Transport of Water and Solutes in Plants

Core Concepts

Water potential: the driving force

Water moves from higher water potential (Ψ) to lower. Its main components are solute potential (Ψs) and pressure potential (Ψp):

Ψ = Ψs + Ψp

Pure water at standard conditions has Ψ = 0. Dissolved solutes lower Ψ, so Ψs is always negative or zero (more solute = more negative). Pressure potential can be positive (turgor) or negative (tension in a xylem vessel). Water always flows toward the more negative total Ψ — and a turgid cell can have very negative Ψs yet a high total Ψ, a common exam trap.

Getting water into roots

Water enters through root hairs, which enlarge the epidermal surface, by three routes: apoplast (through cell walls, no membranes), symplast (through cytoplasm via plasmodesmata), and transmembrane (crossing membranes cell by cell). At the endodermis, a waterproof Casparian strip blocks the apoplast route, forcing all water and solutes through a living plasma membrane — the plant's selective checkpoint. Root pressure, from active ion pumping into the xylem, pushes sap a short distance — visible as guttation, liquid droplets at leaf margins on humid nights — but is far too weak to lift water to a treetop.

The xylem and the cohesion–tension mechanism

Xylem consists of tracheids and vessel elements — dead, hollow, lignified cells forming continuous pipes. The water column is pulled, not pushed:

1. Transpiration: water vapor evaporates from leaf cell walls and exits through stomata, creating a water deficit in the leaf.
2. Tension: water is drawn from the xylem into the leaf cells, putting the column under negative pressure.
3. Cohesion: water molecules stick together via hydrogen bonds, transmitting tension down the column like a rope being pulled.
4. Adhesion: water sticks to the hydrophilic vessel walls, helping hold the column.
5. Result: water streams continuously from roots to leaves.

The mechanism needs an unbroken liquid column: an air bubble (embolism) breaks it and stops flow. Transpiration rises with light, warmth, low humidity, and wind; stomatal closure (e.g., under drought, via ABA) slows the stream.

Phloem: translocation by pressure flow

Phloem is made of sieve-tube elements — living cells that lose their nuclei at maturity — connected through sieve plates, with companion cells providing metabolic support. The pressure-flow hypothesis:

1. Loading at the source (e.g., a leaf): sugars are actively transported into sieve tubes (often with companion-cell help), raising solute concentration.
2. Osmosis: water follows from the xylem, raising pressure at the source end.
3. Bulk flow: the pressure difference pushes sap toward lower pressure.
4. Unloading at the sink: sugars are removed, solute concentration falls, water leaves the phloem, and pressure drops.

Two key points: loading is active (requires energy) while transport is passive bulk flow; and direction is set by sources and sinks, not gravity — in spring, stored sugars may move up from roots to buds, and in summer down from leaves to roots. Phloem sap also carries amino acids and hormones, making it a communication highway.

Mineral transport

Minerals taken up by roots ride the transpiration stream. Mobile minerals (nitrogen, potassium, phosphorus) are re-mobilized from old leaves via the phloem, so their deficiency appears first in old leaves; immobile minerals (calcium, iron) appear deficient in new growth.

Common Confusions