explain how hydrogen bonding of water molecules is involved with movement of water in the xylem by cohesion-tension in transpiration pull and by adhesion to cellulose in cell walls
Cambridge A-Level Biology 9700 – Transport Mechanisms
Transport Mechanisms – Role of Hydrogen Bonding in Water Movement
Learning Objective
Explain how hydrogen bonding of water molecules is involved with the movement of water in the xylem by cohesion‑tension (transpiration pull) and by adhesion to cellulose in cell walls.
Key Concepts
Water is a polar molecule; each molecule can form up to four hydrogen bonds.
Hydrogen bonds give water high cohesion (water‑water attraction) and adhesion (water‑surface attraction).
These properties are essential for the continuous column of water from roots to leaves.
Cohesion–Tension Theory (Transpiration Pull)
The cohesion‑tension mechanism relies on two physical processes:
Cohesion: Hydrogen bonds between adjacent water molecules create a strong, continuous column within the xylem vessels and tracheids.
Tension: Evaporation of water from stomatal pores generates a negative pressure (tension) that pulls the water column upward.
where the negative pressure component (\$\Psi_{pressure}\$) is produced by transpiration and transmitted through the cohesive water column.
Adhesion to Cellulose in Cell Walls
While cohesion keeps water molecules together, adhesion allows the water column to remain attached to the walls of xylem conduits, preventing collapse under tension.
These groups form hydrogen bonds with water molecules, anchoring the water to the vessel walls.
Adhesion also contributes to capillary rise in narrow pores, described by the equation:
\$h = \frac{2\gamma \cos\theta}{\rho g r}\$
where \$h\$ is the height of rise, \$\gamma\$ the surface tension of water, \$\theta\$ the contact angle (small due to adhesion), \$\rho\$ the density of water, \$g\$ gravitational acceleration, and \$r\$ the radius of the conduit.
Comparison of Cohesion and Adhesion
Property
Cohesion (water‑water)
Adhesion (water‑cellulose)
Primary force
Hydrogen bonds between water molecules
Hydrogen bonds between water and hydroxyl groups of cellulose
Effect on water column
Maintains continuity of the column
Anchors column to vessel walls, prevents breakage
Role in transpiration pull
Transmits tension from leaf to root
Provides resistance to gravitational pull, aids capillary action
Observed in
All xylem vessels and tracheids
Inner surfaces of xylem conduits, especially narrow pores
Step‑by‑Step Flow of Water from Roots to Leaves
Root hairs absorb water from the soil; adhesion to cell wall polysaccharides helps water enter root cortex.
Water moves radially through the cortex via apoplast (cell wall) and symplast pathways, aided by adhesion to cell walls.
At the endodermis, the Casparian strip forces water into the xylem vessels.
Within xylem, hydrogen‑bonded water molecules create a cohesive column.
Transpiration from stomata creates negative pressure; tension is transmitted up the column.
Adhesion to cellulose prevents the column from breaking under tension, allowing continuous ascent.
Common Misconceptions
“Water is pulled by a suction pump in the leaves.” – The pull is a physical tension transmitted through the cohesive water column, not a biological pump.
“Cohesion alone is enough for ascent.” – Without adhesion to the vessel walls, the column would slip and break under tension.
“Root pressure is the main driver of water movement.” – Root pressure contributes only modestly; transpiration pull dominates in most plants.
Suggested diagram: A cross‑section of a leaf showing water movement from root to leaf, highlighting hydrogen bonds between water molecules (cohesion) and bonds to cellulose in xylem walls (adhesion).
Summary
Hydrogen bonding gives water its unique cohesive and adhesive properties. Cohesion creates a continuous water column that can transmit the negative pressure generated by transpiration, while adhesion to cellulose in the xylem walls anchors this column, allowing it to resist gravity and maintain integrity. Together, these mechanisms enable the efficient transport of water from roots to leaves in vascular plants.