Mass transport-plants

• Mass Transport


  Movement of large volumes of fluid (water) carrying dissolved molecules/ions all moving at same rate in same direction movement caused by pressure gradient
• Mass transport needs-

  • Closed vessels - Xylem & phloem
  • Pressure differences
• Why does a plant need water?

  • Solvent
  • Fills vacoule,makes cell turgid so plant is supported
  • Reactant in photosynthesis
  • Hydrolysis reaction
  • Transport medium-ions from roots to leaves
• Vessels

  • Xylem transports water & dissolved ions up the stem from roots to leaves
  • Phloem transports sucrose (& amino acids) from source to sink
• • Source-Site of production of sucrose (leaves)
  • Sink-Site of use of sucrose (growing/storage areas)
• Xylem


  Transports water (and dissolved mineral ions) from roots up through the stem and leaves of plants
• Phloem


  Transports food (sugars) produced in the leaves to other parts of the plant
• Xylem and Phloem

  A) Water & mineral ions

  B) 1-way only

  C) Outer cells are not living

  D) organic molecules

  E) end walls (sieve plates)

  F) 2-way movement

  G) Cells are living but need support

  H) No end walls between cells

  
• Xylem vessels


  Transport water (and dissolved mineral ions) from roots up through the stem and leaves of plants
• Transport in xylem vessels:

  • Dead tissue,end walls broken (forming hollow “pipe to carry H2O”)
  • Transport H2O in a continuous,unbroken column
  • Vertical cell walls are thickened with lignin to withstand tension
  • Lignin is waterproof to keep water inside
• Cohension-tension theory of water transport

  • Transpiration-water evaporates from leaf mesophyll cells & water vapour diffuses into the air via stomata
  • water is drawn from xylem vessels,to replace that lost from transpiration
  • Water enters the xylem at the roots
• • The transpiration "pull" stretches the column of water molecules, it is under tension (negative pressure)
  • Lignin withstands the tension- vessels become narrower, but walls don't collapse
  • Cohension water molecules are “held together” by many weak hydrogen bonds (giving cohesion between the water molecules)
  • Adhesion of water molecules to walls helps the column to remain unbroken & rise upwards

  
• Why does a plant need sucrose?

  • In growing areas-hydrolysed to glucose so can be used in respiration or converted to cellulose
  • In storage organs-condensed to starch for storage
  • Transports better than glucose-less reactive so less likely to be used up along the way

  
• • Living cells
  • Sieve tube cells-little cytoplasm & few organelles
  • Companion cells-carry out active transport

  
• Translocation

  1. Sucrose made after photosynthesis-glucose+fructose condense
  2. FD into companion cell
  3. AT into sieve tube cell
  4. Sucrose lowers WP
  5. water enters sieve cells from xylem
  6. Creates high HP in sieve cell near the source
  7. Mass flow to sink occurs-down a pressure gradient
  8. Fd of sucrose into companion cell
  9. AT of sucrose into sink cell-sucrose used
  10. Loss of water from sink region lowers HP in sieve tube-maintainin pressure differences

  
• Evidence for & against mass flow in phloem

  1. Using ringing experiments
  2. Using aphids
  3. Using radioactively labelled substances (CO2 or sucrose)
• Using ringing experiments

  Ringing…

  Cutting a ring of bark to cut phloem

  • Bulge forms above the cut
  • Fluid contains a higher sucrose concentration than below the cut

  
• Using aphids


  Aphids…

  Pierce the phloem with mouth parts

  • Sap comes out under pressure
  • more so at leaves than further down stem so,must be a pressure gradient

  
• Using radioactively labelled substances (CO2 or sucrose)


  Radioactive tracers…

  • Used to track the movement of molecules over time
  • Shows movement from source to sink-leaves to roots

  
• Evidence for mass flow:

  • sucrose reaches roots faster than by diffusion alone
  • pressure in phloem vessels (sap is pushed out when cut)
  • Sucrose conc is higher in leaves than root
  • Sap moves down when light but stops at night
  • Sucrose conc rises in phloem shortly after sucrose conc has risen in leaves
  • Respiratory poisons/lack of oxygen inhibits translocation
• Evidence against mass flow:

  • Sieve plates create a barrier to mass flow
  • Dissolved molecules/ions don’t all move at the same rate
  • sucrose reaches all parts of the plant at the same rate-should go more quickly to areas with lowest concentration
• Comparison of mass transport in xylem and phloem

  A) Direction is from roots to leaves up the stem

  B) Water transports dissolved ions

  C) Tension created by transpiration pull

  D) Xylem-dead tissue

  E) No ATP (active transport) required for transport

  F) Direction is from source to sink

  G) Water transports sucrose

  H) Phloem-living tissue

  I) ATP (active transport) required