The diagram below shows part of the carbon cycle.

In the carbon cycle, which substance is absorbed by producers?

There is evidence that the concentration of carbon dioxide in the Earth’s atmosphere is increasing.

Which change could explain this?

Describe the roles of different living organisms in the carbon cycle.

The diagram below shows the carbon cycle.

Name the processes A to E.

A student was helping plant cabbages in the school allotment. 

The student observed there were earthworms in the soil.

He also saw a mouse.

Explain how cabbages, earthworms and mice contribute to the carbon cycle.

Decomposers play an important role in the carbon cycle.

They release carbon dioxide.

Which process is responsible for this?

Describe how human activities are affecting the carbon cycle.

A gardener made his own compost by using fruit and vegetable scraps, dry leaves, grass and plant clippings, and mixing them with paper, cornstalks and twigs.

He then allowed for the scraps to decay in a compost bin.

The gardener made sure the compost bin had no base and sat on the soil.

The soil contains organisms that cause decay.

Which two types of organism cause decay?

The rate of decay in the compost heap depends on several environmental factors.

Explain how the rate of decay would vary with a temperature increase from 0°C to 25°C.

Give one environmental factor needed for decay.

Do not refer to temperature in your answer.

The dead plant material in the compost bin does not decay completely.

Explain why the gardener might spread the remaining dead plant material onto his plants.

Nitrogen is cycled through the environment.

The diagram below shows the nitrogen cycle.

Label processes A, B, C and D.

Name a type of organism that performs process C.

Describe the roles of bacteria in the nitrogen cycle.

Farmers use crop rotation to reduce the need to add nitrate fertilisers to the soil.

Certain plants have a mutualistic relationship with nitrogen fixing bacteria.

Explain why farmers use these plants in their crop rotation cycle.

Decomposition is the breakdown of dead organic material.

Explain why the decomposition of dead plants and animals is a crucial process for ecosystems.

A student investigated the decomposition of dead leaves collected from a park.

The student followed the method below:

1. Collect four samples of dead leaves each with a mass of 1.0 kg.
2. Label the samples A, B, C and D.
3. Grind sample A and keep it at 4°C.
4. Grind sample B and keep at 25°C.
5. Do not grind sample C and keep at 4°C.
6. Do not grind sample D and keep at 25°C.
7. Measure the mass of each sample after 10 weeks.

The graph shows the student's results.

State two biotic variables that the student should have controlled in her investigation.

Calculate the difference between the rate of decomposition in sample A and in sample B.

Give your answer in kg per week.

State and explain how bacteria are important in cycles within ecosystems.