Plant Transportation

• Xylem - transports water a minerals through vessel elements and tracheids, which are dead at maturity and have a primary and secondary wall. In pits, the secondary wall is thin or missing, allowing water to flow laterally
• Structure and Function

  A) xylem

  B) phloem

  C) tracheids

  D) vessel elements

  E) xylem fibers

  F) xylem parenchyma

  G) sieve-tube elemnts

  H) companion cells

  I) phloem parenchyma

  J) phloem fibers

  K) tension

  L) food

  M) sugars

  N) fat

  O) starch

  P) support

  Q) strenght

  R) water

  
• Phloem transports sugars and other items. In angiosperms, sieve tube elements, contain the sugar solution. Sieve tube elements are surrounded by support cells.
• Transport Mechanisms in Plants

  • Sugar made by leaves
  • Water from roots up the stem/trunk --> leaves
• Diffusion

  • movement from higher concentration to a region of lower

  concentration
• Apoplast

  • Everything outside the plasma membrane of living cells
  • Water & minerals move thru cell walls but not into the cell
• Symplast

  • Entire mass of cytosol of living cells
  • Moves from cytoplasm of one cell to the next via open channels between cells
• Transmembrane

  • Movement thru plasma membrane from one cell to another
• Root pressure

  • moves water up the plant for a few meters
  • Created by roots constantly absorbing water from soil
  • Constant movement of water into roots --> force that pushes water into xylem and up the plant
  • Not enough to drive water movement up the plant
• Transpiration

  • Drives movement of water in xylem
  • Loss of water from plant thru water evaporation on the stomata
  • Regulated by opening & closing of stomata
  • Constant water loss from leaves --> negative pressure --> water is pulled up the stem
• Cohesion-tension hypothesis - Water moves from areas of higher water potential to areas of lower water potential.

  • Cohesion - allows water molecules to stick together
  • Adhesion - allows water to adhere to the walls of the xylem
• Xylem sap movement is unidirectional

  • Roots --> leaves
• Phloem sap movement is multidirectional

  • Sites of sugar production --> sites of sugar use/storage
• Translocation - transport of photosynthesis products

  • Sugar is actively transported from source cells into the phloem tissues
• Bulk flow

  • Long distance transport
  • Movement of liquid (xylem & phloem sap) due to a pressure gradient
  • Faster than diffusion
  • Perforation plates connecting sieve tube elements enhance bulk flow --> ensure transport of water & nutrients