A-LevelBiologyTransport in mammals9700/21 · Oct/Nov 2020 · Q39 Marks

(a) The circulatory system of mammals is a double circulation.

(a)(i)Explain what is meant by the term double circulation.[1]
(a)(ii)Fig. 3.1 is a photograph showing one valve in the mammalian heart. [Figure 3.1] Identify the structures labelled A in Fig. 3.1 and describe their role during the cardiac cycle. structure A ____________________________________________________ role of structure A ____________________________________________________[3]
(b)(i)The endocardium is a thin layer of tissue lining the chambers of the heart. A serious condition called endocarditis results if bacteria infect this tissue. Endocarditis is treated with a combination of antibiotics. This increases the effectiveness of the treatment and reduces the risk of antibiotic resistance in bacteria. Table 3.1 shows the action of two antibiotics used together to treat endocarditis. [Table 3.1] With reference to Table 3.1, explain why treating endocarditis with a combination of gentamicin and penicillin G reduces the risk of developing antibiotic resistance.[2]
(b)(ii)Describe how the bacteria that cause endocarditis could become resistant to gentamicin.[3]

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 (Oct/Nov 2020). The mark or mark type is shown in brackets.

[1] blood, travels through the heart twice for every one complete, circuit (of the body) / circulation or blood, travels through the heart twice and through pulmonary and systemic, circulations / systems / circuits ; A description of both circulations
[3] chordae tendineae / tendinous cords; A tendons / 'heart strings' any two from: idea that (atrioventricular) valve, held in position / prevented from inverting, during ventricular, systole / contraction ; A AV valve for atrioventricular valve connect the papillary muscle to the (atrioventricular / AV), valve ; ref. to tension, in the chordae tendineae / AW, (during ventricular contraction); (allows valve to) prevent backflow of blood into atrium / ensure one way blood flow (through the heart); ecf max 2 marks for function if structure A is a correctly named valve (atrioventricular/tricuspid/bicuspid)
[2] idea of using both antibiotics makes sure all bacteria are killed ; if, resistance against one antibiotic occurs then, the other antibiotic will be effective / bacteria still susceptible to the other antibiotic or if, some bacteria are less susceptible to one antibiotic, then, the other antibiotic will be effective / bacteria still susceptible to the other antibiotic ; any one from: antibiotics / gentamicin and penicillin G, have different sites of action / AW; A suggestion of how the antibiotics have different ways of killing ref. to mutations, qualified ; e.g. need two mutations to occur to be resistant to both antibiotics probability / AW, of these two mutations occurring in the same bacterium is very small mutations must be in different genes
[3] any three from: course of antibiotic is not completed; less susceptible bacteria survive; A not all bacteria destroyed replication/population growth, increases chance of mutation; ref. to random mutation, ref to gene / allele, for resistance to gentamicin ; suggestion of outcome of different mutations ; e.g. changed ribosome structure so gentamicin cannot bind enzyme to breakdown gentamicin ref. to transmission of allele for resistance; e.g. vertical transmission / described horizontal transmission / described (conjugation / transformation) e.g. frequency of allele for resistance increases in the population; when antibiotic next prescribed antibiotic in the environment is the selection pressure / resistant bacteria selected for / susceptible bacteria selected against;