Rucete ✏ Campbell Biology In a Nutshell
Unit 6 PLANT FORM AND FUNCTION — Concept 36.3 Transpiration Drives the Transport of Water and Minerals from Roots to Shoots via the Xylem
Plants transport large volumes of water from roots to shoots without pumps or muscles. This is achieved by transpiration, a process powered by the sun that pulls water upward through the plant’s xylem via cohesion, adhesion, and negative pressure.
Water Absorption in Roots
- Water enters primarily through root tips with abundant root hairs.
- Water pathways:
- Apoplastic: along cell walls and spaces
- Symplastic: through plasmodesmata
- Transmembrane: across cell membranes
- The Casparian strip in the endodermis blocks apoplastic flow, forcing selective uptake into the vascular cylinder.
Xylem Sap Transport
- Water and minerals move via bulk flow through tracheids and vessel elements.
- Transport is rapid—up to 45 meters/hour.
- Flow path: soil → root cortex → xylem → shoots.
Root Pressure and Guttation
- Nighttime ion pumping lowers xylem water potential.
- Water enters by osmosis, generating root pressure that can push water upward.
- Guttation: water droplets form at leaf tips in the morning.
- However, root pressure is limited and not sufficient for tall trees.
The Cohesion-Tension Mechanism
- Main driver of long-distance water transport.
- Evaporation from leaves pulls water upward via:
- Cohesion: water sticks to itself
- Adhesion: water sticks to xylem walls
- Surface tension: creates negative pressure as water evaporates from mesophyll walls
Process:
- Water evaporates from mesophyll surfaces.
- Water film retreats, increasing curvature and tension.
- Tension pulls water from surrounding cells into the xylem.
- A continuous column of water is pulled from roots to leaves.
Water Potential Gradient
- Water moves from higher to lower water potential.
- Leaves have the lowest potential due to evaporation.
- This gradient pulls water upward from the soil through the plant.
Cavitation and Repair
- Cavitation (air bubble formation) disrupts xylem flow.
- Occurs during drought or freezing.
- Bypassed via pits between adjacent xylem cells.
- Secondary growth forms new xylem to restore flow.
Bulk Flow vs. Diffusion
- Bulk flow:
- Driven by pressure, not solute concentration
- Occurs in non-living xylem cells
- Moves the entire fluid, not just components
- Much faster than diffusion or osmosis
In a Nutshell
Transpiration pulls water up from the roots through the xylem using cohesion, adhesion, and negative pressure. While root pressure contributes slightly, it is the cohesion-tension mechanism that moves water across great heights. This elegant, energy-efficient system is vital for nutrient transport and cooling and is powered entirely by sunlight and water potential differences.