SCIENCE · INVESTIGATE AND EXPLAIN

Connect roots, leaves and a water budget

Follow water uptake, transpiration and changing stores without inventing a negative amount.

Learning outcome: Reconcile plant water fluxes and explain how stomatal responses connect water balance with gas exchange.

Starting recap

A rate in mL/h becomes an amount in mL when multiplied by time in hours. A store changes by inflows minus actual outflows.

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Try

A plant begins with 40 mL accessible water. Uptake is 3 mL/h and potential transpiration is 5 mL/h for five hours.

Open the experiment

Predict

Predict its final store. If demand exceeds available water, can actual transpiration continue unchanged?

Your prediction and reason

Experiment

Compare uptake 3 and potential loss 5 with uptake 0 and potential loss 12. Inspect hourly losses and unmet demand. Then increase uptake while holding demand fixed.

Account for plant water gains and losses

Worked teaching model · synthetic values · The plant begins with a 40 mL accessible store. Five one-hour steps add root uptake, then lose water up to the potential transpiration or available amount, whichever is smaller. The model has no storage capacity, stomatal regulation, hydraulic resistance or soil depletion.

Root uptake per hour: 3 mL/h Potential transpiration per hour: 5 mL/h

Reset keeps your prediction, captures and practice history.

Root input → accessible store → leaf loss

Five-hour input: 15 mL. Initial store: 40 mL. Potential five-hour loss: 25 mL. Actual hourly loss is limited by water available after uptake.

Hourly water account in mL
Hour	Uptake	Actual loss	Store
0	0	0	40
1	3	5	38
2	3	5	36
3	3	5	34
4	3	5	32
5	3	5	30

Calculated values on the same fixed scale
Row	Quantity	Value (mL)
1	Initial water store	40
2	Total root uptake	15
3	Total transpiration	25
4	Final water store	30
5	Unmet potential transpiration	0

Final store: 30 mL. Water balance: 40 + 15 − 25 = 30 mL.

Transpiration is capped by available water, so the ledger never creates a negative store. A shortfall indicates that the specified potential flux cannot be sustained; it does not predict a safe threshold or real plant survival.

Observation to attach to a capture

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

Understand

Reconcile a store with two fluxes

With initial store 40 mL, uptake 3 mL/h and transpiration 5 mL/h for five hours, the final store is 40 + 5×3 − 5×5 = 30 mL. This assumes the potential outflow can actually be supplied.

At uptake zero and potential loss 12 mL/h, stores after successive hours are 28, 16, 4, 0 and 0 mL. Actual losses are 12, 12, 12, 4 and 0. The model reports 20 mL of unmet potential demand rather than a negative water store.

The cap is an accounting condition, not a description of every response a real plant makes. It prevents an impossible outflow but does not predict when tissues wilt, when hydraulic transport fails or whether the plant survives.

Connect the organs

Water moves through roots and xylem toward leaves under water-potential differences. Transpiration from leaf surfaces contributes to the tension involved in that movement. Stomata regulate a major route for water loss and gas exchange. Closing them can conserve water while restricting CO₂ entry. Background: OpenStax, plant transport.

Water transport and sugar transport are related parts of the organism but are not identical: xylem and phloem have different roles. The lab does not simulate either tissue’s anatomy or pressure distribution.

Design a comparison

Hold plant amount, observation time and environmental conditions comparable while examining water loss. Distinguish soil evaporation from plant transpiration, and measure initial as well as final stores when possible. An educator-approved physical investigation would need an appropriate protocol; the browser supplies only a model ledger.

Explain why a plant can change stomatal conductance rather than maintain the constant rates selected here. Connect your water result with the earlier carbon-fixation lesson, but do not infer an exact photosynthetic rate from water loss alone.

Apply

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

Explain how closing stomata can reduce water loss while also affecting carbon uptake. State why this water ledger alone cannot determine photosynthetic rate or survival.

Your explanation and limitations

Continue

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

Skipping retains experiments and awards no mastery.

Previous: Trace a negative-feedback response Next: Challenge an unlimited-growth prediction

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