SCIENCE · INVESTIGATE AND EXPLAIN

Track carbon through fixation and respiration

Reconcile plant carbon gain or loss with exchanges across a system boundary.

Learning outcome: Construct a conserved carbon ledger and distinguish organic-carbon change from wet mass or energy.

Starting recap

A system boundary says what is included. An amount in mmol C counts carbon atoms regardless of whether they are in CO₂ or organic molecules.

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Try

A plant begins with 20 mmol organic carbon. During one interval it fixes 12 mmol C and respires 8 mmol C.

Open the experiment

Predict

Predict its final organic-carbon store and the change in the external CO₂-carbon pool. Is carbon lost from the combined system?

Your prediction and reason

Experiment

Capture fixation 12 and respiration 8, then reverse the amounts. Test equal fixation and respiration. Verify both final stores add to 40 mmol C each time.

Keep a plant carbon ledger

Worked teaching model · synthetic values · The plant starts with 20 mmol carbon in organic stores; an external CO₂ pool starts with 20 mmol C. Fixation moves carbon into the plant and respiration returns it. No litter, feeding, import or export occurs. All rates are integrated over one common interval; this is carbon accounting, not a growth-rate law.

Carbon fixed during interval: 12 mmol C Carbon respired during interval: 8 mmol C

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Two stores, one conserved carbon total

Into plant: 12 mmol C fixed. Out of plant: 8 mmol C respired. Combined carbon remains 40 mmol C. Widths share a fixed scale.

Calculated values on the same fixed scale
Row	Quantity	Value (mmol C)
1	Initial plant organic carbon	20
2	Carbon fixed into plant	12
3	Carbon respired out	8
4	Final plant organic carbon	24
5	Final external carbon	16
6	Conserved combined carbon	40

Plant carbon change: +4 mmol C; final plant store: 24 mmol C.

Final plant = initial plant + fixation − respiration. External CO₂ carbon changes by the opposite amount. Carbon does not turn into energy. Organic-carbon accumulation alone does not specify wet mass, height, nutrient sufficiency or energy-conversion efficiency.

Observation to attach to a capture

Captures preserve recipes and replay their calculated results. They are synthetic model evidence, not physical measurements.

Understand

Put the boundary on the page

This model includes a plant organic-carbon store and an external CO₂-carbon store. Each starts with 20 mmol C, so the combined system contains 40 mmol carbon atoms. Fixation transfers carbon into organic molecules; respiration transfers it back to the external pool. No feeding, litter loss or other exchange is included.

For fixation F = 12 and respiration R = 8 mmol C during one interval, final plant carbon is 20 + F − R = 24. External carbon is 20 − F + R = 16. Their sum remains 40.

Reverse F and R. The plant ends with 16 and the external pool with 24. The plant’s decline is a transfer across its boundary, not destruction of carbon. At equal F and R, both stores remain unchanged while exchanges can continue.

Do not confuse carbon and energy

Organic molecules carry carbon and participate in energy transformations, but carbon atoms do not turn into energy during these biochemical processes. Energy can be transferred to work and ultimately heat while carbon remains in material compounds. Food-web descriptions therefore need both material accounting and energy-transfer reasoning. Background: OpenStax, ecosystem energy flow.

The ledger uses amounts over one common interval. If a measurement is a rate, multiply or integrate it over the relevant time before combining it with a store. Do not add “mmol per hour” directly to “mmol.”

A larger seedling is not a complete carbon measurement

A germinating seed can take up water while respiring stored organic compounds. Wet mass and height can increase even when the total organic-carbon store falls. Plant tissue also includes elements other than carbon. This model cannot convert carbon gain into a unique height, wet mass or dry mass.

For a seed-growth investigation, state which outcome you measure and which mechanism it can support. Record initial seed differences, observation times and water conditions. A future project should compare repeated observations with alternatives; a single final height cannot reveal the whole carbon budget.

Apply

Public practice checks a specific claim. It does not establish independent mastery.

A seedling gains wet mass while its organic-carbon store falls. Give a possible explanation and identify a measurement needed to distinguish water uptake from carbon gain.

Your explanation and limitations

Continue

Completion requires a saved prediction, at least one capture and a reflection. Presence checks do not grade your reasoning.

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