A-LevelBiologyInfectious diseases9700/53 · Oct/Nov 2025 · Q212 Marks

Compounds containing nitrite ions (NO₂⁻) are present in many foods eaten by humans. Scientists tested the effect of different concentrations of nitrite ions on the population growth of 5 species of bacterium. All 5 species are pathogenic and can infect the human digestive system. For each species of bacterium, the scientists placed suspensions of the bacteria in a microwell plate. A microwell plate is a plastic plate containing 96 wells, which are similar to small test-tubes. In each well, the scientists added: • a suspension of the bacterial species • a solution containing nitrite ions • nutrient broth. The scientists incubated the microwell plate at 37°C for 24 hours. After 24 hours, the scientists estimated the number of bacteria in each well by measuring optical density, using a microwell plate reader. For each species of bacterium, the scientists repeated the experiment with several different concentrations of solution containing nitrite ions. The scientists determined the lowest nitrite concentration at which no bacterial population growth had occurred after 24 hours.

(a)The scientists standardised temperature, pH (pH4.8) and time in the investigation. State two other variables that the scientists should standardise in this investigation.[2]
(b)(i)The results of the investigation are shown in [Fig. 2.1]. State the type of data represented by the species of bacterium and the lowest nitrite concentration in [Fig. 2.1]. species of bacterium lowest nitrite concentration[2]
(b)(ii)The 2 species of Salmonella shown in [Fig. 2.1] are closely related. Calculate the percentage difference between the results shown in [Fig. 2.1] for the 2 species of Salmonella. Show your working.[3]
(b)(iii)Use [Fig. 2.1] to state: • the species of bacterium that is most affected by nitrite ions • the species of bacterium that is least affected by nitrite ions. most affected least affected[1]
(c)A student looked at the results and stated: Consuming a lot of nitrites would improve the health of people because it would help to prevent diseases in the digestive system caused by pathogenic bacteria. Suggest four reasons why this conclusion might not be valid.[4]

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Mark scheme

Marking points from the official mark scheme (Oct/Nov 2025). The mark or mark type is shown in brackets.

[2] any two from: 1 (initial) volume / population / number, of bacteria (in bacterial suspensions) or volume of (bacterial) suspension or (original) optical density of (bacterial) suspensions ; 2 volume of nitrite (ion solution); 3 concentration / volume, of nutrient broth ;
[2] (species of bacterium:) nominal ; (lowest nitrite concentration:) continuous ;
[3] 1 correct reading of values from Figure 2.1 (Salmonella enteritidis =) 0.7 and (Salmonella typhimurium =) 5 ; 2 correct working ; 3 correct value for percentage difference ; 614 (%) / 86 (%)
[1] (most affected =) Yersinia enterocolitica (least affected =) Salmonella typhimurium ;
[4] any four from: 1 idea that investigation, not carried out in a person / carried out in a laboratory; 2 idea that investigation only carried out for 24 hours / no information about longer term effects ; 3 idea that experiment, only used 5 species of (pathogenic) bacteria / did not use other bacteria ; 4 pH in the, digestive system / stomach / intestines / AW, is not 4.8 / is different ; 5 idea that nitrites may, kill / reduce growth / AW, of / beneficial/AW, bacteria (in the digestive system) ; 6 nitrites could, cause side effects / be toxic / AW (in humans); 7 no evidence of control experiment ; 8 no statistical, test / analysis; 9 idea that (bacterial) population sizes may not have decreased (as only measured when no bacterial population growth occurred);

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