Notes for Teachers s4

Key Stage 5 – Nodule experiment

Notes for teachers

At a glance

Some plants, such as peas, are able to develop a symbiotic relationship with nitrogen-fixing rhizobia bacteria from the soil. The bacteria infect the root cells and grow inside structures called root nodules.

In this activity students carry out an investigation to find out how the concentration of nitrate in the soil affects the growth of root nodules on peas. This is a non-core practical but will help students to develop skills in following a method, collaborating to gather results and writing an analysis and conclusion.

Learning Outcomes

· Students make up molar solutions.

· Students gather results, analyse data and write a scientific conclusion.

Each student will need

· 1 copy of the pupil worksheet

· Equipment for the experiment:

- Balances

- Potassium nitrate (OXIDISING)

- Spatula

- Weighing boat

- Stirring rod

- Measuring cylinder (500 cm3 or as large as you have)

- 1 litre stoppered bottle (an clean soft drink bottle would be suitable)

- Large plastic container for mixing soil

- Tablespoon or small trowel for mixing soil

- 3 polystyrene or plastic cups

- Soil (this needs to be taken from the ground, not bagged compost as this will not contain soil bacteria)

- Vermiculite

- Pea seeds

Note: before carrying out the activity make sure you carry out a full risk assessment.

Possible Lesson Activities

1. Starter activity

· Show the video ‘Give peas a chance’ to the class.

· Discuss why researchers at the University of Oxford are looking at how to increase the uptake of rhizobia bacteria by pea plants: what use will this have for us?

2. Main activity: Setting up the experiment

· Ask students to work in pairs and follow the method on the Pupil Worksheet to set up the experiment. Assign each pair a different concentration of potassium nitrate which they will use to water their peas (1 mM, 5 mM, 10 mM , 20 mM or 30 mM).

· Students should be able to work out the mass of potassium nitrate needed to make up 1 litre of their solution.

· Peas need to be left in a warm, sunny location for at least 3 weeks to allow growth. Students should water the peas using the solution enough to keep the soil moist but not waterlogged.

3. Main activity: Students gather results and analyse

· After three weeks students remove the pea plants from the soil, gently rinse under tap water, count the number of root nodules on each plant and calculate a mean for their concentration of potassium nitrate.

· The class should collaborate in order to gather results for the range of concentrations.

· Students then work individually to analyse the results and write a conclusion.

4. Plenary

· Discuss with the class what their results showed. They should see that the higher the concentration of potassium nitrate, the fewer nodules formed on the roots. This might not be the result they expected.

· This happens because if the plant has access to a high enough concentration of nitrates in the soil it has no need for the symbiotic relationship with rhizobia bacteria so root nodules will not form.