Fig. 4.1 is a transmission electron micrograph of the bacterium that causes cholera, Vibrio cholerae. The flagellum shown in Fig. 4.1 allows movement of the bacterium within the gut and may also function to help it to bind to an intestinal epithelial cell. The organism does not enter the cell but the toxin it releases can enter and cause damage. Large quantities of water, chloride ions and sodium ions are lost from the cell. People with symptoms of cholera have severe watery diarrhoea and as a result can become very dehydrated. [Figure 4.1]

Mark scheme

Marking points from the official mark scheme (Oct/Nov 2017). The mark or mark type is shown in brackets.

1. [2] two from
2. (loss of ions) increases / AW, water potential within cell; ora, A Ψ for water potential, I ref. to solutes / solute potential
3. water moves out of cell, down water potential gradient / from high(er) to low(er) water potential ; R from high to low water potential gradient
4. (out) by osmosis / through the partially permeable membrane ; A selectively permeable membrane I osmotic gradient
5. [4] four from
6. capillary side sodium ions
7. 1 sodium ions out (of cell), by active transport / with use of ATP ; A sodium ions pumped out
8. 2 (so) lowers concentration of sodium ions within cell or sodium ion concentration gradient, set up / maintained ;
9. intestinal lumen sodium ions and glucose
10. 3 sodium ions enter by facilitated diffusion ; A diffusion / high to low concentration, through, SGLT1/cotransporter I glucose enters by facilitated diffusion
11. 4 glucose, cotransported with sodium ions into cell (through SGLT1) ; A sodium ions cotransported with glucose A glucose enters by secondary active transport, A idea of glucose only able to enter if moving with sodium ions (i.e. sodium drives the process)
12. 5 (cotransport means) glucose enters against concentration gradient ;
13. capillary side glucose
14. 6 glucose out of cell (towards capillary) by facilitated diffusion ; A by diffusion if stated through, membrane protein / GLUT2
15. water uptake from lumen
16. 7 (higher concentrations of) sodium ions / glucose / solutes, within cell lowers water potential ;
17. 8 water follows, sodium ions / glucose / solutes (osmotically) or so water enters cell (down water potential gradient) ; must have idea that it follows inward movement of solutes
18. [1] any one valid e.g.(if not stated artery or vein, assume vein)
19. high(er) pressure of artery (will not allow drip)
20. or artery may be deeper to reach to insert needle for drip / easier to find vein A vein more, visible / superficial
21. or greater risk / more complications / greater blood loss, associated with intra arterially AW
22. [1] one from
23. no/reduced, polypeptide / protein, synthesis
24. or mRNA not translated / no translation / reduced translation; A detail of translation e.g. tRNA cannot bind R DNA not translated
25. or no/few, enzyme-catalysed reactions ;
26. [3] three from
27. 1 volume / AW, decreases over time for all groups;
28. 2 compared to no antibiotic antibiotic groups, steep(er) / faster, decrease to, 32/48 hours ;
29. 3 idea that diarrhoea, stops / is 0 dm³, at / after, 64 hours, for one dose 1 g/ A, or, multiple dose / C ; A recovers after 64 hours / AW
30. 4 after 48 hours, one dose 2 g / B, fluctuation / decreases then (slight) increase then decrease / AW ;
31. 5 no antibiotic / D, higher volumes diarrhoea than antibiotics (to approx. 110h) or no antibiotic / one dose 2 g / B, took 128 hours (for diarrhoea) to, reach 0 dm³/ stop ;
32. 6 multiple dose / C, higher volumes than, A (all readings) / B (to 48 hours) ora or A has steepest decrease in context of 16-32 hours or overall
33. [2] alternative ways to refer to decrease in volumes of diarrhoea may be in terms of recovery, destroying bacteria, decreasing loss of glucose and salts
34. two from
35. support treatment
36. there is a difference between antibiotic and no antibiotic treatment or fast(er) decrease in volume of diarrhoea with antibiotics / AW or (generally) faster recovery with antibiotics; I ref. to one dose 2 g
37. use of Fig. 4.3 to support;
38. e.g. use (1 dose) 1 g or multiple dose
39. time, to recover/reach 0 dm³, is halved
40. use of numerical data from Fig. 4.3
41. does not support treatment
42. (in all cases) volume decreased to, same level / zero or all patients recovered ;
43. use of Fig. 4.3 to support;
44. e.g. by 128 hours all patients 0 dm³
45. one dose of 2 g same trend from 112 hours as no antibiotic
46. one dose of 2 g patients relapse after 64 hours
47. one dose of 2 g took 128 hours (for recovery)
48. not able to say
49. limited information available / small number of patients;
50. ref. to one dose of 2 g antibiotic ; e.g. does not reach 0 dm³ until same time as no antibiotic
51. also see arguments above – allow once only here or for does not support
52. [2] answer may be from point of view of single dose or multiple dose
53. allow AW - note mp 3 is for starting with susceptible bacteria and mp 4 is for starting with resistant bacteria
54. penalise once if use virus throughout
55. single dose
56. 1 easier to be sure patient has taken complete dose
57. 2 if (bacteria are all susceptible and) treatment completed, all bacteria killed / no reservoir of bacteria
58. 3 (susceptible so) no bacteria survive to, mutate / become resistant
59. 4 idea that (if resistance is already present) single stronger dose has greater chance of killing resistant bacteria
60. 5 (if all killed with single dose) idea that resistance not transferred (if all killed) e.g. no vertical / horizontal, transmission this could be suggested as follow up to mp 2/4
61. multiple dose
62. course may not be completed
63. treatment may not be completed so some (susceptible) bacteria survive
64. (bacteria replicating so) increased chance of, mutation / becoming resistant
65. weaker dose spread over time, resistant bacteria, more likely to survive / have less chance of being killed
66. if resistant / if develop resistance, this could be transferred A vertical / horizontal, resistance
67. 6 AVP
68. e.g.
69. one dose may mean, no / less, antibiotic enters environment (in faeces)
70. (more effective so) bacteria passed out for shorter time, so reduces risk of transmission (of pathogen)
71. idea that multiple low dose antibiotics may increase mutagenesis
72. suggestion that if resistant and not killed by antibiotic, there may be less of an effect on (good) gut bacteria with single dose
73. [3] three from
74. 1 ref. to different antigens (in context of, flagellum / whole cell / toxin); A ref. to epitopes instead of antigens
75. 2 specificity ; in correct context (B-lymphocytes/plasma cells/antibodies/antigen binding sites)
76. 3 detail of B-lymphocytes; e.g. specific B-lymphocytes activated (by each different antigen) A clonal selection form plasma cells that release specific antibody, A B-lymphocytes release specific antibody
77. 4 detail of antibody; I ref. to receptor
78. e.g. antibody complementary (shape) to antigen, antigen binding sites on antibody, variable regions different for each antibody
79. [1] passive natural / natural passive ;