7. Transport in plants

• What tissue carries water?

  Xylem
• Why do xylem vessels contain lignin?

  Strengthen the walls and makes them waterproof
• The walls of Xylon vessels contain tiny holes called pits, why?


  If a vessel becomes blocked or damage, the water can be diverted
• Why is the lying in xylem vessels spiral?

  Takes less energy and gives more flexibility
• What is the cohesion tension theory?

  When water is a evaporated from the mesophyll cells, the water potential is reduced. This forms, a water potential gradient against the leaf . water then leaves the island vessels in the leaf and diffuses into mesophyll cells by osmosis this creates a negative pressure at the top of the island vessels causing water to be pulled up in continuous columns due to cohesion between the water molecules.
• What is the evidence for cohesion tension theory?

  Tension has been measured in the xylem, the diameter of trees decreases when they are transpiring
• What are xerophytes?

  Xerophytes are plants that have adapted to survive in dry environments by reducing water loss through various mechanisms such as having thick waxy leaves, storing water in their stems or leaves, or having deep root systems.
• Why do xerophytes have hair on the lower epidermis of the leaf?

  Traps water vapour and reduces the water potential gradient
• Why do xerophytes have stomata in sunken pits

  Water vapour is held above the stomata, so reduces the water potential gradient
• Why are non reducing sugars transported in the phloem rather than reducing sugars?

  Reducing sugars are too reactive and unstable
• What is the structure of the phloem tissue?

  Sieve tubes and companion cells.
• What is the mass flow theory?

  The mass flow theory explains how sugars are transported in plants through the phloem.
• Describe the mass flow theory

  Active transport is used to pump sucrose from the mesophyll cells into the phloem via companion cells. increases the concentration and lowers water potential. Water enters by osmosis and increases the volume which increases the hydrostatic pressure. There is a hydrostatic pressure gradient so mass flow
• What is the evidence for mass flow theory

  High hydrostatic pressure as shown by release of sap

  companion cells have mitochondria

  sucrose levels are higher in the leaves than the sink
• How do we investigate transport in plants?

  ringing experiments - sugars accumulate in the bark above the cut

  radioactive tracer experiment- radioactively labelled carbon can be traced