IB MYP Sciences – Cycles in science 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 – the IB holds copyright in its own papers. Use these alongside the official past papers available through your school or the IB store.

This practice set covers the carbon, water, nitrogen and rock cycles for IB MYP Sciences, aligned to the International Baccalaureate Organization, Middle Years Programme Subject Brief – Sciences, from 2014. It suits MYP years 4 and 5 (no SL/HL split). MYP has no prescribed content list; schools design their own units, and “cycles” is one of the topics the IB’s brief lists for the sciences on-screen examinations.

Each question is labelled with the criterion it trains. Real MYP work is judged against criterion level descriptors (achievement levels 1–8), so the [1] points in the answers are a revision aid, not IB marks. All data in the questions are fictional.

Study first with the cycles study guide and revision notes. Also: course hub, checklist, criteria in practice questions.

Questions

1. (Criterion A) In the carbon cycle, name two processes that release carbon dioxide into the atmosphere and one process that removes it. [3]

2. (Criteria A and C) A fictional upland catchment receives 900 mm of precipitation in one year. Evapotranspiration returns 540 mm to the atmosphere.

(a) Define transpiration. [1] (b) Calculate the depth of water left for runoff and groundwater, and express it as a percentage of the precipitation. [2] (c) Suggest how clearing the catchment’s forest would change surface runoff. Explain your answer. [2]

3. (Criterion A) Explain the role of each of these in the nitrogen cycle: nitrogen-fixing bacteria, nitrifying bacteria and denitrifying bacteria. [3]

4. (Criteria A and C) A layer of shale lies above a layer of sandstone in a fictional cliff.

(a) Describe how the sediment that formed the sandstone became rock. [2] (b) The sand was deposited at an average rate of 0.25 mm per year. Calculate how long a 1.5 m layer took to deposit. [1] (c) The shale is later buried deeply and heated, but does not melt. Name the type of rock that forms and one example. [1]

5. (Criterion A) Nitrogen gas makes up most of the air, yet crops can be short of nitrogen.

(a) Explain why plants cannot use nitrogen gas directly, and state the form in which most plants absorb nitrogen. [2] (b) Describe two ways nitrogen gas can be converted into compounds without living organisms. [2]

6. (Criterion B) Design an investigation into how temperature affects the rate at which leaf litter decomposes. Include a testable research question, a hypothesis with a scientific reason, the independent and dependent variables, and two controlled variables with how you would control them. [6]

7. (Criterion C) A student places 20.0 g of dry leaf litter in mesh bags in soil kept at three temperatures. After 8 weeks the bags are dried and reweighed.

Temperature / °C	Final dry mass / g
10	17.2
20	14.6
30	12.1

(a) Calculate the percentage mass lost at each temperature. [3] (b) Calculate the mean rate of mass loss at 30 °C in g per week. Give your answer to 2 decimal places. [1] (c) Explain the trend using your knowledge of decomposers. [2]

8. (Criterion C) A monitoring station on the fictional island of Norland records these annual mean carbon dioxide concentrations.

Year	1	2	3	4	5
CO₂ / ppm	402.0	404.4	406.8	409.0	411.6

(a) Calculate the mean annual increase from year 1 to year 5. [2] (b) Assuming the same rate continues, predict the concentration in year 8. [1] (c) Within each year the monthly readings rise in winter and fall in summer. Explain this pattern. [2]

9. (Criteria B and C) A student uses a bubble potometer to compare water uptake by a leafy shoot in still air and with a fan blowing. The bubble moves 42 mm in 6 minutes in still air and 90 mm in 6 minutes with the fan.

(a) Calculate the rate of bubble movement in each condition in mm per minute, and the percentage increase caused by the fan. [3] (b) The capillary tube has a cross-sectional area of 0.8 mm². Calculate the volume of water taken up in 6 minutes with the fan. [1] (c) State why a potometer measures water uptake rather than water loss. [1]

10. (Criteria A and D) A river next to heavily fertilised farmland has frequent fish deaths in summer. Explain, in sequence, how the fertiliser can lead to the death of fish. [5]

11. (Criterion D – extended response) Two approaches are proposed for reducing the carbon dioxide added to the atmosphere by a fictional country, Norland:

• Approach 1: plant new forest on unused farmland.
• Approach 2: fit carbon capture and storage (CCS) to its coal-fired power stations, storing the captured carbon dioxide in porous rock underground.

Evaluate the two approaches. Explain how each works in carbon cycle terms, discuss strengths and limitations, consider at least one economic, social or environmental factor, and reach a justified conclusion. [10]

Answers

1. Any two releasing processes: respiration, combustion, decomposition (respiration by decomposers) [1] [1]. Removing: photosynthesis (or dissolving in the oceans) [1]. [3] Examiner insight: “Decay” alone may not score; saying decomposers respire names the process that releases the CO₂.

2. (a) Loss of water vapour from the leaves of a plant, mainly through the stomata [1]. (b) 900 − 540 = 360 mm [1]; 360 ÷ 900 × 100 = 40% [1]. (c) Runoff would increase [1]: fewer leaves intercept rain, transpiration falls and less water infiltrates without roots [1]. Examiner insight: Show the unit on the depth; a bare “360” can lose the accuracy point despite correct method.

3. Nitrogen-fixing bacteria convert nitrogen gas into ammonium compounds [1]. Nitrifying bacteria convert ammonium into nitrite and then nitrate [1]. Denitrifying bacteria convert nitrate back to nitrogen gas, in oxygen-poor soil [1]. [3] Examiner insight: Each role needs the starting and final substance; “they help plants get nitrogen” names no conversion and earns nothing.

4. (a) Sand was deposited in layers and compacted by the weight of layers above [1], then the grains were cemented together by minerals [1]. (b) 1,500 ÷ 0.25 = 6,000 years [1]. (c) Metamorphic rock, e.g. slate [1]. Examiner insight: In (b), convert 1.5 m to 1,500 mm first; mixed units give an answer 1,000 times too small.

5. (a) The two nitrogen atoms in N₂ are held by a very strong triple bond, which plants cannot break [1]; plants absorb nitrogen mainly as nitrate ions through their roots [1]. (b) Lightning provides energy for nitrogen to react with oxygen, forming compounds that dissolve in rain [1]; the Haber process in industry makes ammonia from nitrogen and hydrogen, used in fertilisers [1]. Examiner insight: The question says “without living organisms”, so nitrogen-fixing bacteria earn nothing here even though the chemistry is correct.

6. Research question: how does soil temperature (10, 20, 30 °C) affect the percentage mass of leaf litter lost in 8 weeks? [1] Hypothesis: mass loss will increase with temperature [1] because decomposers’ enzymes work faster, so they respire and break down material faster [1]. Independent variable: temperature; dependent variable: percentage loss in dry mass [1]. Controlled: same leaf species and starting dry mass, weighed on a balance [1]; same soil and moisture, using soil from one source and adding a set volume of water weekly [1]. [6] Examiner insight: A controlled variable earns credit only with a method of controlling it; a list of variables alone gains little under criterion B.

7. (a) 10 °C: 2.8 ÷ 20.0 × 100 = 14.0% [1]; 20 °C: 5.4 ÷ 20.0 × 100 = 27.0% [1]; 30 °C: 7.9 ÷ 20.0 × 100 = 39.5% [1]. (b) 7.9 ÷ 8 = 0.99 g per week [1]. (c) Mass loss increases with temperature [1] because the enzymes of bacteria and fungi work faster, so decomposers respire more and break down the litter faster, releasing carbon as CO₂ [1]. Examiner insight: Percentage loss uses the starting mass (20.0 g) as the denominator; dividing by the final mass is a common error that loses the accuracy points.

8. (a) Increase = 411.6 − 402.0 = 9.6 ppm over 4 years [1]; mean = 9.6 ÷ 4 = 2.4 ppm per year [1]. (b) 411.6 + 3 × 2.4 = 418.8 ppm [1]. (c) In summer, plants photosynthesise more (longer days, more leaves), removing more CO₂ [1]; in winter, photosynthesis falls while respiration and combustion continue, so CO₂ builds up [1]. Examiner insight: There are four intervals between five years, not five; dividing by 5 gives 1.92 ppm and loses the accuracy point.

9. (a) Still air: 42 ÷ 6 = 7.0 mm per minute [1]; fan: 90 ÷ 6 = 15.0 mm per minute [1]; increase = (15.0 − 7.0) ÷ 7.0 × 100 = 114% [1]. (b) 90 × 0.8 = 72 mm³ [1]. (c) Some water taken up is used by the plant (e.g. in photosynthesis), so uptake does not equal transpiration loss [1]. Examiner insight: The percentage change is compared with the original (still-air) value; using 15.0 as the base gives 53% and scores no accuracy credit.

10. Rain washes fertiliser nitrate into the river [1]. Algae grow rapidly on the surface (algal bloom) [1]. Light is blocked, so plants below cannot photosynthesise and die [1]. Decomposers break down the dead matter, using up dissolved oxygen in respiration [1]. Fish die from lack of dissolved oxygen [1]. [5] Examiner insight: “Algae use up the oxygen” misses the key step; the oxygen fall is due to respiring decomposers, and naming them earns the fourth point.

11. Model plan with indicative points (real criterion D work is judged against level descriptors, not point by point):

• Approach 1: growing trees remove CO₂ by photosynthesis and store carbon in wood and soil [1].
• Strength: low cost; also gives habitats and reduces runoff and soil erosion [1].
• Limitation: slow (decades), and the carbon returns to the air if the forest burns or is felled [1].
• Limitation: needs large areas of land that may be needed for food [1].
• Approach 2: CO₂ is removed from power-station waste gases before release, compressed and stored in porous rock [1].
• Strength: cuts most CO₂ from a large single source immediately [1].
• Limitation: expensive; capture uses energy, so more fuel is burned per unit of electricity [1].
• Limitation: stores must be monitored for leaks; no help for spread-out sources such as vehicles [1].
• Other factor: CCS keeps coal jobs (social/economic), but coal mining still damages land and water (environmental) [1].
• Justified conclusion: e.g. forests are cheaper with wider benefits but slow, so using both while moving away from coal cuts emissions faster than either alone [1]. [10] Examiner insight: Describing both approaches without weighing them, or ending with no judgement, keeps a response in the lower criterion D bands however accurate the science.

Where marks are usually lost

• Naming a cycle process without saying what it converts or moves (for example “nitrification happens”).
• Confusing nitrogen fixation, nitrification and denitrification, or which needs oxygen.
• Dividing by the wrong value in percentage questions (final instead of starting value).
• Counting data points instead of intervals when finding a mean rate of change.
• Listing controlled variables without saying how each is controlled.
• Writing a one-sided criterion D answer, or no justified conclusion.

Next steps

• Cycles revision notes
• Cycles study guide
• IB MYP Sciences course hub
• Printable MYP Sciences checklist
• All free 10-minute diagnostics
• Book a free trial class

Official syllabus

International Baccalaureate Organization, Middle Years Programme Subject Brief – Sciences, from 2014. The brief lists “cycles” among the topics explored in the MYP sciences on-screen examinations; all questions here are original.