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Carbon Cycle and Transfers

Trace carbon dioxide through photosynthesis, respiration, combustion and decomposition, and distinguish stores from transfer processes.

  • SEC G3 Combined Science Chemistry component 2027
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Learning objectives

  • describe the carbon cycle in simple terms, to include — the processes of combustion, respiration and photosynthesis
  • describe the carbon cycle in simple terms, to include — how the carbon cycle regulates the amount of carbon dioxide in the atmosphere (see also 11.1(d))

1. Outcome and prerequisites

By the end, you should be able to follow carbon atoms between atmospheric carbon dioxide, living material and fuels; label photosynthesis, respiration, combustion and decomposition on a cycle; and distinguish a store from a transfer process.

A carbon store is a place where carbon is held, such as atmospheric carbon dioxide, plant material or fuel. A transfer process moves carbon between stores.

2. Modelled example

Required processes in the carbon cycleAn atmospheric carbon dioxide box is connected to plants, living organisms and fuels. Photosynthesis moves carbon dioxide from the atmosphere into plants. Respiration and combustion return carbon dioxide to the atmosphere.Carbon moves between the atmosphere and carbon storesAtmospheric carbon dioxideCO₂Plantscarbon stored in biomassLiving organismsrespiration releases CO₂Fuelscombustion releases CO₂photosynthesisremoves CO₂respirationadds CO₂combustionadds CO₂
Combustion and respiration add carbon dioxide to the atmosphere; photosynthesis removes it and stores carbon in plants.
Starting storeProcessDestination store
atmospheric carbon dioxidephotosynthesisplant material
plant or animal materialrespirationatmospheric carbon dioxide
fuel or biomasscombustionatmospheric carbon dioxide
dead organic materialdecomposition, including decomposer respirationatmospheric carbon dioxide and other stores

Modelled example 1

Repair one reversed arrow

Core

Problem

A cycle diagram shows plant material → photosynthesis → atmospheric carbon dioxide. Identify the error and repair the transfer.

View solution step by step

  1. Classify the labels

    Method

    Plant material and atmospheric carbon dioxide are stores; photosynthesis is a process.

    Reason

    This prevents “photosynthesis” being treated as a place where carbon sits.

    Working

    store → process → store.

  2. Follow the carbon

    Method

    Reverse the arrow so atmospheric carbon dioxide moves into plant material by photosynthesis.

    Reason

    Plants use carbon dioxide to build carbon-containing organic material.

    Working

    atmospheric CO₂ → photosynthesis → plant material.

3. Guided practice

Guided practice 2

Change to a composting context

About 5 min

Problem

Plant cuttings are placed in a compost pile. Explain how some of their carbon can return to atmospheric carbon dioxide.

Name both process and direction

Hints

Hint 1: start store

The carbon begins in dead plant material.

Hint 2: organism process

Decomposers break down the material and respire.

View solution step by step

  1. Identify decomposition

    Method

    Decomposers break down the dead organic material.

    Reason

    Decomposition makes its carbon available in the decomposer food and respiration pathway.

    Working

    dead plant carbon → decomposers.

  2. Complete the atmospheric transfer

    Method

    Decomposer respiration releases some carbon as carbon dioxide.

    Reason

    The carbon atoms are conserved as they move into a different store.

    Working

    organic carbon → respiration → atmospheric CO₂.

4. Plausible error contrast

A list reading “plants, photosynthesis, air, combustion” is not yet a carbon-cycle model. It mixes stores and processes and gives no directions. Repair it by writing transfers, for example atmospheric carbon dioxide → photosynthesis → plant material and fuel carbon → combustion → atmospheric carbon dioxide.

Arrow direction is evidence

Respiration and combustion point towards atmospheric carbon dioxide; photosynthesis points away from it into plant material.

5. Changed-context transfer

Mind stretcher 1: Separate release from reduced removalExtension

A forest is cleared, but the wood is stored and not burned. Explain one carbon-cycle reason atmospheric carbon dioxide may still rise.

Show feedback

Fewer living trees remain to photosynthesise, so less atmospheric carbon dioxide is transferred into plant material. The question removes immediate combustion, so reduced removal is the relevant change.

6. Independent evidence

Complete this representation without headings as clues: atmospheric carbon dioxide → [process A] → plant material → [process B] → atmospheric carbon dioxide. Give one valid process for A and one for B, justify both arrow directions, and then add a different transfer from a fuel store to the atmosphere.

Continue to Greenhouse Gases and the Enhanced Greenhouse Effect.