobulins / antibody as receptors for B-lymphocyte
56. A surface molecules as receptors for T-lymphocytes
57. 3 (different) antigens, stimulate / activate / AW, (different) B-lymphocytes / T-lymphocytes ;
58. A antigens stimulate an immune response
59. 4 idea that different antibodies, synthesised / produced / AW, for different (O) antigens / O polysaccharides / lipopolysaccharides ;
60. 5 memory cells will, not respond to different antigen / only respond to same antigen / AW ;
61. 6 different O-antigens can, be composed of different sugars ;
62. A can have different shapes