Plant Transport

Water Uptake

Root hair cells absorb water from the soil by osmosis (soil water has higher water concentration than root cells). Root hairs increase surface area for absorption.

Water moves through the root cortex by two pathways:

• Symplastic: through the cytoplasm of cells
• Apoplastic: through the cell walls

Transpiration

Transpiration is the loss of water vapour from the leaves through stomata. This creates a transpiration pull that draws water upward through the xylem.

Cohesion-Tension Model

The main mechanism for water transport up a plant:

1. Water evaporates from leaf cells through stomata (transpiration)
2. This creates tension (negative pressure) in the xylem
3. Water molecules are attracted to each other by cohesion (hydrogen bonds)
4. Water molecules are attracted to xylem walls by adhesion
5. A continuous column of water is pulled upward from roots to leaves

Factors Affecting Transpiration Rate

• Temperature: higher = faster evaporation = more transpiration
• Humidity: lower humidity = faster transpiration
• Wind: more wind = faster transpiration
• Light: stomata open in light = more transpiration

Stomata & Guard Cells

Guard cells control the opening and closing of stomata. When turgid (full of water), they open the stoma; when flaccid, they close it. CO₂ concentration also controls stomata - low CO₂ causes them to open.