4.2 Transport of Water and Mineral Salts
Water and dissolved mineral salts are vital for plant survival, providing turgidity to keep plants upright and supplying nutrients needed for biochemical processes such as photosynthesis. In vascular plants, water moves across vast vertical heights against gravity—from the soil into the roots, up through the stem xylem vessels, and finally into the leaves. This continuous movement depends on three coordinated forces: root pressure, capillary action (adhesion and cohesion), and transpirational pull. In addition, herbaceous plants display guttation under specific environmental conditions when transpiration is low.
Learning Objectives
- Explain the roles of root pressure, capillary action, and transpirational pull in the movement of water and mineral salts from the soil to the leaves.
- Describe the pathway of water from the soil into root hairs, cortex, endodermis, pericycle, and xylem vessels via osmosis.
- Distinguish between the simplast pathway and the apoplast pathway across root tissues, including the function of the Casparian strip.
- Describe experiments investigating the effects of root pressure and transpirational pull on water transport.
- Define guttation, compare guttation with transpiration, and explain the consequences to plants if transpiration or guttation does not occur.
Summary of Water Transport Pathways and Guttation vs Transpiration
Trace the step-by-step upward movement of water from root hairs to stomata and contrast the exit points of guttation and transpiration.
Pathway from Soil to Xylem
- 1Root hair cells actively pump mineral ions into their vacuoles, lowering the water potential of the cell sap.
- 2Water from the soil diffuses into root hair cells and epidermal cells by osmosis.
- 3Water moves into adjacent cortex cells by osmosis.
- 4Osmosis continues across the cortex, endodermis and pericycle.
- 5Water enters the root xylem vessels, creating root pressure that pushes water upward.
Guttation vs Transpiration
Guttation releases liquid water droplets rich in dissolved minerals through hydathodes at leaf vein endings, mainly at night or early morning when root pressure is high and transpiration is low.
Transpiration releases pure water vapour through open stomata on hot, sunny and windy days, creating transpirational pull that draws water up the xylem.
Short Explanation
How Water Moves Through a Plant
Water moves into a plant's roots by osmosis because root hair cells actively pump mineral ions into their vacuoles, keeping their internal water potential lower than that of the surrounding soil water. As water diffuses into the root cortex, it can travel through cell walls (apoplast pathway) or through the cytoplasm (simplast pathway). At the endodermis, a waxy barrier called the Casparian strip blocks the apoplast pathway, forcing all water to pass through the living plasma membrane and cytoplasm into the xylem vessels.
Root Pressure
Generated in the roots via osmosis as water moves from the soil into root xylem vessels. Root pressure pushes water upward into the stem, but it is weak and can only move water over short distances.
Capillary Action
Produced by the combination of adhesion forces and cohesion forces, pulling water up narrow xylem vessels against gravity.
Transpirational Pull
A pulling force generated when water evaporates from mesophyll cells into air spaces and exits through open stomata into the atmosphere, creating tension that draws water out of leaf xylem vessels.
Guttation
Secretion of water droplets through special structures called hydathodes at the vein endings of leaf margins without involving stomata, caused by high root pressure.
Try to Answer First
Answer in your mind, then press “Check Answer”.
Why does water enter the root hair cells from the soil via osmosis?
What is the function of the Casparian strip located in the endodermis?
State two differences between guttation and transpiration.
Common Mistakes
- Confusing the roles of adhesion and cohesion: Adhesion is the attraction between water molecules and the xylem wall, whereas cohesion is the mutual attraction between water molecules themselves.
- Assuming guttation occurs through stomata: Guttation occurs through specialised structures at the ends of leaf veins (hydathodes), not through stomata.
- Believing root pressure alone can supply water to the shoots of tall trees: Root pressure is weak and can only push water over short distances; tall trees rely predominantly on transpirational pull.
- Stating that guttation releases pure water: Water released via guttation contains dissolved mineral salts and waste substances, whereas water lost through transpiration evaporates as pure water vapour.
Concept Misunderstandings
Water moves into root hairs purely by passive diffusion of minerals and water.
Root cells expend energy via active transport to pump mineral ions into vacuoles first, deliberately establishing the low water potential required for water to enter by osmosis.
Guttation and dew drops on leaves are the exact same thing.
Dew is atmospheric water vapour that condenses onto leaf surfaces from external air, whereas guttation droplets originate from inside the plant's vascular system due to root pressure.
The apoplast pathway carries water directly into xylem vessels without any barrier.
The apoplast pathway is completely blocked at the endodermal layer by the suberin-rich Casparian strip, forcing all substances through the selectively permeable simplast pathway.
Summary
- Absorption in Roots: Active transport of mineral ions lowers root hair cell sap water potential, driving osmotic water uptake from soil through cortex and endodermis into root xylem.
- Pathways: Apoplast (cell wall spaces) and simplast (cytoplasm and plasmodesmata); Casparian strip in endodermis blocks apoplast, directing flow into simplast.
- Ascent of Sap: Powered by root pressure (push from below), capillary action (adhesion and cohesion), and transpirational pull (pull from above due to stomatal evaporation).
- Guttation: Occurs in herbaceous plants under low transpiration and high humidity, pushing mineral-rich droplets through leaf vein ends via high root pressure.
- Physiological Importance: Transpiration cools the plant and transports minerals; guttation relieves excessive vein pressure and eliminates wastes. Failure of either process leads to wilting, damage, or death.
Short Activity
Objective: test your understanding of water transport mechanisms, pathways and guttation.
A. Objective Quiz
Which force is responsible for the continuous, unbroken column of water moving upward through narrow xylem vessels against gravity due to the mutual attraction between water molecules?
What occurs in plants when root pressure is high but the rate of transpiration is low during humid nights or early mornings?
B. Fill in the Blanks
In the root endodermis layer, the cell walls contain strips that are impermeable to water, forcing water to travel exclusively through the simplast pathway.
The pulling force produced when water evaporates from mesophyll cells and diffuses out through open stomata is called pull.
C. Matching / Drag and Drop
Drag each water transport mechanism or structure to its correct textbook description. On a phone, tap a choice first, then tap the matching box. Tap the Answer Choices area to return a selected item.
C. Match the Mechanism or Structure to Its Description
Write the correct mechanism or structure next to each description.
| Description | Mechanism / Structure |
|---|---|
| 1. Water movement through spaces between cellulose fibres at the cell wall | |
| 2. Water movement through cytoplasm and plasmodesmata across cells | |
| 3. Release of mineral-rich water droplets through vein endings due to high root pressure | |
| 4. Evaporation of pure water vapour from open stomata on hot, windy days |
Answer choices: Apoplast pathway • Simplast pathway • Guttation • Transpiration
