Water Movement through plants

• What process primarily drives the movement of water through the xylem?

  Transpiration
• Where does water evaporate from in the leaf before diffusing out through stomata?


  From mesophyll cell walls into the air spaces
• Why does water move out of the stomata?

  Due to a water potential gradient
• How does water move across leaf cells?

  By osmosis along a water potential gradient
• What property of water molecules allows them to stick together in the xylem?

  Cohesion (due to hydrogen bonding)
• What is the cohesion-tension theory?

  It explains how water is pulled up the xylem
• What happens to the xylem diameter during the day and why?

  It decreases due to tension from transpiration pull
• What causes water to move into a mesophyll cell from its neighbour?

  A lower water potential in the mesophyll cell
• What happens if a xylem vessel is broken and air enters?

  Water can no longer be drawn up the xylem
• What structural features make xylem vessels suitable for water transport?

  Long hollow tubes, dead cells, lignin walls
• What is the role of lignin in xylem walls?

  Provides strength and prevents collapse under tension
• How does water move out through stomata?

  • Stomata open for gas exchange
  • Water vapor diffuses out due to a gradient
  • Water lost is replaced by:
  • Evaporation from mesophyll cell walls
  • Drawn out by osmosis
• What are the steps in the movement of water across the leaf?

  1. Mesophyll cells lose water to air spaces
  2. Cells become less turgid, lowering water potential
  3. Water enters by osmosis from neighboring cells
  4. Creates a water potential gradient across the leaf
• What are the pathways for water movement in the leaf?

  • Apoplastic pathway: through cell walls
  • Symplastic pathway: through cytoplasm via plasmodesmata
• How does water move up the xylem?

  • Driven by transpiration pull
  • Water evaporates from mesophyll
  • Cohesion forms a continuous column
  • Creates negative pressure in the xylem
• What evidence supports the cohesion-tension theory?

  • Tree trunks shrink during high transpiration
  • Air enters xylem when broken, disrupting water column
• What are the structural features of xylem vessels?

  • Long, hollow tubes with no end walls
  • Dead cells with lignified walls for strength
  • Provide a low-resistance pathway for water flow
• What is the function of lignin in xylem vessels?

  • Provides structural strength
  • Prevents collapse under tension