7.2.4—Hydrogen bonding in water movement

The cohesion-tension theory explains how evaporation at leaves can help move a continuous column of water up xylem. Evaporation creates tension (negative pressure) in the xylem; cohesion transmits that pull through water molecules, while adhesion helps keep the column against the xylem walls.

1. Leaf event: Water evaporates from mesophyll surfaces into the leaf air spaces and exits by transpiration. This lowers water potential at the leaf end and creates tension in nearby xylem water.
2. Transmission: Hydrogen bonds create cohesion between adjacent water molecules. Because the molecules resist separating, the tension is transmitted down the continuous water column through the xylem towards the roots.
3. Wall support: Adhesion between water and cellulose in xylem walls helps stop the column pulling away from the vessel wall. Lignified xylem walls resist collapse under the pressure differences.
4. Long-distance result: The water-potential gradient between roots and leaves, maintained by root uptake and leaf water loss, supports upward movement through roots, xylem and leaves as the transpiration stream.

Causal cue: evaporation → xylem tension/transpiration pull → cohesion transmits pull → adhesion and lignified walls support the column → water moves upward.

Root pressure can assist entry and raise xylem pressure, but it is not the complete explanation for normal long-distance ascent.

Hydrogen bonding does not act as a pump by itself: evaporation supplies the initial pull, while cohesion and adhesion transmit/support it. This mechanism card explains the water column and xylem ascent; card 4590 defines the transpiration process, and card 4592 concerns xerophytic water-loss adaptations. Staff-only visual brief: show leaf evaporation, tension arrow down a continuous xylem water column, cohesion links, adhesion to cellulose wall and lignified-wall support; do not generate or bind an image.