Photosynthesis moves carbon from the atmosphere into living things; respiration and decomposition move it back. These huge flows nearly balance, so the air's CO₂ stays steady until something upsets them.
A big gum tree can weigh many tonnes. Almost all of that mass came out of the air.
You know the four spheres. This lesson follows carbon between two of them.
Engage: Commit to a prediction before you test anything. A wrong guess is fine: it shows you what to look for.
Explore: Try it on the board and compare what happens with your prediction.
Explain: Use the particle model to explain what you saw.
Elaborate: Use the same idea in a new context.
Evaluate: Look back at your first ideas and what changed.
WidthMore contexts, same idea.
Make your prediction first. Help opens once you've locked it in.
EngageCommit to a prediction before you test anything. A wrong guess is fine: it shows you what to look for.
Engage: where does a tree's mass come from?
ExploreTry it on the board and compare what happens with your prediction.
Explore: switch the flows
First press Run 10 years with everything on. Then switch Respiration and decomposition off and run again. Then switch it back on, switch Photosynthesis off, and run again. Press Start again whenever you like.
Try it yourselfYour edits are saved in this browser.
Make the CO₂ in the air fall, then make it rise, by switching processes on and off.
Use the board. This ticks when you get there.
ExplainUse the particle model to explain what you saw.
Explain: two flows, opposite directions
In your book: draw the atmosphere and biosphere as boxes, with arrows for photosynthesis, respiration and decomposition, labelled with their direction.
Which way does each process move carbon?
Photosynthesis
A kangaroo breathing out
Fungi rotting a fallen logDecomposers respire: decomposition returns carbon to the air.
A tree respiring at nightPlants respire too, day and night.
Grass growing
Elaborate TypicalUse the same idea in a new context.
Elaborate: a carbon atom's journey
Put a carbon atom's journey in order.
CO₂ in the air
Taken in by a grass leaf in photosynthesis
Eaten by a kangaroo, part of its body
The kangaroo dies and fungi decompose it
Respired by the fungi back into the air as CO₂
Elaborate WiderUse the same idea in a new context.
Elaborate: seasons in the air
EvaluateLook back at your first ideas and what changed.
Evaluate: look back
Write one sentence: "At first I thought… Now I think… because…"
Revision summary
Photosynthesis: carbon moves from the atmosphere (CO₂) into the biosphere (sugars, then wood, leaves and animals).
Respiration (plants, animals, decomposers) and decomposition move carbon back to the atmosphere.
When the flows balance, CO₂ in the air stays steady; change one and the store changes.
For teachers
Students explain photosynthesis and respiration (including decomposition) as opposite flows of carbon between the atmosphere and biosphere, and explain balance between them (AC9S9U03, node 3, a hinge node). About 22 minutes.
I can explain that photosynthesis moves carbon from the air into living things.
I can explain that respiration and decomposition move it back.
I can explain why the air's CO₂ stays steady when the flows balance.
Before class
A concept tutorial for introducing or revising the idea. Set the width with ?width=minimum, typical (default) or wider.
The hinge is representing each process as a directional flow between spheres. The misconception that plants are made from soil is raised in Engage.
Board notes: stores in billions of tonnes of carbon (GtC); the air starts at a balanced 280 ppm (before 1800). Photosynthesis takes about 120 GtC a year; respiration and decomposition return about the same; the oceans exchange about 80 each way. Switching a process off is a "what if": with respiration and decomposition off, plants draw the air's CO₂ down within years; with photosynthesis off, it climbs. The model's numbers are rounded and its rules simple (plants take up a little more CO₂ when there is more of it).
Running the lesson
Engage: count how many chose the soil.
Explore: ask what happened in each run, and why.
Explain: the balance question is the checkpoint.
The hard part (on purpose)
Seeing photosynthesis and respiration as flows of carbon that nearly balance. When a student says only animals respire, ask whether plant cells need energy at night.
When students get stuck
A student says only animals respire. Ask: Do plant cells need energy at night? Fix: Yes: plants respire all the time, and so do decomposers.
A student says carbon is used up when plants grow. Ask: Where did it go? Fix: Into the plant's body; it returns to the air when the plant respires, rots or burns.
Stretch and support
Stretch: If photosynthesis and respiration nearly balance, how did fossil fuels form?
Support: Give a two-box diagram (atmosphere, biosphere) with three blank arrows to label.
Engage: where does a tree's mass come from?: Students who think plants are built from soil miss photosynthesis as a carbon flow; the board's arrows show it.
Explore: switch the flows: Switching photosynthesis or respiration off in turn, and watching CO₂ in the air fall or rise, shows each as a directional flow between spheres.
Explain: two flows, opposite directions: Stating each process as a flow from one sphere to another, and seeing balance, is what the carbon cycle diagram needs.
Elaborate: a carbon atom's journey: Tracing one atom through a food chain connects the carbon cycle to food webs.
Elaborate: seasons in the air: Reading a real pattern as evidence of the photosynthesis–respiration balance applies the model.
Now you can
Explain that photosynthesis moves carbon from the air into living things.
Explain that respiration and decomposition move it back.
Explain why the air's CO₂ stays steady when the flows balance.