1.1. Nitrogen is an essential element for plant growth. Most plants can only use nitrogen in the form of nitrate ions from the soil. Only legumes that have bacteria living in root nodules, as shown in the Figure 1 below, can use nitrogen from the air.

Figure 1

(i) Explain how nitrate ions help plants to grow.

(2)

(ii) Describe how nitrogen in the air is converted into nitrates which can be used by the plants.

(2)

(iii) The relationship between the plant and the root-nodule bacteria in the diagram above is described as 'mutualistic', meaning that both organisms in the relationship benefit from living together.

Use the information in the diagram to explain how the relationship between the plant and the bacteria is mutualistic.

(2)

1.2. Figure 2 shows the nitrogen cycle. Different stages have been numbered 1 to 8.

Figure 2

(i) Complete the table to identify the stages in the nitrogen cycle that correspond to each number from the diagram.

One stage has been filled in for you.

(4)

(ii) Explain how nitrates are absorbed into plants at stage 8.

(3)

1.3. Earthworms are invertebrates that live in the soil. As they move through the soil they create air pockets between the soil particles.

Suggest why this may lead to an increase in nutrients, such as nitrates, in the soil.