Transpiration and Water Movement in Plants

Overview

• Transpiration: Evaporation of water from the aerial parts of the plant (leaves and stems).
• Transpiration Pull: Created by water evaporating from leaves, drawing water up through the plant.

Water Movement Process

Roots

• Root Hairs: Increase surface area for water absorption.
• Osmosis: Water passes from soil into root hair cells, moving down the concentration gradient.
• Xylem Vessels: Water moves from roots to stems and leaves.

Transpiration Stream

• Cohesion of Water Molecules: Facilitates the pull of water through the plant.
• Photosynthesis Support: Water necessary for photosynthesis and cellular turgidity.
• Mineral Transport: Mineral ions dissolved in water are transported from roots to leaves.

Factors Affecting Transpiration Rate

Environmental Factors

• Temperature
• Humidity
• Wind
• Light Intensity

Physical Factors

• Waxy Cuticle
• Stomata Number
• Guard Cells
• Leaf Surface Area and Shape

Regulation of Transpiration

Water Scarcity or Root Damage

• Reduces transpiration by closing stomata.

Guard Cells Function

• Daytime:
  • Chloroplasts produce sugar, lowering water potential.
  • Water enters guard cells by osmosis, making them turgid.
  • Turgid guard cells open stomata for water loss.
• Nighttime:
  • Sugar used up, increasing water potential.
  • Water leaves guard cells by osmosis, making them flaccid.
  • Flaccid guard cells close stomata, reducing water loss.

Conclusion

• Water moves through plants via the xylem and is driven by the transpiration pull.
• Plants regulate water loss and maintain balance through stomata control based on guard cell turgidity.

Next Topic

• Movement of glucose in plants (to be covered in the next section).