3.3.4.2 Plants

• Mass transport 


  Movement of large volumes of fluid carrying dissolved molecules/ions at the same rate and moving in the same direction

  Caused by a pressure gradient
• Xylem


  Transports water and dissolved ions up the stem from roots to leaves
• Phloem


  Transports sucrose and amino acids from source to sink
• source 


  site of production of sucrose (leaves)
• Sink


  site of use of sucrose (growing/storage areas)
• Why does a plant need water?

  Fills vacuole, makes cells turgid so the plant is supported

  Reactant in photosynthesis

  Hydrolysis reactions

  Transport medium - ions from roots to leaves

  Solvent
• Xylem vessels structure

  • Dead tissue, end walls broken down
  • Hollow pipes to carry water in a continuous, unbroken column
  • Vertical cell walls are thickened with lignin to withstand tension
  • Lignin is waterproof so will keep water inside
• Cohesion-tension theory - Transpiration

  • Water evaporates from the mesophyll cells and diffuses into the air via stomata
  • Water is drawn from xylem vessels to replace what is lost from transpiration - water enters the xylem at the roots
  • Water molecules are held together by many weak hydrogen bonds giving cohesion between the molecules
  • Transpiration stretches the column of water molecules putting it under pressure which the lignin can withstand
  • Adhesion of water molecules helps the column remain unbroken and rise upwards
• Factors affecting rate of transpiration:

  • Air currents
  • Temperature
  • Light intensity
  • Air humidity
• Why does a plant need sucrose?

  In growing areas - hydrolysed to glucose so can be used in respiration or converted into cellulose

  In storage organs - condensed to starch for storage

  Transports better than glucose as it is less reactive so less likely to be used up along the way
• Phloem vessel structure:

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

  1. Sucrose made after photosynthesis - glucose and fructose condense
  2. Facilitated diffusion into companion cell
  3. Active transport into sieve tube cell lowering the water potential
  4. Water enters sieve cell from xylem - creates a high hydrostatic pressure in the sieve cell near the source
  5. Mass flow to sink occurs down a pressure gradient
  6. Facilitated diffusion into companion cell
  7. Active transport into sink cell - sucrose used
  8. Loss of water from sink region lowers hydrostatic pressure in sieve tube - maintaining pressure difference

  
• Experimental evidence for phloem transport:

  Ringing - cutting a ring of bark out of a tree will form a bulge above the cut, this shows that the fluid contains a higher sucrose concentration above than below the cut

  Aphids - pierce the phloem with mouthparts, sap will come out under pressure more so at leaves than further down the stem. Shows that there is a pressure gradient

  Radioactive tracers - used to track the movement of molecules over time, showing the movement from source to sink
• Evidence for mass transport:

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

  • 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 the lowest concentrations