The water cycle shows the continuous movement of water within the Earth and atmosphere.

The image below shows the water cycle.

Which process forms part of both the carbon and water cycles?

Give two uses of water in plants.

State the process by which water is absorbed into plant roots.

Surface run-off water passes through soil and back into the lake.

Give one reason why surface run-off water is important to organisms living in the lake.

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.

Explain the results obtained by the student.

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.

A farmer decided to turn the manure in his field into compost.

He made the compost by mixing straw and manure and allowing them to rot down.

State three things needed for the straw and manure to rot.

The farmer noticed that the straw and manure mixture felt warm.

Explain why.

The farmer noticed that it takes less time to produce compost in the summer.

Suggest why.

Some substances, like plastic, are not biodegradable.

Define biodegradable.

Milk contains bacteria.

Milk can be used to investigate decay caused by bacteria.

A student followed the method below:

1. Put milk in a sterilised conical flask.
2. Put a lid on the conical flask.
3. Leave the conical flask for 3 days at 20 °C.
4. Measure the pH of the milk each day using universal indicator.

Suggest why the student put a lid on the conical flask.

Give one reason why the student sterilised the flask before adding the milk.

The student's results are shown in the table below.

Give two control variables the student should have used in this investigation.

The student decided to change his method and repeat the investigation.

He used a pH meter to measure the pH, and left the milk to decay for 6 days.

Suggest why these changes improve the investigation.

The student's results were plotted on the graph below.

Give one reason why:

• the pH stayed the same during the first day
• the pH decreased after day 1
• there was no change in pH between days 5 and 6

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