AQA GCSE Combined Science: Trilogy Biology Paper 2 (Foundation), 2022: Question 5

16 marks · Standard Demand difficulty · Extended Answer

Identify processes in the water cycle, calculate population statistics in standard form, and explain how microorganisms recycle carbon from fallen leaves.

Practise this question

Question

A multi-part exam question about ecosystems. It includes a diagram of the water cycle (Figure 7) with numbered arrows showing movement between a lake, clouds, trees, and the sun. Question parts ask to identify reactions producing water, name water cycle processes, calculate a percentage of the world population in standard form, identify causes of water shortage, and explain carbon recycling by microorganisms.
Question text

05 This question is about the cycling of water and carbon in ecosystems.

05.1 Which reaction produces water?

[1 mark]

Tick ( ) one box.

Aerobic respiration

Anaerobic respiration

Photosynthesis

The water cycle provides water for plants and animals on land before the water goes

into lakes and seas.

Figure 7 represents the water cycle.

Figure 7

05.2 Name the processes 1 to 5 shown on Figure 7.

[5 marks]

05.3 In 2007 the population of the world was 6 000 000 000

A study found that 4.5% of the population had severe water shortage.

Calculate how many people had severe water shortage.

Give your answer in standard form.

[3 marks]

Number of people (in standard form) =

05.4 Why do more people have severe water shortage now than in 2007?

[2 marks]

Tick ( ) two boxes.

Climate change has increased the area of deserts.

Each person drinks less water.

More water is used to grow crops.

Sea levels have risen because the ice caps are melting.

Some countries have built de-salting factories for seawater.

Leaves on a tree contain carbon compounds.

In autumn the leaves fall to the ground.

05.5 Microorganisms in the soil recycle carbon from the leaves so that the carbon is used

for new plant growth.

Explain how.

[4 marks]

05.6 What is one benefit of fallen leaves for living plants?

[1 mark]

Tick ( ) one box.

*24* Energy is released for living plants.

Insect pests in the soil are killed.

Nitrates are released into the soil.

Oxygen is supplied to root cells.

Mark scheme

Show the mark scheme The mark scheme for question 5. It lists 'aerobic respiration' for 05.1. For 05.2, it names processes 1-5: evaporation, condensation, precipitation, draining, and transpiration. For 05.3, it shows the calculation 6,000,000,000 times 0.045 equals 270,000,000, which is 2.7 x 10^8 in standard form. For 05.5, it explains carbon recycling through decay, respiration by microorganisms releasing CO2, and photosynthesis using that CO2.

Question 5

AO /

Question Answers Extra information Mark

Spec. Ref.

05.1 aerobic respiration 1 AO1

4.4.2.1

4.8

AO /

Spec. Ref.

05.2 1. evaporation allow evaporate(s) / evaporating 1 AO1

4.7.2.2

2. condensation allow condense(s) / condensing 1

3. precipitation allow rain(ing) / rainfall 1

allow named precipitation

ignore precipitates

4. draining / drainage allow run-off / percolation / 1

infiltration

allow groundwater /

underground flow

5. transpiration 1

AO /

Spec. Ref.

05.3 AO2

4.5 4.7.2.2

6 000 000 000 × allow 6 000 000 000 × 0.045 1

270 000 000 1

2.7 ×108 allow an incorrectly calculated 1

number of people given in

correct standard form

– OMBINED SCIENCE: TRILOGY – –

AO /

Spec. Ref.19

05.4 climate change has increased 1 AO3

the area of deserts 4.7.2.2

more water is used to grow 1

crops

AO /

Spec. Ref.

05.5 decay / decomposition of leaves allow leaves are broken down 1 AO2

respiration (by microorganisms / ignore aerobic / anaerobic 1 AO1

decomposers) ignore respiration by worms /

detritivores / insects / leaves

respiration releases carbon 1 AO1

dioxide

or

microorganisms release carbon

dioxide

carbon dioxide is used in do not accept carbon dioxide 1 AO1

photosynthesis (for new plant absorbed in the roots

growth) 4.7.2.2

4.4.1.1

AO /

Spec. Ref.

05.6 nitrates are released into the soil 1 AO2

4.7.2.2

4.4.1.3

Total Question 5 16

How to answer it

Cycling of Water and Carbon

What this question tests

This question assesses your ability to identify biological processes that produce water, label the stages of the water cycle, perform percentage calculations in standard form, and explain how microorganisms recycle carbon through decay and respiration.

Part (05.1)

Identifying Water Production

Correct Answer

Aerobic respiration

Key Knowledge

The equation for aerobic respiration is:
Glucose + Oxygen → Carbon Dioxide + Water

Exam Technique

Don't confuse this with photosynthesis, which uses water as a reactant. Anaerobic respiration in humans produces lactic acid, not water.

Part (05.2)

The Water Cycle

Correct Answers

  1. Evaporation (Liquid to gas from the lake)
  2. Condensation (Gas to liquid/clouds)
  3. Precipitation (Rain/snow falling)
  4. Draining / Surface run-off (Water moving to the lake)
  5. Transpiration (Water loss from plants)

Examiner Insight

For process 3, "Rain" is accepted, but "Precipitation" is the scientific term. For process 4, "Percolation" or "Infiltration" are also acceptable as they describe water moving through soil.

Part (05.3)

Percentage Calculation & Standard Form

Step-by-Step Calculation

  1. Identify the values: Population = 6,000,000,000. Percentage = 4.5%.
  2. Calculate the number: 6,000,000,000 × (4.5 / 100) = 270,000,000. [2 marks]
  3. Convert to Standard Form: Move the decimal point 8 places to the left.
    Answer = 2.7 × 10⁸ [1 mark]

Common Errors

  • Miscounting the number of zeros (there are 9 zeros in 6 billion).
  • Giving the answer as 27 × 10⁷. In standard form, the first number must be between 1 and 10.

Part (05.4)

Water Shortage Factors

Correct Answers

  • Climate change has increased the area of deserts.
  • More water is used to grow crops.

Logic Check

The question asks why shortages have increased. While sea levels rising is a fact, it doesn't provide more fresh drinking water. De-salting factories actually reduce shortages, so that would be the opposite of what the question asks.

Part (05.5)

The Carbon Cycle: Recycling Leaves

How Carbon is Recycled (4 Marks)

  1. Decay/Decomposition: Microorganisms (bacteria/fungi) break down the fallen leaves.
  2. Respiration: These microorganisms respire while breaking down the organic matter.
  3. CO₂ Release: Respiration releases carbon dioxide into the atmosphere.
  4. Photosynthesis: This CO₂ is then taken in by new plants to make glucose for growth.

Common Misconceptions

Students often lose marks by saying carbon is "absorbed by the roots." Carbon is taken in as CO₂ through the leaves via photosynthesis, not through the roots. Roots absorb water and minerals (like nitrates), not carbon.

Part (05.6)

Benefits of Fallen Leaves

Correct Answer

Nitrates are released into the soil.

Key Fact

Decomposition doesn't just recycle carbon; it also recycles mineral ions. Nitrates are essential for plants to build proteins for growth.

Topics

Biology · B4: Bioenergetics · B7: Ecology

Question and mark scheme from the AQA GCSE Combined Science: Trilogy examination, Biology Paper 2 (Foundation), 2022. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.