Edexcel IGCSE Biology Cycles within ecosystems questions

Cycles within ecosystems syllabus

Questions on cycles within ecosystems can test recall, calculations, explanations, diagrams, data handling and practical skills. You should be able to:

4.10 describe the carbon cycle, including respiration, photosynthesis, decomposition and combustion.
4.11B describe the nitrogen cycle, including nitrogen-fixing bacteria, decomposers, nitrifying bacteria and denitrifying bacteria. Specific bacterial names are not required.

How to answer cycles within ecosystems questions

Identify the population, community or environmental factor before interpreting the ecological relationship.
Quote values from graphs or tables and distinguish correlation from a supported causal explanation.
For sampling methods, describe placement, repeats, measurements and how the result is made representative.
Trace energy or matter through each trophic level or cycle without implying that energy is recycled.

Cycles within ecosystems example questions and worked answers

These examples show how information in a diagram, graph or experimental context becomes part of a complete exam answer.

Example 1: Comparing decomposition in two mesh sizes

Adapted from June 2011, Paper 2B · Question 3(b)

Question 1

Scientists in Malaysia studied the rate of decomposition of leaf discs placed in bags in the soil. Leaf discs from a forest tree were placed in bags of two different mesh sizes. At intervals of four weeks, some of the bags were removed from the soil and the dry mass of the leaves was measured. The experiment was continued over a period of 20 weeks.

Two tied mesh bags containing leaf discs; the left bag has smaller mesh and the right bag has larger mesh.

The data they obtained are shown in the table.

Weeks	Dry mass of leaves remaining (% of original mass)
Weeks	Small mesh	Large mesh
0	100	100
4	92	88
8	84	78
12	74	66
16	72	60
20	68	58

Plot a graph of the data, using straight lines to join consecutive points in each series.

Final answer

Use a linear horizontal scale from 0 to 20 weeks and a linear vertical scale from 50% to 100%. Label the horizontal axis Weeks and the vertical axis Dry mass of leaves remaining (% of original mass). Plot these points:

Weeks	Small mesh (%)	Large mesh (%)
0	100	100
4	92	88
8	84	78
12	74	66
16	72	60
20	68	58

Join the points for each mesh size with neat straight lines, and include a key identifying the two lines.

Mark scheme points

M1 Use linear scales on both axes.
M2 Put weeks on the horizontal axis and dry mass remaining (%) on the vertical axis; label both axes.
M3 Plot the data points accurately for both mesh sizes.
M4 Join the points for each series with neat straight lines through the points.
M5 Include a key to identify the small-mesh and large-mesh lines.

The mark scheme notes that a graph with only one line does not gain the line, plotting or key marks.

Explanation

A linear scale means equal distances on an axis represent equal changes in value. Put time in weeks along the horizontal axis and the percentage of dry mass remaining on the vertical axis. Plot each pair of values from the table at its corresponding week. The two series share the point (0, 100), so use a key to show which line represents each mesh size. Join each series separately with straight line segments.

Common mistakes

Do not omit either data series: one line alone misses the line, plotting and key marks.
Check that the axes are not swapped and that both are labelled, including weeks and %.
Do not draw bars; the mark scheme allows only the axes and key marks for a bar chart.

Example 2: Explaining long-term carbon storage in roots

Adapted from June 2022, Paper 2B · Question 1(b)

Question 2

Scientists plan to produce transgenic crop plants that remove carbon dioxide from the atmosphere and store more carbon in their roots.

Coastal plants that have their roots in seawater contain a substance called suberin in the cell walls of the outer layer of the roots. Suberin is a waterproof substance that contains a high proportion of carbon atoms. Suberin is decomposed very slowly so remains in the soil for a long time.

The photograph shows a coastal plant called a mangrove.

Photograph of a coastal plant called a mangrove, with its roots in seawater.

(Source: © Snow At Night/Shutterstock)

The scientists intend to transfer a gene for high suberin production from a coastal plant into perennial crop plants. Perennial plants live for many years rather than dying each winter.

Explain why producing genetically engineered plants with additional suberin in their roots could reduce atmospheric carbon dioxide.

Final answer

The plants absorb carbon dioxide from the atmosphere for photosynthesis. They use its carbon to make suberin and store it in their roots. Suberin decomposes slowly, so the carbon remains stored for a long time and less carbon dioxide is released back into the atmosphere by decomposition. Because the plants are perennial, they also remain for many years rather than dying and decomposing each year.

Mark scheme points

Any four of these six points are needed for full marks; the alternatives do not all have to be included.

M1 The plants take in or absorb carbon dioxide.
M2 They take in carbon dioxide for photosynthesis.
M3 Carbon from carbon dioxide is converted into and stored as suberin in the roots.
M4 Suberin decomposes slowly or remains for a long time.
M5 Perennial plants remain for many years or do not die off each year.
M6 Decomposition therefore releases carbon dioxide more slowly or releases less carbon dioxide.

Explanation

First, the plants remove carbon dioxide from the air by absorbing it for photosynthesis. The carbon from this carbon dioxide is then used to make suberin and is stored in the roots.

Because suberin decomposes slowly, its carbon stays stored for a long time, so decomposition returns carbon dioxide to the atmosphere more slowly. The plants’ perennial nature also means they do not die and decompose each year.

Common mistakes

The examiner report noted that some candidates omitted photosynthesis or gave vague restatements of the passage. State that the plants absorb carbon dioxide for photosynthesis, then explain how it is stored.
Saying only that “carbon” is taken in is too vague: identify carbon dioxide as the gas removed from the atmosphere.

Practise Cycles within ecosystems questions

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