Water Movement Through Plants

• Root hairs are long and thin - provides a large surface area and thin diffusion pathway for water

•  
  • 1. - Soil around the roots has a very high water potential
  • 2. - Root hair cells contain sugars, mineral ions, etc so have a lower water potential
  • 3. - Water moves by osmosis, down the water potential gradient. 
  • 4. - Moves up the cells of the root cortex after absorption by the apoplastic or symplastic pathways

• Water moves between cell membrane and cell wall

• Water has cohesive properties, so more water drags more water down the water potential gradient, along cell walls of the root cortex

• Water reaches the impermeable Casparian ***** just before xylem, and is forced back into the cytoplasm.

• Then travels by the symplastic pathway

• Moves across the cytoplasm of cells in the root cortex
• Passes from cell to cell across plasmodesmata which creates a continous cytoplasm

• Water entering the root hair cell raises the water potential so it's above the first cell in the cortex
• Water moves down the water potential gradient, along the symplastic pathway

• Active transport of mineral ions into xylem by carrier proteins
• Creates a lower water potential in the xylem than in the root cortex
• Water moves down the water potential gradient into the xylem
• Root pressure is created - contributing to water moving up plants

•  Root pressure
  • Pressure increases with rise in temperature
  • Metabolic inhibitors cease root pressure
  • Decrease in abailability of oxygen reduces root pressure

Cohesion Tension Theory

• Water evaporates from leaves in transpiration

• Water molecules form hydrogen bonds and stick together - cohesion

• They form a continuous column up the xylem

• Water evaporates from mesophyll cells so more water is drawn up

• Creates a transpiration stream

• Puts xylem under tension

• Creates negative pressure within the xylem - pulls more water into leaf

Adhesion

• Adhesion to the sides of the xylem prevents backflow

Diameter Change

• Day time - transpiration stream at greatest, more negative pressure so diameter shrinks

Light

• Photosynthesis only occurs in light
• Stomata are open to release CO2 so transpiration occurs

Humidity

• Affects water potential gradient outside of the stomata

Temperature

• Rise in temperature increases kinestic energy so increases evaporation
• Rise in temperature decreaes humidity, increases water potential gradient

Air Movement

• Water vapour accumulates around stomata
• Wind disperses water vapour
• Maintains water potential gradient