IB DP Biology: Production and the carbon cycle: production, sinks, sources and recycling (DP Biology) – 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.

These questions are for IB Diploma Programme Biology, aligned to the IB Biology guide, first assessment 2025, statements C4.2.15 to C4.2.22 (production, the carbon cycle, sinks and sources, the Keeling Curve and recycling). They apply at standard level and higher level; C4.2 has no AHL content. All data are supplied in the questions and are illustrative.

Links: study guide, revision notes, course hub, printable checklist.

Questions

1. Define primary production and state the units in which it is usually measured. [2]

2. A forest and a tundra site were compared. In the forest, plants fixed 2800 g C m⁻² yr⁻¹ and released 1350 g C m⁻² yr⁻¹ in their own respiration. In the tundra, plants fixed 350 g C m⁻² yr⁻¹ and released 200 g C m⁻² yr⁻¹.

(a) Calculate the net primary production of each site. [2] (b) Calculate how many times greater the forest value is, and suggest one reason why the tundra value is low. [2]

3. Explain why secondary production in an ecosystem is lower than primary production. [3]

4. Describe how you would construct a diagram of the carbon cycle in an ecosystem, using photosynthesis, feeding and respiration. Refer to the direction of arrows. [4]

5. Two ecosystems were studied (all values g C m⁻² yr⁻¹).

Ecosystem	Photosynthesis	Respiration by autotrophs	Respiration by heterotrophs and decomposers
A: managed woodland	2000	900	1050
B: drained wetland	400	180	520

(a) Calculate the net carbon dioxide uptake of each ecosystem. [3] (b) State which ecosystem is a carbon source and suggest why its respiration is so high relative to photosynthesis. [2]

6. Coal, peat and natural gas are all stores of carbon.

(a) Distinguish between peat and natural gas in terms of the organisms they came from and the time taken to form. [2] (b) Explain why combustion of fossil fuels increases atmospheric carbon dioxide concentration even though lightning fires also occur naturally. [2]

7. The table shows monthly carbon dioxide concentration (ppm) at a monitoring station in one year, and annual means in three years.

Month	Jan	Feb	Mar	Apr	May	Jun	Jul	Aug	Sep	Oct	Nov	Dec
CO₂ (ppm)	415.5	416.5	417.0	418.5	419.5	418.5	416.5	414.5	413.5	413.8	415.0	416.2

Year	1980	2000	2020
Annual mean (ppm)	338	369	414

(a) State the months with the highest and lowest concentration. [1] (b) Calculate the range of the annual fluctuation. [1] (c) Explain the annual fluctuation. [3] (d) Calculate the mean rate of increase in ppm per year for 1980 to 2000 and for 2000 to 2020. [2] (e) Comment on what the two rates show about the long-term trend. [1]

8. Explain how aerobic respiration and photosynthesis depend on each other. [3]

9. Identical dried leaves were placed in litter bags and buried for 12 weeks. Coarse-mesh bags allowed small invertebrates, fungi and bacteria to enter. Fine-mesh bags excluded the invertebrates. The mean mass remaining was 38% of the starting mass in coarse-mesh bags and 71% in fine-mesh bags.

(a) Calculate the mean percentage mass lost in each type of bag. [2] (b) Suggest why mass loss differed. [2] (c) Explain why decomposition matters for the recycling of elements such as phosphorus. [2]

10. The table gives net ecosystem carbon exchange for a temperate deciduous forest, where positive values mean net CO₂ uptake (g C m⁻² per month).

Month	Jan	Feb	Mar	Apr	May	Jun	Jul	Aug	Sep	Oct	Nov	Dec
Net uptake	−20	−22	−15	10	65	110	100	75	30	−25	−30	−25

(a) List the months in which the forest is a carbon source. [1] (b) Calculate the net uptake for the whole year, and state whether the forest is a sink or a source over the year. [2] (c) Explain why the forest is a source in winter. [2] (d) Explain why net uptake is greatest in June. [2] (e) The forest is clear-felled and the wood is burned. Predict, with reasons, how the annual net uptake would change. [2]

11. A company proposes to plant a forest to offset carbon dioxide released by burning natural gas. Evaluate this proposal. [6]

Answers

1. Primary production is the accumulation of carbon compounds in the biomass of autotrophs [1]. It is measured as carbon mass for each square metre in each year, usually g m⁻² yr⁻¹ [1]. [2] Examiner insight: A definition that mentions only photosynthesis and not accumulation in biomass is incomplete, and units without “per area” or “per time” tend to lose the second point.

2. (a) Forest: 2800 − 1350 = 1450 g C m⁻² yr⁻¹ [1]. Tundra: 350 − 200 = 150 g C m⁻² yr⁻¹ [1]. [2] (b) 1450 ÷ 150 = 9.7 times [1]. Low temperature, short growing season or frozen soil limits photosynthesis (accept low light in winter or limited nutrients) [1]. [2] Examiner insight: Subtracting the plants’ own respiration is the step the data invite, so show it. Quote units on both answers.

3. Respiration converts carbon compounds to carbon dioxide and water [1], so carbon is lost from biomass at each stage of feeding [1]. Not all biomass is eaten, and some of what is eaten is egested rather than assimilated [1]. [3] Examiner insight: Answers that say only “energy is lost as heat” miss the carbon-based reasoning this question asks for. Refer to carbon compounds and respiration.

4. Draw boxes for CO₂ in air or water, producers, consumers, dead organic matter and decomposers. Draw an arrow from CO₂ to producers labelled photosynthesis [1]. Draw feeding arrows from the organism eaten to the feeder, for example producers to consumers [1]. Draw respiration arrows from producers, consumers and decomposers back to CO₂ [1]. Show death and egestion arrows from living organisms to dead organic matter, then to decomposers [1]. [4] Examiner insight: Reversed feeding arrows and a missing decomposer box are two common ways a diagram loses credit. Label every arrow with its process.

5. (a) A: 2000 − 900 − 1050 = +50 [1]. B: 400 − 180 − 520 = −300 [1]. Units g C m⁻² yr⁻¹ given [1]. [3] (b) B is a source because respiration exceeds photosynthesis [1]. Drainage lets oxygen in, so decomposers respire stored organic matter (peat) rapidly [1]. [2] Examiner insight: Both types of respiration must be subtracted. Link the sign of the answer to “sink” or “source” explicitly.

6. (a) Peat forms from partly decayed plant material in waterlogged, acidic conditions over thousands of years [1]. Natural gas forms from buried remains of marine organisms over millions of years [1]. [2] (b) Human activities have greatly increased the rate of combustion compared with natural fires [1]. Fossil fuel carbon was removed from the atmosphere long ago, so burning returns it faster than photosynthesis can take it up [1]. [2] Examiner insight: Answers gain credit for the idea of rate and for the age of the stored carbon, not for restating that “burning releases CO₂”.

7. (a) Highest in May, lowest in September [1]. [1] (b) 419.5 − 413.5 = 6.0 ppm [1]. [1] (c) Most land and vegetation is in the northern hemisphere [1]. In spring and summer photosynthesis exceeds respiration, so CO₂ falls to a minimum by September [1]. In autumn and winter respiration (and decomposition) exceeds photosynthesis, so CO₂ rises to a peak by May [1]. [3] (d) 1980 to 2000: (369 − 338) ÷ 20 = 1.55 ppm per year [1]. 2000 to 2020: (414 − 369) ÷ 20 = 2.25 ppm per year [1]. [2] (e) The rate of increase has risen, so the long-term rise is accelerating, consistent with increasing combustion of fossil fuels [1]. [1] Examiner insight: Keep the two features apart: the fluctuation is explained by photosynthesis and respiration, the long-term rise by combustion. Quote data when describing trends.

8. Aerobic respiration uses oxygen that was released by photosynthesis [1]. Photosynthesis uses carbon dioxide released by respiration (and combustion) [1]. The yearly fluxes are enormous, so this is a major link between autotrophs and heterotrophs [1]. [3] Examiner insight: Show both directions of dependence. A one-way answer usually earns only part of the credit.

9. (a) Coarse: 100 − 38 = 62% [1]. Fine: 100 − 71 = 29% [1]. [2] (b) Invertebrates in coarse bags fragment the leaves, increasing surface area for fungi and bacteria [1]. Greater decomposer activity means faster digestion and respiration of the leaf carbon compounds [1]. [2] (c) Decomposers digest dead organic matter and release phosphate and other ions into the soil [1]. Autotrophs take these up again, so the element is recycled rather than locked in dead material [1]. [2] Examiner insight: Link each observation to a named process. Naming decomposers as bacteria and fungi, rather than “microbes”, is more precise.

10. (a) January, February, March, October, November, December [1]. [1] (b) Sum = −20 − 22 − 15 + 10 + 65 + 110 + 100 + 75 + 30 − 25 − 30 − 25 = +253 g C m⁻² [1]. Positive, so a sink [1]. [2] (c) Leaves have fallen or light is low, so photosynthesis is low [1]. Respiration by trees, heterotrophs and decomposers continues and exceeds photosynthesis, releasing CO₂ [1]. [2] (d) Long days, high light and full leaf cover give a high rate of photosynthesis [1]. This greatly exceeds respiration, so net uptake peaks [1]. [2] (e) Net uptake would fall, possibly becoming negative [1]. Photosynthesis would be removed or reduced, and burning and decomposition of residues release CO₂, so the site may become a source [1]. [2] Examiner insight: Show the sum of positive and negative values. Describe the forest in a given month as a source or sink using the net exchange, not the amount of photosynthesis alone.

11. Points that can be credited: growing trees take up CO₂ by photosynthesis, so a young forest is a sink [1]. Fossil fuel carbon was stored for millions of years, but the forest stores carbon only while biomass accumulates [1]. As the forest matures, respiration approaches photosynthesis and net uptake falls [1]. Fire, disease or felling can convert the forest into a source [1]. The rate of release from burning gas is rapid, and growth of trees is slow, so offsetting takes years and a large area [1]. A judgement is made, such as supporting the proposal only as a partial, temporary measure alongside reduced combustion [1]. [6] Examiner insight: Evaluation tends to gain credit when there are advantages, limitations and a final judgement. A list of facts with no conclusion is likely to fall short.

Where marks are usually lost

• Leaving out units or using energy units for production.
• Subtracting only one form of respiration when the table has two.
• Confusing a sink with a large store of carbon, rather than a net uptake.
• Explaining the seasonal zigzag and the long-term rise with the same reasoning.
• Drawing feeding arrows backwards and omitting decomposers.
• Saying “respiration” without naming the products (CO₂ and water) when explaining lower secondary production.
• Not quoting data in graph descriptions.
• Writing that decomposers “create” nutrients. They release inorganic ions from dead organic matter.

Next steps

• Re-read the revision notes and the study guide.
• Visit the course hub and tick off the printable checklist.
• Take all free 10-minute diagnostics.
• Book a free trial class at /trial/.

Official syllabus

IB Diploma Programme Biology guide, first assessment 2025, published by the International Baccalaureate Organization. Statements C4.2.15 to C4.2.22.