A-LevelBiologyCell structure9700/22 · Feb/Mar 2017 · Q220 Marks

The infectious disease cholera is caused by a bacterium. (a) Fig. 2.1 shows a transmission electron micrograph of this bacterium. ×30 000 [Figure 2.1]

(a)(i)Name the bacterium that causes cholera.[1]
(a)(ii)The bacterium in Fig. 2.1 is an example of a prokaryotic cell. Each of the descriptions A to C describes a cell structure found in prokaryotic cells and in plant cells. For each of the descriptions A to C: • name the cell structure described • state one difference in this structure between a prokaryotic cell and a plant cell. A the site of polypeptide synthesis cell structure difference B the genetic material of the cell cell structure difference C the structure that provides a rigid shape to the cell and prevents osmotic lysis cell structure difference[6]
(b)Cholera is an example of an infectious disease. Explain what is meant by an infectious disease.[2]
(c)The symptoms of cholera are caused by choleragen, a toxin released by the bacterium. Choleragen is a protein made up of six polypeptides: • a single copy of a polypeptide known as the A subunit that includes an extended alpha helix • five polypeptides that together make the B subunit. The B subunit of choleragen binds to a cell surface membrane component, known as GM1, of an intestinal epithelial cell. The complete choleragen protein then enters the cell by endocytosis. Once inside the cell, the A subunit of the protein acts as an enzyme, disrupting the normal functioning of the cell. List the levels of protein structure present in choleragen.[2]
(d)Outline the mechanism by which choleragen enters the cell. You may use the space for annotated diagrams.[3]
(e)(i)Using genetic engineering, it is possible to produce a form of choleragen consisting of only subunit B. This can be combined with inactivated bacterial cells to produce a vaccine against cholera. Suggest why subunit B, rather than subunit A, is used in the vaccine.[1]
(e)(ii)Outline how this vaccine can give protection against cholera.[5]

This question uses a diagram from the original paper, which isn't shown here. Open the full paper from the past papers archive to see it.

Mark scheme

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

[1] 2(a)(i) Vibrio cholerae ;
[6] 2(a)(ii) A 1 cell structure: ribosome; R RER 2 difference: 70S/smaller/18nm v 80S/larger/25–30 nm ; B 3 cell structure: DNA/chromosome; I RNA 4 difference: circular/(closed) loop v linear OR no histone proteins/naked v histone proteins OR not surrounded by nuclear envelope v surrounded by nuclear envelope ; A in a nucleus v not in a nucleus C 5 cell structure: cell wall; 6 difference: murein/peptidoglycan v cellulose; I lignin
[2] 2(b) two from 1 caused by, a pathogen/a bacterium/V. cholerae ; 2 transmissible/AW OR reference to faecal-oral route; 3 reference to reduced effectiveness of functions/AW;
[2] 2(c) primary, secondary, tertiary; quaternary; A 1º, 2º, 3º A 4º
[3] 2(d) three from: 1 choleragen, fits into/complementary to, receptor/GM1; A complementary shape 2 membrane pinches in/invaginates/AW; A engulfs/envelops 3 membrane fusion ; 4 (endocytotic) vesicle/vacuole, formed; 5 ATP/energy, required; A points from an annotated diagram
[1] 2(e)(i) one from: 1 portion that binds to cell; 2 (antibodies produced) prevent binding to cell/prevent entry to cell; 3 safer as not the toxic portion; 4 A subunit, causes damage to cell/less safe/AW; 5 AVP e.g. larger so more likely to provoke immune response/AW;
[5] 2(e)(ii) five from: 1 vaccine contains (subunit B/bacterial) antigen(s); 2 primary immune response occurs ; 3 correct ref to B-lymphocytes/formation of plasma cells; A B cells 4 secretion of, antibody/immunoglobulin (against cholera antigens)/ antitoxins; 5 T-helper lymphocytes secrete cytokine ; 6 (cytokine) increases humoral response/stimulates T-killer cells/stimulates macrophages ; 7 memory cell production ; 8 secondary (immune) response/response on further infection, is faster ; 9 higher levels of antibodies produced (during further infection); 10 active artificial immunity (against cholera) ; 11 AVP e.g. idea of specific antibody against each of the different vaccine antigens ;