4.7 transport plants

• structure of xylem

  • long hollow tubes - made from dead tissue, allows transport of water
  • pits - allows water to move between vessels
  • thickened with lignin - provides strength
• structure of phloem

  • sieve tube cells - transport sugars in the plant
  • companion cells - for active transport of sugars into tubes
  • plasmodesmata - links cytoplasm. allows substances to travel from cell to cell
• differences in structure of xylem and phloem?

  • xylem has lignin, phloem doesn't
  • xylem is hollow, phloem contains sieve plates
  • xylem doesn't have companion cells, phloem does
• xylem tissue - transports water and ions

  phloem tissue - transports sucrose and amino acids
• transpiration - cohesion tension model

  • water in leaves diffuses down the water potential gradient from the spongy mesophyll
  • through the stomata
  • water lost from the cells in the mesophyll is replaced by water from the xylem
  • cohesion between water molecules causes water to be pulled up
  • water pulling up causes tension - causing vessels to go inwards
• water is pulled up from the root and crosses the root via the apoplast and symplast pathway
• apoplast pathway

  • osmosis through the roots
  • water moves through the cell walls of the cells and the intracellular spaces
  • until it reaches the endodermis - containing the casparian strip
• symplast pathway

  • osmosis through roots
  • water moves through the cytoplasm of the cells via the plasmodesmata
• casparian strip

  • in the endodermis
  • waxy layer of cells in cell wall
  • prevents water passing via apoplast
  • causes water to enter symplast pathway to pass endodermis
• • the endodermal cells secrete mineral ions into xylem
  • lowers the w.p
  • water moves into xylem via osmosis
  • hydrostatic pressure increases - causing root pressure
  • water enters root hair cells via osmosis - large surface area and uses active transport to take up mineral ions
  • maintaining water potential in root hair cells
• how does root pressure affect water movement?

  • high mineral content
  • root has low w.p
  • weak push effect - water moves from roots into stem
• factors affecting rate of transpiration

  • temperature
  • humidity
  • light intensity
  • surface area
• how does temperature affect rate of transpiration

  • higher temperature increases rate of evaporation of water
  • from surface of spongy cells - increased kinetic energy of water molecules
• how does humidity affect rate of transpiration?

  • higher water potential in the air
  • reducing the concentration gradient
  • less transpiration
• how does light intensity affect transpiration?

  • increases rate of photosynthesis
  • more open stomata
  • for gas exchange
  • increasing rate of transpiration
  • issue of open stomata - water loss during the day
• translocation - the mass flow hypothesis

  at the source:

  • sucrose formed in photosynthesis
  • loaded into sieve tube via active transport
  • lowering water potential
  • which causes water to move into sieve tube from xylem

  at the sink:

  • sucrose converted to starch (is stored)
  • sucrose removal increases water potential
  • water moves out by osmosis
  • decreasing hydrostatic pressure of cells
• translocation - the mass flow hypothesis

  
  

  at the source:

  • sucrose formed in photosynthesis
  • loaded into sieve tube via active transport
  • lowering water potential
  • which causes water to move into sieve tube from xylem

  at the sink:

  • sucrose converted to starch (is stored)
  • sucrose removal increases water potential
  • water moves out by osmosis
  • decreasing hydrostatic pressure of cells

  
• evidence to support mass flow hypothesis

  • sap taken from source has higher conc of sucrose than taken from sink (water potential gradient formed)
  • sap released when stem is cut (showing pressure in phloem)
• evidence against mass flow hypothesis

  • transport of sucrose solutes occurs at different rates. if mass flow was correct, the rate for all solutes should be same
  • solutes found to move in different directions in plant. if mass flow was correct, they should be moving down pressure gradient.