OxfordAQA International GCSE Biology: Decay, the carbon cycle and human effects on the environment (9201) – Practice Questions

These are original questions written for Marlbridge, for revision and practice on this content. They are not reproduced past-paper questions, and they do not replicate the exam’s exact structure, question count or mark tariffs – examination boards hold copyright in their own papers. Use these alongside the official past papers from your board or school.

These questions cover sections 3.3.3 and 3.3.4 of the OxfordAQA International GCSE Biology (9201) specification, Version 5.2, for exams May/June 2018 onwards. Read the study guide first, and keep the revision notes beside you. Course pages: Biology hub and printable checklist. For a quick check of your level, use the free diagnostics.

Questions

1. State three conditions in which microorganisms digest dead material fastest. [3]

2. A student puts 500 g dry mass of grass clippings into each of four heaps with different water content. After 14 days she measures the dry mass left.

Water content of heap (%)	20	45	60	85
Dry mass remaining (g)	470	395	310	455

(a) Calculate the percentage of dry mass lost in the heap with 60% water content. [2] (b) Explain why the heap with 85% water content lost less mass than the heap with 60%. [2]

3. Complete the account of the carbon cycle. Name the process at each letter. Green plants take carbon dioxide from the air by A. Carbon compounds pass to animals by B. Plants, animals and microorganisms return carbon dioxide to the air by C. Burning wood or coal also returns carbon dioxide by D. [4]

4. The table shows the yearly carbon flows in a pond community, in arbitrary units.

Flow	Units per year
Carbon taken in by algae and plants	86
Released by algae and plant respiration	31
Released by animal respiration	9
Released by respiration of microorganisms	36

(a) Calculate the total carbon released by respiration. [1] (b) Calculate the net change in carbon in the water and air, and say what this shows about the stability of the community. [2] (c) State how carbon from the algae becomes part of the fats and proteins in a fish. [1] (d) Describe how carbon in a dead alga is returned to the environment. [2]

5. Dead plants in a waterlogged peat bog decay much more slowly than dead plants on a woodland floor. Explain why. [3]

6. Fish die in a lake that receives run-off from fertilised fields. Explain why. [6]

7. For each source of waste, state the medium it pollutes and one consequence. (a) Sulfur dioxide from burning fuel [1] (b) A herbicide sprayed on a field [1] (c) Untreated sewage [1] (d) Smoke from a fire [1]

8. A tropical region has 60 000 km² of forest. Over ten years 4 800 km² are cleared. (a) Calculate the percentage of the forest cleared. [2] (b) Describe two effects of this clearance on biodiversity and two effects on carbon dioxide levels in the atmosphere. [4]

9. The table shows data from an island monitoring station.

Year	1990	2005	2020
Carbon dioxide (ppm)	354	379	413
Mean air temperature (°C)	14.1	14.5	14.9

(a) Name two gases in the atmosphere that are increasing and contribute to global warming. [1] (b) Calculate the percentage increase in carbon dioxide from 1990 to 2020. [2] (c) A student says the data prove that carbon dioxide causes the rise in temperature. Evaluate this statement. [1] (d) Give two possible effects of a rise of a few degrees Celsius in the Earth’s temperature. [2]

10. Volunteers at four coastal sites record the day of the year on which a summer-visiting bird is first seen. The mean first-sighting day was 112 for the years 1985 to 1994 and 104 for the years 2015 to 2024. (a) Calculate how many days earlier the bird arrives in the later period. [1] (b) Suggest what the data may show about the effect of climate change on the bird. [1] (c) Evaluate the method used to collect the data, considering validity and reliability. [4]

11. A tropical forest is cleared and burned to make pasture for cattle. Describe and explain the effects on biodiversity, on the carbon cycle and on global warming. [8]

12. A lake is sampled each week after a heavy rainstorm washes fertiliser into it.

Week	0	1	2	3	4	5
Nitrate (mg per dm³)	1.8	5.2	7.9	7.1	6.5	6.0
Algae (thousand cells per cm³)	9	60	210	340	70	25
Dissolved oxygen (mg per dm³)	9.0	9.8	10.4	7.5	3.6	1.2

(a) Describe the change in the number of algae over the six weeks. [2] (b) Explain why the dissolved oxygen fell between week 2 and week 5. [3] (c) Calculate the percentage fall in dissolved oxygen from week 2 to week 5. [2] (d) All the water samples came from one spot near the shore. Suggest one improvement. [1]

Answers

1. Warm [1]; moist [1]; aerobic, so oxygen is present [1]. [3] Examiner insight: “Hot” is risky, because very high temperatures stop decay.

2. (a) Mass lost = 500 − 310 = 190 g [1]. Percentage = 190 ÷ 500 × 100 = 38.0% [1]. (b) At 85% the water fills the spaces between the clippings, so there is less oxygen [1]. Aerobic respiration of the microorganisms is limited, so they digest the clippings more slowly [1]. [4] Examiner insight: A comparison needs the reason as well as the observation. Saying “too much water” without linking it to oxygen and respiration leaves the explanation incomplete.

3. A photosynthesis [1]; B feeding (eating) [1]; C respiration [1]; D combustion [1]. [4] Examiner insight: Each blank needs a process word, not a gas or an organism.

4. (a) 31 + 9 + 36 = 76 units [1]. (b) Net = 86 − 76 = 10 units more taken in than released [1]. Input is greater than output, so the community is not perfectly balanced; carbon is accumulating in biomass [1]. (c) The fish eat algae (or animals that ate algae) and the carbon compounds are digested and built into the fish [1]. (d) Microorganisms (and detritus feeders) feed on the dead alga [1]. The microorganisms respire and release carbon dioxide [1]. [6] Examiner insight: When a question asks what a value shows, state the meaning in words as well as the number.

5. The bog is waterlogged, so there is little oxygen [1]. Microorganisms are therefore less able to respire aerobically and are less active [1]. Digestion of the dead plants is very slow, so the carbon stays in the dead material [1]. [3] Examiner insight: Naming oxygen as the limiting factor and then linking it to the microorganisms’ activity is the chain examiners typically look for. “It is wet so it decays slowly” contradicts the idea that moisture helps.

6. Fertiliser raises the concentration of mineral ions in the water [1]. Algae and/or plants grow faster [1]. Dense growth blocks light, so some cannot photosynthesise and die [1]. The population of microorganisms feeding on the dead material increases [1]. Their respiration uses up oxygen, so the oxygen concentration falls [1]. Fish and other aerobic organisms die because they cannot get enough oxygen [1]. [6] Examiner insight: Sequence questions reward answers in the correct order. Saying that the algae use up the oxygen, rather than the microorganisms feeding on them, is a common slip.

7. (a) Air; contributes to acid rain [1]. (b) Land; can be washed into water and harm organisms [1]. (c) Water; causes eutrophication [1]. (d) Air; the smoke can harm organisms that breathe it in [1]. [4] Examiner insight: Each part asks for two things, so give both the medium and the consequence. A single word may not be enough.

8. (a) 4 800 ÷ 60 000 [1] × 100 = 8.0% [1]. (b) Biodiversity: habitats and food sources are lost [1]; species die or leave, so fewer species remain [1]. Carbon dioxide: less photosynthesis, so less carbon dioxide is removed from the air [1]; burning or decay releases carbon dioxide [1]. [6] Examiner insight: The question asks for two of each, so they should be different points. Repeating “fewer trees” under both headings gains only once.

9. (a) Carbon dioxide and methane [1]. (b) Increase = 413 − 354 = 59 ppm [1]. Percentage = 59 ÷ 354 × 100 = 16.7% [1]. (c) Both rise together, which fits the idea, but a correlation alone does not prove cause; other factors might be involved and only three years are shown [1]. (d) Any two: rise in sea level [1]; changes in climate; changes in species distribution; changes in migration patterns [1]. [6] Examiner insight: An evaluation of “prove” is usually a statement about what the data can and cannot show. Agreeing with the student outright is unlikely to be enough.

10. (a) 112 − 104 = 8 days earlier [1]. (b) The bird arrives earlier, which may be linked to a warming climate changing migration patterns, although this is not proved [1]. (c) Any four points about validity or reliability: records from four sites and ten-year means reduce the effect of one unusual year [1]; a first sighting depends on how many people are looking, so changes in effort could affect the result [1]; observers may differ in skill, so there may be bias [1]; other factors such as food supply were not controlled, so the cause cannot be linked to climate alone [1]. [6] Examiner insight: Evaluation points score best when each names a feature of the method and says how it affects validity or reliability.

11. Any eight points from: habitats and food sources are lost [1]; many species cannot survive so biodiversity falls [1]; trees were storing carbon in their bodies [1]; fewer trees means less photosynthesis, so less carbon dioxide removed from the air [1]; burning is combustion, which releases carbon dioxide [1]; decay of the remaining material by microorganisms releases carbon dioxide through their respiration [1]; carbon dioxide is a gas that contributes to global warming [1]; global warming may cause climate changes, sea level rise or changes in species distribution [1]. [8] Examiner insight: Extended answers are usually credited for a range of points. Covering biodiversity, the carbon cycle and global warming at least once each is safer than five points on one theme.

12. (a) Algae rise from 9 to a peak of 340 thousand cells per cm³ in week 3 [1], then fall to 25 by week 5 [1]. (b) Dense algae shade one another and many die (the count falls from 340 to 25 after week 3) [1]. The microorganisms that feed on the dead algae increase in number [1]. Their respiration uses oxygen faster than it is replaced [1]. (c) Fall = 10.4 − 1.2 = 9.2 mg per dm³ [1]. Percentage = 9.2 ÷ 10.4 × 100 = 88.5% [1]. (d) Take samples from several places and depths and calculate a mean [1]. [8] Examiner insight: A good description quotes values with units, for example the peak and when it occurs, instead of saying “it goes up then down”.

Where marks are usually lost

• Saying microorganisms “eat” material without mentioning digestion.
• Listing “hot” as a condition for decay, rather than warm.
• Forgetting oxygen, or not linking waterlogging to a lack of oxygen.
• Leaving respiration out of the carbon cycle, or forgetting that microorganisms respire.
• Attributing the oxygen loss in eutrophication to algae instead of microorganisms.
• Stating a calculated value but not interpreting it when the question says “what this shows”.
• Using the wrong starting value for a percentage change, or leaving out units.
• Claiming that correlation proves cause.

Next steps

• Revise with the revision notes and re-read the study guide.
• Tick off the topics on the printable checklist and return to the Biology hub.
• Take all free 10-minute diagnostics.
• Book a free trial class.

Official syllabus

OxfordAQA International GCSE Biology (9201), Version 5.2, for exams May/June 2018 onwards, sections 3.3.3 and 3.3.4. Published by OxfordAQA.