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

These are condensed revision notes for IB Diploma Programme Biology, aligned to the IB Biology guide, first assessment 2025, statements C4.2.15 to C4.2.22 (the second part of C4.2, Transfers of energy and matter). All of it applies at standard level and higher level; C4.2 has no AHL content. For full explanations, read the study guide, then test yourself with the practice questions.

Navigation: course hub, printable checklist, and the free diagnostics.

Key definitions

Term	Meaning
Primary production	Carbon compounds building up in the biomass of autotrophs
Secondary production	Carbon compounds building up in the biomass of heterotrophs
Units of production	Carbon mass for each square metre in each year, usually g m⁻² yr⁻¹
Carbon sink	Takes up more CO₂ than it releases (photosynthesis exceeds respiration)
Carbon source	Releases more CO₂ than it takes up (respiration exceeds photosynthesis)
Decomposers	Bacteria and fungi that digest dead organic matter and release inorganic nutrients and CO₂
Keeling Curve	Record of atmospheric CO₂ concentration at Mauna Loa, Hawaii, since 1958

Helpful extra vocabulary from data tables (not required by the guide): gross production is all carbon fixed by photosynthesis, and net production is gross production minus the autotrophs’ own respiration.

Production: the essentials

• Biomass accumulates only when organisms grow or reproduce.
• Net primary production = carbon fixed − carbon released in autotroph respiration.
• Secondary production is the smaller figure because cell respiration breaks carbon compounds down to CO₂ and water, and because some biomass is not eaten or is egested.
• Biomes differ: warm, wet, bright places accumulate more biomass than cold or dry ones.
• Egested faeces and dead material are not lost to the ecosystem. They supply decomposers.

Method in steps: secondary production from a budget

1. Assimilated carbon = ingested carbon − egested carbon.
2. Secondary production = assimilated carbon − carbon released in respiration.
3. Express as a percentage of what the question asks for (ingested, assimilated or net primary production).
4. Give units, g C m⁻² yr⁻¹.

Worked reminder: ingested 80, egested 30, respired 35 (all g C m⁻² yr⁻¹). Assimilated = 50. Secondary production = 15. As a percentage of assimilated carbon: 15 ÷ 50 = 30%.

Carbon cycle diagram: checklist

Stores: CO₂ in air and water, producers, consumers, dead organic matter, decomposers, and (optionally) fossil fuels.

Processes to label:

• Photosynthesis: CO₂ to producers.
• Feeding: arrows point to the feeder.
• Death and egestion: living organisms to dead organic matter.
• Decomposition: dead organic matter to decomposers.
• Cell respiration: every living box, including decomposers, to CO₂.
• Combustion: biomass and fossil fuels to CO₂.

Sinks and sources

Net CO₂ uptake = photosynthesis − (respiration by autotrophs + respiration by heterotrophs and decomposers).

Result	Meaning
Positive	Net uptake, so a sink
Negative	Net release, so a source
Near zero	Roughly balanced

An ecosystem can switch. A forest that burns, or a peatland that is drained, can change from sink to source.

Combustion

Fuel	Origin	Age of the carbon store
Biomass	Recent plants	Years to centuries
Peat	Partly decayed plants in waterlogged, acidic soil	Thousands of years
Coal	Buried plant remains	Hundreds of millions of years
Oil and natural gas	Buried remains of marine organisms	Millions of years

Lightning fires occur naturally. Human activities have greatly increased combustion rates.

The Keeling Curve

Feature	Pattern	Explanation
Annual fluctuation	Peak about May, minimum about September or October	Northern hemisphere growing season: photosynthesis exceeds respiration, so CO₂ falls. Autumn and winter: respiration exceeds photosynthesis, so CO₂ rises
Long-term trend	Annual mean rises every year, and the rate has increased	Combustion of fossil fuels adds carbon that photosynthesis does not fully offset

When analysing a graph: quote start and end values, calculate the change, give a rate per year, comment on whether the rate is constant, then explain.

Interdependence and recycling

• Aerobic respiration needs atmospheric oxygen, produced by photosynthesis.
• Photosynthesis needs atmospheric CO₂, replenished by respiration and combustion.
• The yearly fluxes are enormous, which makes this a major link between autotrophs and heterotrophs.
• All elements needed by organisms are recycled, not only carbon. Decomposers are the key group.
• Matter is recycled in ecosystems, but energy flows through and leaves as heat.
• Detailed nitrogen-cycle and other nutrient-cycle knowledge is not needed.

Small worked reminders

Sink or source. Photosynthesis 1250, autotroph respiration 520, heterotroph and decomposer respiration 640 (g C m⁻² yr⁻¹). Net uptake = 1250 − 520 − 640 = +90. Positive, so a sink.

Keeling Curve rate. Annual means of 354 ppm and 414 ppm, 30 years apart. Rise = 60 ppm. Mean rate = 60 ÷ 30 = 2.0 ppm per year. Percentage rise = 60 ÷ 354 × 100 = 17%.

Seasonal drop. A peak of 419.5 ppm in May and a minimum of 413.5 ppm in September. Fall = 6.0 ppm over about four months, roughly 1.5 ppm per month.

Litter decomposition. Mass falls from 200 g to 150 g. Loss = 25%. If a second sample loses 62%, it decomposed about 2.5 times faster (62 ÷ 25 = 2.48).

Describing and explaining data

Use this order for any graph or table question in this unit.

1. Describe: name the variable and quote values with units, for example the highest and lowest points.
2. Calculate: a difference, percentage change or rate per year if the data allow it.
3. Compare: say whether the rate stays the same, speeds up or slows down.
4. Explain: link each pattern to named processes, which here are photosynthesis, cell respiration, combustion and decomposition.
5. Judge: if the command term is evaluate or suggest, add a limitation or alternative.

Command terms that matter here

Command term	What a good answer does
Define	Gives the precise meaning, with units where relevant
Describe	Gives the pattern, with data quoted
Explain	Gives the reason, using a named process
Calculate	Shows the working and gives the unit
Distinguish	States a difference for both items
Suggest	Offers a sensible reason from the context
Evaluate	Weighs strengths and limitations and reaches a judgement

Must-know distinctions

• Primary production (autotrophs) versus secondary production (heterotrophs).
• Sink versus source, judged on net CO₂ exchange, not on the size of the store.
• Annual fluctuation (seasonal, small, repeating) versus long-term trend (the yearly mean, large, rising).
• Respiration (releases CO₂ in all organisms) versus combustion (burning, releases CO₂ rapidly).

Quick self-test

1. State the usual units of primary production.
2. A woodland fixes 1500 g C m⁻² yr⁻¹ and its plants release 650 g C m⁻² yr⁻¹ in respiration. Calculate net primary production and the percentage of fixed carbon retained.
3. Explain why secondary production is lower than primary production.
4. In which direction does a feeding arrow point in a carbon cycle diagram?
5. An ecosystem has photosynthesis of 900, total respiration of 940 (both g C m⁻² yr⁻¹). Is it a sink or a source, and by how much?
6. Name the two features of the Keeling Curve.
7. Explain the annual fluctuation in CO₂ concentration.
8. A site records an annual mean of 405 ppm and rises by 2.4 ppm per year for 25 years. Predict the concentration.
9. Why can peat be a sink in one condition and a source in another?
10. Which organisms release phosphate and other ions from dead material?
11. A leaf litter sample loses mass from 200 g to 150 g. Calculate the percentage loss.

Answers

1. g m⁻² yr⁻¹ (mass of carbon per unit area per year).
2. 1500 − 650 = 850 g C m⁻² yr⁻¹. 850 ÷ 1500 = 56.7%, so 57% to 2 significant figures.
3. Carbon compounds are converted to CO₂ and water in cell respiration at each stage, so biomass is lost. Not all biomass is eaten, and some eaten material is egested.
4. From the organism that is eaten to the feeder.
5. 900 − 940 = −40, so a source, releasing a net 40 g C m⁻² yr⁻¹.
6. The annual fluctuation and the long-term upward trend.
7. In the northern hemisphere growing season, photosynthesis exceeds respiration and CO₂ falls. Outside it, respiration exceeds photosynthesis and CO₂ rises.
8. 2.4 × 25 = 60 ppm rise, so 405 + 60 = 465 ppm, if the rate stays constant. In the real record the rate has increased, so the true value could be higher.
9. Waterlogged peat decomposes very slowly, so carbon accumulates (sink). If drained, oxygen allows decomposers to respire it and release CO₂ (source).
10. Decomposers (bacteria and fungi).
11. (200 − 150) ÷ 200 × 100 = 25%.

Where marks are usually lost

• Giving a production figure without units, or with energy units.
• Subtracting only one type of respiration when the question gives two.
• Saying only plants photosynthesize and only animals respire.
• Describing the Keeling Curve rise without a calculated change or rate.
• Explaining the zigzag with fossil fuels, or the long-term rise with seasons.
• Drawing a carbon cycle with no decomposers, or with arrows reversed.
• Writing “decomposers recycle energy”. Matter is recycled, energy is not.
• Assuming a large store of carbon automatically means a sink. Sink and source depend on the net flux.

Official syllabus

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