PMT

• xylem transports

  water

  dissolved minerals
• xylem vessel feature:


  They transport water and minerals, and also serve to provide structural support.
• xylem vessel feature:


  • They are long cylinders made of dead tissue with open ends,

  • therefore they can form a continuous column.
• xylem vessel feature:

  • Xylem vessels contain pits which enable water to move sideways between the vessels.
• xylem vessel feature

  • They are thickened with a tough substance called lignin,

  • which is deposited in spiral patterns
  • to enable the plant to remain flexible.
• Briefly list 4 xylem vessel feature:

  • transport. structural support
  • continuous column
  • pits - water can move sideways
  • lignin- thickens. flexible
• What is transpiration?


  the process where plants ABSORB water through the roots

  • which then moves up through the plant
  • and is released into the atmosphere as water vapour
  • through pores in the leaves
• Transpiration - more details:

  Carbon dioxide enters, while water and oxygen exit through a leaf’s stomata.
• Define XYLEM?

  The tissue that transports water in the stem and leaves of plants.
• Define xerophyte 


  A type of plant that is adapted to survive in places with very little water.
• Define stomata

  Small holes found on leaves that can be opened or closed by guard cells to control the amount of water loss and gas exchange.
• Define spongy mesophyll

  A type of loosely packed mesophyll tissue

  • with air pockets found in plant leaves
  • which is specialised for gas exchange
• Cohesion tension theory:

  • Water molecules form hydrogen bonds with each other,
  • causing them to 'stick' together (COHESION)
  • The surface tension of the water creates this sticking effect.
  • Therefore, as water is LOST through transpiration, more can be drawn up the stem.
• how is water transported in plants

  through xylem vessels:

  • long ,
  • continuous,
  • columns
  • that also provide structural support to the stem
• What is the transpiration stream?

  the movement of water up the stem
• What does the transpiration stream enable?

  processes such as photosynthesis, growth and elongation

  • as it supplies the plant with water
  • which is necessary for all of these processes
• What does the transpiration stream provide the plant with?

  required minerals

  • whilst enabling it to control its temperature
  • via evaporation of water.
• What does transpiration involve?

  Osmosis

  • where water moves from the xylem to the mesophyll cells
• What else does transpiration involve?

  • evaporation from the surface of mesophyll cells
  • into intercellular spaces
  • the diffusion of water vapour
  • down a water vapour potential gradient out of the stomata.
  • AND osmosis
• Factors affecting rate of transpiration include:

  • number of leaves
  • number/size or position of stomata
  • presence of waxy cuticle
  • the amount of light present
  • the temperature
  • humidity of the air
  • air movement
  • water availability.
• How can xerophytes live in such dry conditions?

  various adaptations which help minimise water loss
• Xerophyte adaptations:


  • smaller leaves
  • thick waxy cuticle
  • densely packed mesophyll
• Xerophyte adaptation


  smaller leaves to reduce the surface area for water loss
• xerophyte adaptation


  Both densely packed mesophyll and thick waxy cuticle prevent water loss via evaporation
• How do xerophytes respond to low water availability?

  by closing the stomata to prevent water loss.
• xerophyte adaptations- concentration gradient

  They contain hairs and pits

  • which serve as a means of trapping moist air
  • thus reducing the water vapour potential gradient
• Xerophyte adaptation- to trap moist air:

  Xerophytes also roll their leaves in order to reduce the exposure of the lower epidermis to the atmosphere, thus trapping air that is moist.