9700 Biology November 2025 Question Paper 31

1 The bacterium Streptococcus pneumoniae is commonly found in the throat. S. pneumoniae produces hydrogen peroxide as it grows. A sample can be taken from a patient’s throat and tested to measure the concentration of hydrogen peroxide. This can be used as a measure of the growth of the bacteria.

You will determine the growth of bacteria by measuring the concentration of hydrogen peroxide in a solution that represents a sample taken from a patient. You will do this by measuring how long it takes for a sample of hydrogen peroxide to cause a colour change in a reaction mixture. The faster the mixture changes to a blue‑black colour, the higher the concentration of hydrogen peroxide, and the greater the growth of bacteria.

You will use a range of known concentrations of hydrogen peroxide to estimate the concentration of hydrogen peroxide in a sample.

You are provided with the materials shown in Table 1.1. Table 1.1 labelled contents hazard volume / cm3 R1 dilute sulfuric acid irritant 100 R2 starch solution low 10 R3 potassium iodide solution low 10 R4 sodium thiosulfate solution low 10 H 2.0% hydrogen peroxide solution irritant 25 U solution representing patient sample irritant 10 W distilled water low 100

If any solution comes into contact with your skin, wash off immediately with cold water.

It is recommended that you wear suitable eye protection.

You will need to carry out a serial dilution of the 2.0% hydrogen peroxide solution, H, to reduce the concentration by half between each successive dilution.

You will need to prepare four concentrations of hydrogen peroxide solution in addition to the 2.0% hydrogen peroxide solution, H.

After the serial dilution is completed, you will need to have 10 cm3 of each concentration available to use.

(a) (i) Complete Fig. 1.1 to show how you will prepare your serial dilution.

Each beaker should have: • a labelled arrow to show the volume of hydrogen peroxide solution transferred • a labelled arrow to show the volume of distilled water, W, added • a label under the beaker to show the concentration of the hydrogen peroxide solution. , , 0 cm3 of W , ,

2 J1 is a slide of a stained transverse section through a plant stem.

(a) (i) Draw a large plan diagram of the whole section on J1. Use a sharp pencil.

Use one ruled label line and label to identify the xylem.

[5] , ,

(ii) Observe the epidermis of the stem on J1.

Select a group of four adjacent epidermal cells.

Each cell must touch at least one other cell. • Make a large drawing of this group of four epidermal cells and waxy cuticle. • Use one ruled label line and label to identify the waxy cuticle.

[5] , ,

(iii) Fig. 2.1 is a photomicrograph of a stained transverse section of a stem from a different type of plant from J1. Fig. 2.1

Identify three observable differences, other than colour, between the stem section on J1 and the stem section in Fig. 2.1.

Record these three observable differences in Table 2.1. Table 2.1 feature J1 Fig. 2.1 1 2 3

[3] , ,

(b) Fig. 2.2 is a photomicrograph of a stained transverse section of a stem from a different type of plant.

You will calculate the density of vascular bundles in the stem section. The circle represents the area of the stem. sector P θ Fig. 2.2

(i) To calculate the density of vascular bundles you will first need to count the number of whole vascular bundles in sector P.

number of whole vascular bundles in sector P = [1]

(ii) Use your answer to (b)(i) to estimate the total number of vascular bundles in the stem section shown in Fig. 2.2.

angle θ = 15°

Show your working.

total number of vascular bundles = [2] , ,

(iii) The stem section shown in Fig. 2.2 has an actual area of 44 mm2.

Use your answer in (b)(ii) to calculate the density of vascular bundles in the stem section.

Give your answer to two significant figures.

Show your working.

vascular bundle density = mm–2

[2]

[Total: 18] , ,