What you need to know
- 5.18Knowledge of the carbon cycleThe movement of carbon between the atmosphere, organisms, oceans and rocks by photosynthesis, respiration, decomposition and combustion., as the foundation for methods that reduce atmospheric carbon dioxide.
Reforestation, biofuels and the conflict of needs are completed in 5C 10. Greenhouse physics is 5C 4. This lesson is the cycle you must be able to draw and apply.
By the end, you should be able to…
- Name photosynthesisThe process in which plants use light energy to make glucose from carbon dioxide and water, releasing oxygen., respirationThe breakdown of respiratory substrates such as glucose in cells to release energy, which is used to make ATP., decomposition, feeding, combustion and oceanic dissolving as carbon transfers.
- Distinguish fast biological cycling from long-term stores in fossil fuelCoal, oil or natural gas, formed from organisms over millions of years. Burning it releases stored carbon as CO₂., peat and sediments.
- Explain why cutting combustion or increasing plant stores changes atmospheric CO₂.
Build your understanding
Carbon is conserved; form changes
photosynthesisThe process in which plants use light energy to make glucose from carbon dioxide and water, releasing oxygen. removes carbon dioxide from the atmosphere and stores carbon in producerAn organism, usually a plant, that makes its own organic molecules by photosynthesis. Producers form the first trophic level. biomassThe mass of living material, often measured as dry mass per unit area. — the same chemistry as 5A, now seen as a global transfer. Feeding moves that carbon into consumerAn organism that gets its energy by eating other organisms.. respirationThe breakdown of respiratory substrates such as glucose in cells to release energy, which is used to make ATP. in plants, animals and microorganisms returns carbon dioxide. Decomposition by microorganisms does the same to dead tissue and faeces. The carbon cycleThe movement of carbon between the atmosphere, organisms, oceans and rocks by photosynthesis, respiration, decomposition and combustion. is those transfers together, not a separate mysterious process.
Oceans dissolve carbon dioxide; some is used by phytoplankton, some forms sediments over long timescales. Peat and fossil fuelCoal, oil or natural gas, formed from organisms over millions of years. Burning it releases stored carbon as CO₂. are stores that locked carbon away from the air. Combustion of those stores returns carbon dioxide much faster than it was buried.
Fast cycle, slow stores
NASA’s Earth Observatory distinguishes a fast biological cycle (life spans) from a slow geological cycle (rocks and deep ocean). You do not need their tonnage figures. You do need the idea that burning fossil carbon adds a store that was not in the annual plant–air loop.
A carbon sinkSomething that takes up more carbon than it releases, such as a growing forest, a peat bog or the oceans. takes in more carbon than it releases over the period of interest: growing woodland, an accumulating peat bogA waterlogged habitat where dead plants decompose very slowly, so their carbon is stored as peat., or the ocean in many years. A source does the reverse: a wildfire, ploughing peat, or a power station. The same forest can be a sink while it grows and a source if it is burned.
Why the cycle is a toolkit
Specification 5.18 later asks you to apply this knowledge to cut atmospheric carbon dioxide. The levers are already visible: more photosynthesis and long-lived stores (reforestationPlanting trees where forest has been cut down., peat protection), less combustion, and keeping carbon in soils. You cannot “destroy” the extra carbon dioxide; you can change the transfers.
Do not write that trees “eat pollution” or that the ocean “cleans the air” without naming dissolving, photosynthesis or sedimentation. Name the process.
NOT: plants do not only take up carbon dioxide. Producers respire as well.
- Combustion of fossil carbon returns CO₂ much faster than burial
- A growing wood can be a sink; the same wood burned is a source
- Oceans dissolve CO₂; sediments are a long store
- You cannot destroy the extra CO₂; you can change the transfers
- Name photosynthesis, respiration, decomposition or combustion, not a vague clean-up
Two traps this diagram blocks
- Do not write that only animals respire.
- Do not say trees destroy pollution without naming photosynthesis or a store.
From the diagram alone
Question
Describe two processes that return carbon dioxide to the atmosphere and one process that removes it. [3]
Show a model answer
Respiration and decomposition, or combustion of fossil fuels, return carbon dioxide. Photosynthesis removes it by fixing carbon into producer biomass. Any two returns and photosynthesis answer the question.
| Process | Carbon moves from | Carbon moves to |
|---|---|---|
| Photosynthesis | Atmospheric CO₂ | Producer biomass |
| Respiration | Organic carbon in organisms | Atmospheric CO₂ |
| Decomposition | Dead organic matter | Atmospheric CO₂ (and soils) |
| Combustion | Fossil fuels, peat, biomass | Atmospheric CO₂ |
| Dissolving | Atmosphere | Ocean |
Make your reasoning visible
Every box needs an arrow in and out
A cycle with photosynthesisThe process in which plants use light energy to make glucose from carbon dioxide and water, releasing oxygen. but no respirationThe breakdown of respiratory substrates such as glucose in cells to release energy, which is used to make ATP. is incomplete.
Fossil carbon is old NPP
Coal and oil are stored producerAn organism, usually a plant, that makes its own organic molecules by photosynthesis. Producers form the first trophic level. carbon; burning them is not a new kind of atom.
Key explanations
- Feeding moves that carbon into consumerAn organism that gets its energy by eating other organisms.. Death moves it into remains and wastes.
- Producers are part of the return, because they respire. A mature forest is not a one-way sink of its whole GPP.
- A sink gains carbon over the period you are discussing. A source loses it. One forest can be either, depending on growth or fire.
- Methods that lower atmospheric CO₂ change these transfers. They do not destroy carbon.
Avoid this misconception
The claim: Respiration only happens in animals, so plants just soak up CO₂ forever.
The correction: Producers respire. A mature forest still photosynthesises, but much of the carbon is cycled; net storage is NPP that accumulates, not GPP.
Follow one carbon atom
The carbon cycle branches, so follow one defined route at a time: first the fast biological loop, then the slow route through a fossil store.
Order the steps
What to do
- Pick a chain with Chain, just below these steps: Fast loop or Fossil route.
- Reorder the cards below: drag a card (on a touch screen, drag its number), tap a card then tap where it goes, or use the ↑ and ↓ arrows at its right.
- Press Check under the cards to mark each one green (right place) or red; the count shows how many are right.
- Stuck? Hint first outlines the first wrong place, then gives a clue, then places that step for you.
- Shuffle mixes the cards again; Start again returns them to the starting order. Keyboard: Space picks up, arrow keys move, Space drops.
Follow one carbon atom from the air, through a food chain, and back to the air.
0 of 6 in the right place
Oceans dissolving carbon dioxide and sediments are other routes. Oil and gas form mainly from marine organisms, not peat.
Check your understanding
Try each question before opening the suggested answer.
Practice question
Retrieval and explanation · suggested answer
Name the process that removes CO₂ from the air into plant biomass.
Show suggested answer
Photosynthesis.
Practice question
Retrieval and explanation · suggested answer
How does combustion of peat change the cycle compared with slow peat formation?
Show suggested answer
Formation locks carbon out of the air over long periods; combustion returns that carbon as CO₂ much faster than it was stored.
Practice question
Retrieval and explanation · suggested answer
Why is a growing woodland described as a carbon sink?
Show suggested answer
Over that period it takes in more carbon, via photosynthesis stored as biomass, than it releases by respiration and decomposition.
Take the idea further
Apply the same specification knowledge to an unfamiliar problem. Any extra assumptions are supplied in the question.
Stretch question
A headline says “the ocean absorbs our emissions so planting trees is pointless”. Use the cycle to write a more careful claim, with one assumption.
Show a hint
Sinks can saturate; both transfers exist.
Explore the reasoning
Oceans dissolve CO₂ and marine producers photosynthesise, so they are a sink, but they do not remove all emissions and acidification is a cost. Reforestation is another transfer into biomass. Assume “pointless” was not a measured flux comparison.
Recall this lesson
Lesson recall questions
Question 1 of 5: Compare reducing fossil-fuel use with reforestation as ways to cut atmospheric CO₂.
Carbon cycle and reducing CO₂
Practice question
Retrieval and explanation · suggested answer
Compare reducing fossil-fuel use with reforestation as ways to cut atmospheric CO₂.
Lesson: 5C 10: What Can Be Done?
Try this from memory first.
Put your biology to work
Explore where these ideas appear in the world. Enrichment, not extra required recall.
Fast cycle, slow cycle
5C · The Carbon Cycle in NatureNASA Earth Observatory’s carbon-cycle feature separates the biological loop you draw in school from the slower rock–ocean loop. Human fossil-fuel emissions are large compared with volcanic return of…Specification 5.18Read & use the scienceClose article
Why this matters
NASA Earth Observatory’s carbon-cycle feature separates the biological loop you draw in school from the slower rock–ocean loop. Human fossil-fuel emissions are large compared with volcanic return of geological carbon. Use that as context for 5.18, not as extra numbers to memorise. Photosynthesis moves carbon from atmospheric or dissolved carbon dioxide into biomass, while respiration and decomposition return it. Burning fossil fuel moves carbon from a geological reservoir into that active cycle much faster.
Back to the specification
Pearson Edexcel IAL Biology · Issue 2, February 2021 · Unit 4, pp. 29–32. The application extends beyond required recall.
- 5.18 understand how knowledge of the carbon cycle can be applied to methods to reduce atmospheric levels of carbon dioxide
Use the science
Why does adding volcanic CO₂ as a “natural excuse” fail as an account of the recent rise?
Explore a possible response
The slow geological return is much smaller than the fast combustion of buried carbon. Recent atmospheric increase tracks human transfers, not a sudden volcanic step.
Explore the source
Carbon sinks into the deep sea
5C · The Carbon Cycle in NatureNOAA researchers deploy sediment traps below the sunlit ocean to measure particles that sink from surface waters. Phytoplankton first fix dissolved carbon dioxide into organic molecules; grazing, respiration…Specification 5.18Read & use the scienceClose article
Why this matters
NOAA researchers deploy sediment traps below the sunlit ocean to measure particles that sink from surface waters. Phytoplankton first fix dissolved carbon dioxide into organic molecules; grazing, respiration and decomposition return much of that carbon near the surface. Only a fraction of the particulate material travels deep enough to stay away from the atmosphere for a long time. This biological carbon pump is an extension of the carbon-cycle arrows students draw. A surface bloom is therefore not a measure of permanent carbon storage. Ocean circulation and the depth of remineralisation also affect how long carbon remains isolated.
Back to the specification
Pearson Edexcel IAL Biology · Issue 2, February 2021 · Unit 4, pp. 29–32. The application extends beyond required recall.
- 5.18 understand how knowledge of the carbon cycle can be applied to methods to reduce atmospheric levels of carbon dioxide
Use the science
Why does a larger phytoplankton bloom not necessarily mean a proportionally larger long-term carbon sink?
Explore a possible response
Most fixed carbon may be grazed or respired near the surface. Storage depends on how much sinks deep and remains there.
Explore the source
- Sediment traps and the biological carbon pumpNOAA Atlantic Oceanographic and Meteorological Laboratory