A carbon-cycle question shows stores and flows of carbon and asks what happens to carbon dioxide in the air. Sort each arrow into adds or removes, then total them.
This lesson completes carbon compounds. Combustion from polymers and fuels is one of the flows.
How do I sort the arrows?
Ask of each arrow: does carbon dioxide enter the air or leave it? Write add or remove beside the arrow before any calculation.
| Process | Effect on carbon dioxide in air |
|---|---|
| Photosynthesis | removes |
| Respiration | adds |
| Decomposition | adds |
| Combustion of fuels | adds |
Worked example: net flow in a model
A simplified model gives yearly flows in arbitrary units: photosynthesis 120, respiration 60, decomposition 30, combustion 15.
Removed from air: 120 (photosynthesis).
Added to air: 60 + 30 + 15 = 105.
Net change in air: 105 − 120 = −15 units. The negative sign means the amount of carbon dioxide in the air falls by 15 units in this model.
Assumptions to state. Only these four flows are counted, the units are the same for every flow, and the period is one year.
Check the direction: more is removed (120) than added (105), so the air store falls. The sign agrees.
The mistake that costs marks
The common slip is treating every arrow as an addition. A student adds 120 + 60 + 30 + 15 = 225, which is meaningless because photosynthesis works in the opposite direction.
| Wrong | Right | |
|---|---|---|
| Arrow handling | add all four numbers | removes and adds kept separate |
| Result | 225 | −15 |
| Statement | “air gains 225” | “air falls by 15 in this model” |
A second slip is saying burning fuel removes carbon dioxide. Combustion adds it.
Check yourself
A second model gives photosynthesis 90, respiration 55, decomposition 25 and combustion 20. Find the net change and say whether carbon dioxide in the air rises or falls.
Answer
Added: 55 + 25 + 20 = 100. Removed: 90. Net = 100 − 90 = +10 units.
The amount of carbon dioxide in the air rises by 10 units in this model, because more is added than removed.
What to study next
Put all four lessons together with the carbon compounds practice set. Use the graph evidence comparison lab to practise judging model assumptions.
For a teacher to go through diagram questions with you, see online one-to-one Science tuition.