9700 Biology November 2025 Question Paper 51

1 Juglone is a chemical produced by plants in the genus Juglans, which includes different species of walnut tree.

Fig. 1.1 shows a black walnut tree. Fig. 1.1

Juglone produced by a black walnut tree can diffuse into the soil surrounding the tree.

Groups of scientists have observed that juglone affects different species of plant growing close to black walnut trees.

Juglone can reduce the percentage of seeds that germinate, delay the start of germination or reduce growth. In some studies, juglone has no effect on germination or growth.

A student planned to investigate the effect of juglone on the cucumber plant, Cucumis sativus.

In the investigation, the student: • placed some damp soil containing juglone in a tray • added some cucumber seeds to the soil • placed the container in a suitable environment for germination and growth. , ,

Fig. 1.2 shows cucumber seedlings (young plants) a few days after germination. Fig. 1.2

The student planned to compare the effect of exposing cucumber seeds to: • juglone solution with a concentration of 1.0 × 10–3 mol dm–3, which is a concentration that has been measured in soils surrounding black walnut trees • a control treatment.

The student planned to determine: • the percentage of cucumber seeds that germinated in soil • the growth of each cucumber seedling 10 days after the appearance of the seedling above the soil.

(a) Identify the independent variable in this investigation [1] , ,

(b) Juglone solutions are prepared using a mixture of solvents.

The student was provided with: • a juglone solution with a concentration of 0.1 mol dm–3 • the mixture of solvents.

Using the solution provided, the student prepared a juglone solution with a volume of 500 cm3 and a concentration of 1.0 × 10–3 mol dm–3.

The student carried out 1 dilution to prepare the 1.0 × 10–3 mol dm–3 solution using measuring cylinders.

(i) State the volumes that the student used to carry out the dilution to produce 500 cm3 of 1.0 × 10–3 mol dm–3 juglone solution [2]

(ii) Outline one improvement the student could make to reduce the percentage error while carrying out the dilution [1]

(c) (i) State a suitable control that the student could use, and explain why the student decided to include a control in the investigation. control explanation [2] , ,

(ii) Describe a method the student could use to investigate the effect of juglone on: • the percentage of cucumber seeds that germinate • the growth of each cucumber seedling 10 days after the appearance of the seedling above the soil.

The description of your method should be set out in a logical way and be detailed enough for another person to follow [7] , ,

(d) Another student investigated the effect of juglone on a different species of plant called crimson clover, Trifolium incarnatum.

The student recorded the number of days required for germination of: • 10 seeds treated with a juglone solution with a concentration of 1.0 × 10–3 mol dm–3 • 10 seeds not treated with a juglone solution.

(i) The student used the t-test to analyse the results. The t-test is appropriate for the number of seeds that were used.

Suggest two other reasons why the t-test is an appropriate statistical test to use for this investigation [2] , ,

(ii) The null hypothesis for this t-test was:

There is no difference between the time taken for seeds to germinate when treated with juglone and the time taken for seeds to germinate when not treated with juglone.

The calculated value of t was 2.090.

The student compared 2.090 to the values in Table 1.1. Table 1.1 degrees of freedom probability level (p) 0.10 0.05 0.01 0.001 17 1.740 2.110 2.898 3.965 18 1.734 2.101 2.878 3.922 19 1.729 2.093 2.861 3.883 20 1.725 2.086 2.845 3.850 21 1.721 2.080 2.831 3.819

Using Table 1.1 and the calculated value of t of 2.090, state and explain what the student can conclude about the results [3]

[Total: 18] , ,

2 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 (pH 4.8) and time in the investigation.

State two other variables that the scientists should standardise in this investigation. 1 2 [2] , ,

(b) The results of the investigation are shown in Fig. 2.1. lowest nitrite concentration at which no bacterial population growth occurred after 24 hours / μmol cm–3 Yersinia enterocolitica Salmonella enteritidis Salmonella typhimurium Shigella sonnei Escherichia coli 0.0 1.0 2.0 3.0 4.0 5.0 6.0 species of bacterium Fig. 2.1

(i) 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]

(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]

(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. 1 2 3 4 [4]

[Total: 12] , ,