Cycles within Ecosystems (Edexcel IGCSE Biology)

The diagram shows a cycle found in ecosystems.

Give the name of the cycle.

(i) In the image in part (a), which process is represented by the letter W?

(1)

(ii) In the image in part (a), which process is represented by the letter X?

(1)

Name a group of organisms that is responsible for decomposition.

Carbon dioxide is released into the atmosphere by the decomposition of organic material.

The rate of this decomposition depends on a number of factors.

Design an investigation to find out if changing the pH of organic material affects the rate of decomposition.

Include experimental details in your answer and write in full sentences.

Separate: Biology Only

Decomposer bacteria are involved in the nitrogen cycle.

The bacteria release an enzyme called urease.

The diagram shows part of one strand of DNA used to make urease.

Complete the diagram by giving the missing bases on the other strand of DNA.

Separate: Biology Only

Urease acts on urine to produce ammonia.

The graph shows how pH affects the activity of urease.

(i) Which of these is the optimum pH for urease?

(1)

(ii) Explain the activity of urease at pH 8.5.

(2)

Separate: Biology Only

Describe the role of the other bacteria involved in the nitrogen cycle.

Separate: Biology Only

The diagram shows the nitrogen cycle with four processes labelled A, B, C and D.

(i) Which process is nitrogen fixation?

(1)

(ii) Which process is decomposition?

(1)

(iii) Which process is nitrification?

(1)

(iv) Name a type of organism that carries out process C.

(1)

A student does an investigation to determine if nitrate ions are required for plant growth.

He uses this method.

(i) Explain why the control solution contains all the mineral ions the plant requires but the test solution contains all the mineral ions the plant requires except nitrate.

(2)

(ii) State the purpose of Step 5.

(1)

(iii) Explain the purpose of Step 7.

(2)

(iv) Explain the measurements that the student could make to determine if nitrate ions are required for plant growth.

(2)

The following table shows changes in atmospheric carbon dioxide levels over time.

Calculate the percentage increase in carbon dioxide concentration between 1960 and 2010. Give your answer to one decimal place.

Use the data provided in part (a) to plot a graph that shows the change in atmospheric carbon dioxide levels over time.

Use a ruler to join the points with straight lines.

Use your understanding of the carbon cycle to suggest how human activities are contributing to the trend shown in the data in parts (a) and (b).

Separate: Biology Only

The diagram shows some parts of the nitrogen cycle.

Name the processes labelled A, B and C.

Separate: Biology Only

The mass of nitrogen that moves within the nitrogen cycle has been estimated and some of the masses are given in the table.

(i) Some of these processes remove nitrogen from the atmosphere.

Calculate the total mass of nitrogen removed from the atmosphere by these processes.

Give your answer in g and in standard form.

(2)

(ii) Calculate the percentage of the nitrogen released into the atmosphere that comes from burning of biomass.

(2)

(iii) Explain how burning biomass returns nitrogen to the atmosphere.

(2)

Explain the effect of nitrous oxide on global warming.

Separate: Biology Only

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 diagram below, can use nitrogen from the air.

(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)

Separate: Biology Only

The diagram below shows the nitrogen cycle. Different stages have been numbered 1 to 8.

(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)

Separate: Biology Only

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.

'Carbon flux' is a term used to describe the transfer of carbon from one store to another in the carbon cycle.

The following data show fluxes in carbon between different storage reserves in an ecosystem, measured in gigatonnes per year (GT yr^-1). A gigatonne equals 1 billion tonnes.

(i) Calculate the difference between the amount of carbon entering the atmosphere and the amount of carbon leaving the atmosphere over the course of a year in this ecosystem.

(3)

(ii) The figure calculated in part (i) is known as the 'net flux' of carbon in the ecosystem; this refers to the overall direction of carbon transfer.

Explain why the net flux of carbon is important at a global level.

(2)

Explain why the combustion of fossil fuels can have a large impact on the carbon balance in the atmosphere.

Estimating global carbon fluxes is of great interest to scientists, even though it may be challenging to make accurate measurements.

(i) Suggest why it is difficult to make accurate measurements when studying the transfer of carbon.

(2)

(ii) Suggest why estimating carbon fluxes is of such interest to scientists.

(2)