Predict root water movement from the complete potential

1. Orient: Predict root water movement from the complete potential

Water movement depends on total water potential, not solute concentration alone. Solute potential lowers the reference potential, while positive pressure can raise it; gravity and interactions with surfaces can also matter. Compare the same reference, units and boundary on both sides of a proposed flow.

2. Connect plant structure and mechanism

The root-cell solute potential is fixed at −1 MPa. Adding its pressure potential gives the modeled total. Soil total potential is supplied separately, so the inward driving difference is soil Ψ minus root Ψ. A positive difference favors inward movement; a negative difference favors outward movement. The signed relative flux is a direction convention, not a negative amount of water.

Root hairs and branching increase contact with soil, but water and ions also encounter tissue barriers. Apoplastic movement through cell walls and symplastic movement through connected cytoplasm are distinct routes. The endodermal Casparian strip interrupts an unrestricted wall pathway and promotes selective membrane crossing before entry into the vascular cylinder.

The model holds conductivity fixed and omits changing solute amount, volume, membrane transport and hydraulic feedback. It therefore predicts an initial direction under stated potentials, not a complete equilibrium cell size or a whole-plant irrigation requirement. More concentrated root solutes do not guarantee inward flow when pressure differs.

3. Work a complete evidence example

At soil Ψ = −0.4 MPa and root pressure +0.2 MPa, root total is −0.8 MPa and inward difference +0.4 MPa. Raising root pressure to +0.8 gives root total −0.2 and difference −0.2, reversing the modeled direction without changing the root solute potential.

4. Predict and investigate

Predict first: Can a pressure change reverse water direction even when the solute potential stays fixed?

Hold soil Ψ at −0.4 and compare root pressures 0.2, 0.6 and 0.8 MPa. Identify inward, zero and outward cases, then distinguish the driving potential from the tissue pathways controlling conductivity.

Retain the original first prediction, stated controls, complete ledger and a conflicting or null comparison. Original supplied calculations are not measured plant specimens or completed practical work.

5. Transfer to a changed plant case

Apply the mechanism to a changed case

At soil Ψ = −0.6 MPa and root pressure +0.4 MPa with root solute Ψ = −1, what is the net driving difference?

Retain your calculation, units and assumptions in the lesson notebook before opening this public practice response.

Compare a worked response

Root total is −0.6 MPa, equal to soil. The ideal driving difference is zero; this does not mean individual water molecules stop moving or that the tissue has no membrane transport.

6. Reflect and defend a bounded conclusion

Connect a tissue or developmental mechanism to the evidence, identify an omitted alternative and propose an independent discriminating observation. Preserve a first report and human critique in the General Biology II workspace. Public practice does not establish independently assessed practical skill, and these conceptual models do not prescribe hormone, fertilizer or field interventions.

Supporting scientific reference, checked 30 October 2026. Original prose, numerical packets and diagrams; no source exercise or figure reproduced. Actual calibrated plant transport or specimen work requires separate supervised evidence.