What is the carbon cycle and why is it important in IB DP Biology?

The carbon cycle is a fundamental ecological process that describes the movement of carbon through the Earth's biosphere, atmosphere, oceans, and geosphere. In IB DP Biology, understanding the carbon cycle is crucial because it explains how carbon is exchanged between living organisms and the environment, influencing global climate patterns and ecosystem dynamics.

How do photosynthesis and respiration contribute to the carbon cycle?

Photosynthesis and respiration are key processes in the carbon cycle. During photosynthesis, plants and other autotrophs absorb carbon dioxide (CO2) from the atmosphere and convert it into organic compounds like glucose, releasing oxygen as a byproduct. Respiration, on the other hand, occurs in both plants and animals, where organic compounds are broken down to release energy, returning CO2 to the atmosphere. These processes help maintain the balance of carbon in ecosystems.

What role do oceans play in the carbon cycle?

Oceans play a significant role in the carbon cycle by acting as major carbon sinks. They absorb large amounts of CO2 from the atmosphere, which is then used by marine organisms for photosynthesis or stored in ocean sediments. This process helps regulate atmospheric CO2 levels and mitigates climate change. Additionally, the ocean's biological pump transports carbon from the surface to the deep sea, further influencing global carbon distribution.

How does human activity impact the carbon cycle?

Human activities, such as burning fossil fuels, deforestation, and industrial processes, have significantly altered the natural carbon cycle. These activities increase the concentration of CO2 in the atmosphere, contributing to global warming and climate change. Understanding these impacts is essential in IB DP Biology to explore sustainable practices and mitigate adverse environmental effects.

What is the significance of carbon sinks and sources in the carbon cycle?

Carbon sinks and sources are critical components of the carbon cycle. Carbon sinks, such as forests, oceans, and soil, absorb more carbon than they release, helping to reduce atmospheric CO2 levels. Conversely, carbon sources, like fossil fuel combustion and deforestation, release more carbon than they absorb. Balancing these sinks and sources is vital for maintaining ecological equilibrium and addressing climate change challenges.

Why is "Drawing the photosynthesis arrow from biomass to atmosphere." a common mistake in Carbon Cycling?

Why it happens: Confusing direction. How to avoid it: Photosynthesis takes CO₂ FROM atmosphere INTO producer biomass.

Why is "Omitting decomposition when outlining the cycle." a common mistake in Carbon Cycling?

Why it happens: Focusing on photo + respiration. How to avoid it: Without decomposers, dead biomass would not return its carbon — they close the cycle.

Why is "Stating that planting trees would 'solve' climate change." a common mistake in Carbon Cycling?

Why it happens: Oversimplification. How to avoid it: Reforestation helps but is limited by available land and slow growth — the dominant emissions need direct reduction.

IB DP Biology (BI)

Carbon Cycling

0% Complete

Short Notes - Carbon Cycling

Page 1/0

Detailed Study Notes

Detailed notes on Ecology for IB DP Biology, covering key concepts, explanations, examples, and exam-focused revision points.

Carbon Cycling — IB Biology SL: photosynthesis, respiration, decomposition, combustion, fossil fuels

Maps to Theme C (Interaction and Interdependence). Covers how carbon moves between atmosphere, organisms, soil, oceans and rocks via photosynthesis, respiration, decomposition, combustion and human activities.

At a glance

PHOTOSYNTHESIS removes CO₂ from atmosphere → glucose in producers.
RESPIRATION returns CO₂ to atmosphere — done by all living organisms.
DECOMPOSITION (by saprotrophs) returns C from dead biomass to CO₂.
COMBUSTION releases CO₂ from biomass or fossil fuels.
OCEANS dissolve CO₂; corals and shellfish form CaCO₃ (calcium carbonate).
FOSSIL FUELS = stored carbon from ancient organisms; burning shifts the balance.
Carbon is RECYCLED (unlike energy, which flows one way).

What you’ll learn

Mapped to the IB DP Biology SL subject guide (2025 onwards (first assessment May 2025)).

C4.3.1 — Outline the main processes in the carbon cycle.
C4.3.2 — Explain the role of decomposers.
C4.3.3 — Describe carbon storage in fossil fuels, biomass and limestone.
C4.3.4 — Link combustion of fossil fuels to atmospheric CO₂ rise.

The carbon cycle

Six main fluxes.

Main processes of the carbon cycle:

Process	Direction
Photosynthesis	Atmospheric CO₂ → producer biomass
Respiration	Biomass → atmospheric CO₂
Feeding	Biomass of one organism → biomass of another
Death + Decomposition	Dead biomass → atmospheric CO₂ (saprotrophs)
Combustion (fire, fossil fuels)	Biomass or fossil fuels → atmospheric CO₂
Dissolution / precipitation	Atmospheric CO₂ ⇌ dissolved bicarbonate / CaCO₃

Carbon stores:

Atmosphere — CO₂ (~800 Gt C).
Oceans — dissolved CO₂ and bicarbonate (~38 000 Gt C).
Living biomass and soil organic matter (~2300 Gt C).
Fossil fuels — coal, oil, natural gas (~4000 Gt C; ancient buried plants).
Carbonate rocks — limestone, chalk (vast; > 60 000 000 Gt C).

Carbon is recycled, in contrast to energy (which flows one way). The same atoms cycle indefinitely between the stores.

Photosynthesis IN; respiration, decomposition, combustion OUT.
Stores: atmosphere, biomass, oceans, fossil fuels, limestone.
Carbon recycled; energy flows.

Human impact and fossil fuels

Unbalancing the cycle.

Fossil fuels are ancient organic matter — plants and plankton buried millions of years ago that fossilised under heat and pressure into coal, oil, and natural gas. Burning them oxidises the stored carbon and releases CO₂ on a timescale far faster than natural processes can re-absorb it.

Atmospheric CO₂ trends.

Pre-industrial: ~280 ppm.
1958 (Keeling curve begins): ~315 ppm.
2024: ~422 ppm (and rising ~2.5 ppm/year).

Higher CO₂ enhances the greenhouse effect → global warming, ocean acidification (CO₂ + H₂O → H₂CO₃ → lower pH; harms coral and shellfish).

Carbon sinks that have helped absorb some emissions:

Forests (especially tropical and boreal) — but deforestation reverses this.
Oceans — absorb ~30% of emitted CO₂ but at the cost of acidification.

Saprotroph decomposition. Bacteria and fungi break down dead matter, releasing carbon back to the atmosphere as CO₂. Anaerobic conditions (waterlogged soil, gut of cattle, rice paddies) instead produce methane (CH₄) — a more potent greenhouse gas. This is why wetland drainage and livestock farming contribute to climate change beyond mere CO₂.

Climate change. The next subtopic covers consequences and mitigation in more depth.

Fossil fuels: ancient buried biomass.
Burning shifts long-buried C to atmosphere → CO₂ rise.
Anaerobic decomposition → CH₄ (potent greenhouse gas).

Quick recap

Carbon cycles via photosynthesis, respiration, decomposition, combustion, ocean exchange.
Stores: atmosphere, biomass, oceans, fossil fuels, limestone.
Burning fossil fuels shifts carbon to atmosphere.
CO₂ rose from 280 to ~420 ppm since industrial era.

Memorise this

Verbatim phrases, formulae and definitions IB DP mark schemes credit (key for AO1 knowledge marks on Paper 1).

CO₂ ⇌ glucose: photosynthesis vs respiration
Decomposers return C from dead biomass
Fossil fuels = ancient stored carbon
Pre-industrial 280 ppm → today >420 ppm
Anaerobic decomposition → CH₄

How it’s examined

Paper 1: MCQ on process direction. Paper 2: 'outline the carbon cycle' (diagram acceptable); 'explain how combustion of fossil fuels affects atmospheric CO₂'.

Sources: IB Diploma Programme Biology subject guide (IBO, 2023 — first assessment 2025). Last reviewed 2026-05-31.

Take this whole topic with you

Step-by-step worked examples — Carbon Cycling

Step-by-step solutions to past-paper-style questions on carbon cycling, written exactly the way a tutor would explain them at the board.

1Four key processes of the carbon cycle
Getting started• cycle
Question
Name FOUR processes that move carbon between atmosphere and organisms / soil. (4 marks)
Step-by-step solution
1. Step 1
  Photosynthesis — atmosphere CO₂ → producer biomass.
2. Step 2
  Respiration — biomass → atmosphere CO₂.
3. Step 3
  Decomposition (by saprotrophs) — dead biomass → CO₂.
4. Step 4
  Combustion (fire, fossil fuels) — biomass / fossil fuels → CO₂.
Answer
Photosynthesis; respiration; decomposition; combustion.
2Why fossil-fuel burning raises CO₂
Building confidence• climate
Question
Explain why burning fossil fuels rapidly raises atmospheric CO₂. (3 marks)
Step-by-step solution
1. Step 1
  Fossil fuels contain carbon that was removed from the atmosphere millions of years ago by photosynthesis and buried.
2. Step 2
  Burning oxidises this carbon to CO₂ on a timescale of decades.
3. Step 3
  Natural sinks (forests, oceans) cannot re-absorb it quickly enough → net atmospheric increase.
Answer
Releases ancient buried C as CO₂ faster than natural sinks can re-absorb it.
3Anaerobic decomposition
Stretch• methane
Question
Explain why waterlogged soils contribute more to climate change per unit biomass than well-drained soils. (3 marks)
Step-by-step solution
1. Step 1
  Waterlogged soils are anaerobic — oxygen cannot diffuse fast enough.
2. Step 2
  Anaerobic decomposers (methanogens) produce methane (CH₄) instead of CO₂.
3. Step 3
  Methane is a far stronger greenhouse gas than CO₂ (~28× over 100 years).
Answer
Waterlogged → anaerobic decomposition → CH₄ (more potent greenhouse gas than CO₂).

Key Definitions and Keywords — Carbon Cycling

Definitions to memorise and the exact keywords mark schemes credit for carbon cycling answers — sharpened from recent examiner reports for the 2026 IB DP Biology SL sitting.

Carbon sink
Examiner keyword
A reservoir (forest, ocean) that absorbs more CO₂ than it releases over a given period.
Decomposition
Examiner keyword
Breakdown of dead organic matter by saprotrophs and detritivores, releasing CO₂.
Fossil fuel
Examiner keyword
Carbon-rich fuel formed from ancient buried organic matter (coal, oil, gas).

Common Mistakes and Misconceptions — Carbon Cycling

The traps other students keep falling into on carbon cycling questions — taken from recent IB DP Biology SL examiner reports and mark schemes — and how to avoid them.

✕Drawing the photosynthesis arrow from biomass to atmosphere.
Why it happens
Confusing direction.
How to avoid it
Photosynthesis takes CO₂ FROM atmosphere INTO producer biomass.
✕Omitting decomposition when outlining the cycle.
Why it happens
Focusing on photo + respiration.
How to avoid it
Without decomposers, dead biomass would not return its carbon — they close the cycle.
✕Stating that planting trees would 'solve' climate change.
Why it happens
Oversimplification.
How to avoid it
Reforestation helps but is limited by available land and slow growth — the dominant emissions need direct reduction.

Carbon Cycling — frequently asked questions

The things students keep getting wrong in this sub-topic, answered.

Carbon Cycling

Short Notes - Carbon Cycling

Detailed Study Notes

At a glance

What you’ll learn

Quick recap

Memorise this

How it’s examined

Take this whole topic with you

1Four key processes of the carbon cycle

2Why fossil-fuel burning raises CO₂

3Anaerobic decomposition

Carbon sink

Decomposition

Fossil fuel

✕Drawing the photosynthesis arrow from biomass to atmosphere.

✕Omitting decomposition when outlining the cycle.

✕Stating that planting trees would 'solve' climate change.

Carbon Cycling — frequently asked questions