Transport in plants

Biology · Transport. The idea in plain words, the rules to remember, the mistake to avoid and a worked example, written and checked by Mega Lecture.

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The idea

Water moves from a region of higher water potential to a region of lower (more negative) water potential. Pure water has a water potential of zero. Solutes lower it and pressure raises it. This one rule explains water entering root hairs, crossing the cortex and moving from cell to cell.

Water rises in the xylem mainly by transpiration pull. Evaporation from the leaves creates tension, and the cohesion of water molecules keeps the column unbroken. Root pressure gives only a small push from below.

Food moves in the phloem by pressure flow. Sugar loaded at the source draws in water by osmosis. The high pressure pushes the sap to the sink, where sugar is unloaded.

Rules to remember

water potential = solute potential + pressure potential (ψ_w = ψ_s + ψ_p); unit MPa. ψ_s is zero or negative; ψ_p is usually positive in a turgid cell.
Routes across the root: apoplast (cell walls), symplast (cytoplasm through plasmodesmata), vacuolar. The Casparian strip of the endodermis blocks the apoplast.
Ions are taken up mainly by active transport; water by osmosis.
Cohesion-tension theory: transpiration pull + cohesion between water molecules + adhesion to xylem walls.
Root pressure causes guttation (liquid water lost through hydathodes).
Stomata open when K⁺ enters the guard cells, water follows and they become turgid. They close when K⁺ leaves.
Transpiration rises with light, temperature and wind; it falls with high humidity. Phloem carries sugar mainly as sucrose.

Common mistake

Students treat −0.7 MPa as higher than −0.4 MPa. With negative values the number closer to zero is higher, so water moves from −0.4 MPa to −0.7 MPa.

Worked example

Cell A has ψ_s = −1.2 MPa and ψ_p = +0.5 MPa. Cell B has ψ_s = −0.8 MPa and ψ_p = +0.3 MPa. The cells touch. Which way does water move?

Cell A: ψ_w = −1.2 + 0.5 = −0.7 MPa.
Cell B: ψ_w = −0.8 + 0.3 = −0.5 MPa.
−0.5 MPa is higher than −0.7 MPa, and water moves from higher to lower water potential.

Answer: Water moves from cell B to cell A.

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Chapter 14: Transport

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