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

Transport Mechanisms – Role of Hydrogen Bonding in Water Movement

Learning Objective

Explain how hydrogen bonding of water molecules contributes to the movement of water in the xylem by (i) cohesion‑tension (transpiration pull) and (ii) adhesion to cellulose in cell walls, and relate these ideas to water‑potential concepts, experimental investigations, xylem anatomy, fluid‑mosaic membranes, cellular transport, and animal circulatory transport (Cambridge International AS & A Level Biology 9700).

1. The Fluid‑Mosaic Membrane (Syllabus 4.1)

• Phospholipid bilayer – amphipathic molecules with hydrophilic heads (‑PO₄⁻) facing water and hydrophobic fatty‑acid tails inward.
• Cholesterol – inserts between phospholipids, modulating fluidity and membrane stability.
• Glycolipids & glycoproteins – carbohydrate‑rich regions that provide recognition sites and contribute to the “mosaic” of proteins.
• Integral & peripheral proteins – channels, carriers, receptors and enzymes that mediate selective transport.
• Relevance to water movement – the semi‑permeable nature of the plasma membrane allows osmosis; membrane proteins generate the ion gradients that drive root pressure and active uptake of solutes.

2. Movement Into and Out of Cells (Syllabus 4.2)

3. Water Potential (Ψ_w)

The net tendency of water to move is described by water potential, the sum of its components:

\$\Psi{w}= \Psi{s}+ \Psi{p}+ \Psi{g}+ \Psi_{m}\$

Water moves from regions of higher (less negative) Ψ_w to lower (more negative) Ψ_w. A typical gradient in a plant is:

• Root water (Ψ≈ ‑0.03 MPa) → Xylem (Ψ≈ ‑0.1 MPa) → Leaf mesophyll (Ψ≈ ‑0.6 MPa) → Atmosphere (Ψ≈ ‑0.9 MPa).

4. Xylem Structure – Form Meets Function (Syllabus 7.1)

Plan‑diagram tip: In exams, draw a transverse section of a stem showing vessels, tracheids, pits and the surrounding lignified tissue. Label each part and indicate the direction of water flow.

5. Cohesion–Tension Theory (Transpiration Pull) (Syllabus 7.2)

1. Cohesion – Hydrogen bonds (O–H···O) between adjacent water molecules create a tensile‑strong, continuous column.
2. Tension – Evaporation of water from stomatal pores generates a negative pressure (Ψ_p < 0) that pulls the column upward.
3. The tension is transmitted unchanged through the cohesive column to the roots, where water is drawn from the soil.

Because liquid water is essentially incompressible, the column can sustain tensions of up to –2 MPa without breaking, provided the walls adhere to the water and prevent cavitation.

6. Root Pressure – A Complementary Mechanism

7. Adhesion to Cellulose & Capillary Rise

• Cellulose microfibrils expose –OH groups that form hydrogen bonds with water → adhesion.
• Adhesion together with the narrow diameter of xylem conduits generates capillary rise, described by:

\$h = \frac{2\gamma \cos\theta}{\rho g r}\$

• γ = surface tension of water (≈ 0.072 N m⁻¹ at 20 °C).
• θ = contact angle; very small for water‑cellulose (≈ 0°, cos θ ≈ 1).
• r = radius of the conduit (tracheid r ≈ 10 µm gives h ≈ 10 m).

Thus adhesion not only prevents the column from slipping but also contributes to the initial rise of water into the narrow xylem vessels.

8. Surface‑Area‑to‑Volume Ratio Effects

In experimental models (e.g., agar blocks, dialysis tubing), the rate of water movement is proportional to the surface area exposed to the water‑potential gradient and inversely proportional to the volume of the material:

• Rate ∝ A / V – a higher surface‑area‑to‑volume (A/V) ratio gives a faster change in Ψ_w.
• Practical implication: young root hairs (high A/V) absorb water more rapidly than mature root cortex cells.

9. Practical Investigation – Estimating Water Potential Using Agar Blocks

10. Link to Animal Transport (Topic 8.1)

Blood plasma is > 90 % water; its movement through capillaries follows the same physical principles of water potential and adhesion. Plasma proteins (e.g., albumin) generate an osmotic (colloid) pressure analogous to Ψ_s in plant cells, drawing water into the circulatory system.

11. Brief Link‑Out: O₂ & CO₂ Transport & The Heart (Topics 8.2 & 8.3)

• Oxygen transport – Dissolved O₂ and oxy‑haemoglobin carry O₂ from lungs to tissues; the binding is reversible and influenced by pH (Bohr effect).
• Carbon dioxide transport – Carried as dissolved CO₂, bicarbonate ions (via the chloride shift), and carbamino compounds.
• Heart cycle – Systole (ventricular contraction) creates the pressure that drives blood through arteries; diastole allows filling. The cyclical pressure changes are analogous to the periodic fluctuations in Ψ_p that occur during day‑night cycles in plants.

These processes illustrate how the same physical concepts—pressure gradients, diffusion, and binding equilibria—underpin transport in both plant and animal systems.

12. Common Misconceptions

• “Water is pumped by a suction device in the leaf.” – The pull is a physical tension transmitted through the cohesive water column; no biological pump is required.
• “Cohesion alone can lift water to any height.” – Without adhesion to the conduit walls, the column would slip or cavitate under tension.
• “Root pressure is the main driver of ascent in tall trees.” – Root pressure contributes only a few kilopascals; transpiration pull provides the dominant negative pressure.
• “All water movement is driven by gravity.” – Water moves from higher to lower water potential, which may be opposite to the direction of gravity.

13. Integrated Step‑by‑Step Flow of Water (Root → Leaf)

1. Root‑soil interface: Soil water (higher Ψ) contacts root hairs; adhesion to cell‑wall polysaccharides (pectin, cellulose) helps water enter the apoplast.
2. Radial movement: Water moves through the cortex via apoplastic (cell‑wall) and symplastic (plasmodesmata) pathways; high A/V of root hairs accelerates uptake.
3. Endodermis: Casparian strip forces water into the stele, ensuring it passes through the xylem.
4. Xylem ascent: Hydrogen‑bonded water molecules form a cohesive column; adhesion to cellulose walls prevents breakage; capillary rise provides the initial lift.
5. Transpiration pull: Evaporation from mesophyll cells creates a strong negative Ψ_p in the leaf; tension is transmitted up the column.
6. Root pressure (optional): At night, active ion uptake generates a modest positive Ψ_p that can push water upward, evident as guttation.
7. Delivery to cells: Water exits the xylem through pits into surrounding parenchyma, where it is used for metabolism or stored.

14. Suggested Diagram for Exam Practice

Draw a cross‑section of a leaf showing:

• Water movement from a root to a leaf vein.
• Hydrogen bonds between water molecules (label “cohesion”).
• Hydrogen bonds between water and cellulose in vessel walls (label “adhesion”).
• Stomatal pore with arrows indicating evaporation and the resulting tension.
• Capillary rise in a narrow tracheid.

15. Summary

Hydrogen bonding gives water its unique cohesive and adhesive properties. Cohesion creates a tensile‑strong column that transmits the negative pressure generated by transpiration (cohesion‑tension theory). Adhesion to cellulose in the xylem walls anchors this column, prevents cavitation, and contributes to capillary rise. Together with water‑potential gradients, surface‑area‑to‑volume effects, and, to a lesser extent, root pressure, these mechanisms enable efficient upward transport of water in vascular plants. The same physical principles—pressure gradients, diffusion, and binding equilibria—underlie water and solute movement in animal circulatory systems, highlighting the universal importance of hydrogen bonding in biology.