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

This study guide is for IB Diploma Programme Biology, aligned to the IB Biology guide, first assessment 2025. It covers the second part of C4.2 (Transfers of energy and matter), statements C4.2.15 to C4.2.22, at both standard level and higher level. The guide has no additional higher level (AHL) material for C4.2, so nothing here is HL only.

Energy flow, trophic levels and energy pyramids (C4.2.1 to C4.2.14) are a separate unit. Here you follow the matter: how carbon is fixed, stored, passed on, returned and burned. Use the course hub and the printable checklist to track your progress. The other two files for this unit are the revision notes and the practice questions. To find your weak spots first, try the free diagnostics.

What this unit covers

Statement	What you must be able to do	Level
C4.2.15	Define primary production; use units of g m⁻² yr⁻¹; compare biomes	SL and HL
C4.2.16	Explain why secondary production is lower than primary production	SL and HL
C4.2.17	Construct a carbon cycle diagram using photosynthesis, feeding and respiration	SL and HL
C4.2.18	Decide whether an ecosystem is a carbon sink or a carbon source	SL and HL
C4.2.19	Describe CO₂ release when biomass, peat, coal, oil and natural gas are burned	SL and HL
C4.2.20	Analyse the Keeling Curve: annual fluctuation and long-term trend	SL and HL
C4.2.21	Explain the mutual dependence of respiration and photosynthesis on each other’s gases	SL and HL
C4.2.22	Explain that all elements are recycled and that decomposers are key	SL and HL

C4.2.15 Primary production

Autotrophs build carbon compounds (sugars, then lipids, proteins and so on) from carbon dioxide. Primary production is the accumulation of these carbon compounds in the biomass of autotrophs. It is measured as carbon mass for each square metre in each year, usually written g m⁻² yr⁻¹.

Biomass builds up only when organisms, whether autotrophs or heterotrophs, grow or reproduce. A plant that photosynthesizes but uses all the sugar in respiration has made no new biomass.

Ecologists often split the idea in two. The guide does not require these terms, but they help you read data tables.

• Gross production: all the carbon fixed by photosynthesis.
• Net production: gross production minus the carbon the autotrophs themselves release in respiration. Net production is the primary production that accumulates as biomass.

Biomes differ in how much biomass they accumulate. Tropical rainforest has warmth, water and light all year, so it accumulates a lot. Tundra and hot desert accumulate little because low temperature, short growing seasons or lack of water limit photosynthesis. Nutrient supply also matters.

Worked example 1: comparing two sites

Data supplied. Site X is a woodland. Site Y is a grassland.

	Carbon fixed by photosynthesis (g m⁻² yr⁻¹)	Carbon released by the plants’ own respiration (g m⁻² yr⁻¹)
Site X	2400	1100
Site Y	900	480

Net primary production:

• Site X: 2400 − 1100 = 1300 g m⁻² yr⁻¹
• Site Y: 900 − 480 = 420 g m⁻² yr⁻¹

Site X accumulates 1300 ÷ 420 = 3.1 times more biomass (to 2 significant figures). The share of fixed carbon kept as biomass is 1300 ÷ 2400 = 54% for X and 420 ÷ 900 = 47% for Y. Always state the unit, because a bare number is incomplete.

C4.2.16 Secondary production

Secondary production is the accumulation of carbon compounds in the biomass of heterotrophs. Heterotrophs take in carbon compounds from other organisms, digest them, then assimilate and rebuild the ones they need.

Secondary production is always lower than primary production in the same ecosystem. In cell respiration, carbon compounds are broken down to give carbon dioxide and water, so that carbon leaves the biomass. Respiration happens at every stage. Not all the biomass is eaten either, and some of what is eaten is egested in faeces.

Worked example 2: a herbivore carbon budget

Data supplied for one stand, per square metre per year. Net primary production is 1300 g C. Herbivores eat 260 g C. Of this, 90 g C is egested as faeces and 120 g C is released as CO₂ by herbivore respiration.

• Assimilated carbon = 260 − 90 = 170 g C m⁻² yr⁻¹
• Secondary production = 170 − 120 = 50 g C m⁻² yr⁻¹
• Fraction eaten = 260 ÷ 1300 = 20%
• Secondary production as a percentage of net primary production = 50 ÷ 1300 = 3.8%
• Share of assimilated carbon kept as new biomass = 50 ÷ 170 = 29%

Notice that the egested 90 g C is not lost from the ecosystem. It feeds decomposers.

C4.2.17 Carbon cycle diagrams

You must be able to draw how carbon is recycled by photosynthesis, feeding and respiration. A diagram needs boxes (stores) and labelled arrows (processes).

Minimum content:

CO2 in air/water --photosynthesis--> Producers
Producers --feeding--> Primary consumers --feeding--> Secondary consumers
Producers, consumers --death, faeces--> Dead organic matter
Dead organic matter --feeding/digestion--> Decomposers
Producers, all consumers, decomposers --cell respiration--> CO2

Checks before you finish:

• Photosynthesis arrow points from CO₂ to producers (one arrow in).
• Feeding arrows point from the organism eaten to the feeder, showing the direction of carbon transfer.
• Respiration arrows go from every living box, including decomposers, to CO₂.
• Dead organic matter and decomposers are included.
• Every arrow carries a process label.

You may add combustion and fossil fuel stores (see C4.2.19), but the core cycle comes first.

C4.2.18 Carbon sinks and carbon sources

A sink takes up more carbon dioxide than it releases. A source releases more than it takes up.

• If photosynthesis exceeds respiration, there is net uptake of carbon dioxide, so the ecosystem is a sink.
• If respiration exceeds photosynthesis, there is net release, so the ecosystem is a source.

An ecosystem can change. A growing forest is a sink. After a fire, or when a peatland is drained so that decomposers respire the stored peat, it may become a source.

Worked example 3: sink or source

Data supplied, g C m⁻² yr⁻¹.

Ecosystem	Photosynthesis	Respiration by autotrophs	Respiration by heterotrophs and decomposers
Young plantation	1800	800	950
Drained peatland	400	180	520

Net CO₂ uptake = photosynthesis − all respiration.

• Plantation: 1800 − 800 − 950 = +50, a net sink.
• Peatland: 400 − 180 − 520 = −300, a net source.

The sign carries the meaning, so write “sink” or “source” as well as the number.

C4.2.19 Combustion and the release of carbon dioxide

Combustion oxidizes carbon compounds and releases carbon dioxide. The fuels you need to know are biomass, peat, coal, oil and natural gas, and burning any of them releases CO₂.

• Biomass (wood, crop waste) is recent carbon, fixed within years to centuries.
• Peat forms where waterlogged, acidic, low-oxygen conditions slow decomposition, over thousands of years.
• Coal formed mainly from the remains of plants buried over very long periods, hundreds of millions of years ago.
• Oil and natural gas formed from the remains of marine organisms buried over millions of years.

These stores of carbon, which are sinks, vary in their date of formation. Burning fossil fuels returns, within a few generations, carbon that was removed from the atmosphere over geological time. Combustion also happens naturally, for example after lightning strikes. Human activities have greatly increased the rate of combustion.

C4.2.20 The Keeling Curve

The Keeling Curve is the record of atmospheric carbon dioxide concentration measured at Mauna Loa in Hawaii since 1958. It has two features you must analyse.

Annual fluctuation (a zigzag). The concentration rises to a peak around May and falls to a minimum around September or October each year. Much of the world’s land and vegetation is in the northern hemisphere. In its growing season (spring and summer) photosynthesis exceeds respiration there, so CO₂ is drawn down. In autumn and winter, leaf fall and low light mean respiration (and decomposition) exceeds photosynthesis, so CO₂ rises.

Long-term trend (a rise). The yearly average increases from year to year, and the rate of increase has itself increased. This is mainly due to combustion of fossil fuels, which adds carbon that is not offset by photosynthesis.

Worked example 4: reading the curve

Annual means supplied (approximate, ppm): 1960, 317; 1990, 354; 2020, 414. Monthly values for one year: peak 419.5 ppm (May), minimum 413.5 ppm (September).

• Rise, 1960 to 2020 = 414 − 317 = 97 ppm.
• Percentage increase = 97 ÷ 317 × 100 = 30.6%.
• Mean rate, 1960 to 1990 = 37 ÷ 30 = 1.2 ppm per year. Mean rate, 1990 to 2020 = 60 ÷ 30 = 2.0 ppm per year, so the rate has risen.
• Annual fluctuation = 419.5 − 413.5 = 6.0 ppm, much smaller than the 97 ppm rise.

When you describe a graph, quote values with units, give the pattern for both features, then explain with named processes.

C4.2.21 Interdependence of photosynthesis and respiration

Most aerobic respiration depends on oxygen in the atmosphere, and that oxygen was produced by photosynthesis. Photosynthesis depends on atmospheric carbon dioxide, which is replenished by respiration (and combustion). Autotrophs and heterotrophs therefore supply each other’s raw materials.

The fluxes are huge: of the order of a hundred billion tonnes of carbon fixed and released each year. Small changes in the balance can therefore alter the atmosphere noticeably.

C4.2.22 Recycling of all elements

Carbon is not special. Every element that organisms need, including nitrogen, phosphorus, sulfur, potassium, calcium and magnesium, is recycled in ecosystems. The supply on Earth is limited, so elements are taken up from soil, water or air, built into organisms, then returned.

Decomposers play the key role. Bacteria and fungi digest dead organic matter, faeces and dead parts, and release inorganic ions and carbon dioxide into the soil and air, where autotrophs can take them up again. Without decomposers, nutrients would stay locked in dead material and primary production would fall.

You do not need the details of the nitrogen cycle or other nutrient cycles.

Unlike matter, energy is not recycled. It enters as light (or from oxidation reactions), is transformed and leaves as heat. That contrast is covered in the energy flow unit.

Common errors

• Writing “plants absorb CO₂ and animals release it”. Plants also respire, and both groups release CO₂.
• Drawing feeding arrows from the feeder to its food.
• Leaving decomposers out of the carbon cycle.
• Giving production in J or kJ. Production here is carbon mass per area per time.
• Saying the Keeling Curve rise is “because of photosynthesis and respiration” without separating the seasonal zigzag from the long-term trend.
• Treating a forest as always a sink.

Official syllabus

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