ICSE Class 10 Biology: Transpiration & Absorption Notes 2026

Absorption of Water by Roots

• Root hair cells have large surface area; absorb water by osmosis (soil water potential > root cell potential)
• Mineral salts absorbed by active transport (carrier proteins, energy required)
• Path of water: root hair → cortex → endodermis → xylem (apoplast and symplast pathways)

Ascent of Sap

• Water rises in xylem against gravity — three theories
• Root pressure: osmotic pressure pushes water upward (guttation at night)
• Transpiration pull (cohesion-tension): evaporation from leaves creates tension; cohesion of water molecules
• Capillarity: water in narrow xylem tubes rises due to adhesion and cohesion

Transpiration — Definition and Types

• Transpiration: loss of water vapour from aerial parts of a plant
• Stomatal (90–95%) — through stomata in leaves; most significant
• Lenticular (through lenticels in bark) and cuticular (through leaf cuticle)

Mechanism of Stomatal Opening and Closing

• Guard cells control stomatal pore; turgor of guard cells determines opening
• Light causes K⁺ influx into guard cells → water enters by osmosis → cells swell → stomata open
• At night or water stress: K⁺ leaves → cells lose turgidity → stomata close (ABA promotes closure)

Factors Affecting Transpiration Rate

• Light: increases transpiration (stomata open wider)
• Temperature: higher temperature → lower relative humidity → faster evaporation
• Wind: removes humid air layer near leaf → increases transpiration; humidity decreases rate

Potometer Experiment

• Measures water uptake (approximate transpiration rate)
• Procedure: cut shoot under water, attach to potometer, measure air bubble movement over time
• Variables to test: fan (wind), lamp (light/temperature), petroleum jelly on leaves (cuticular path)

Significance of Transpiration

• Cooling effect: latent heat of vaporisation cools leaf surface
• Drives ascent of sap and mineral transport
• ICSE question: 'Transpiration is a necessary evil' — explain both sides