mass transport in plants

•  The dissolved mineral ions are transported in the xylem tissue

  up the plant
•  dissolved organic compounds (sugars) are transported in the phloem tissue

  up and down the plant
• • The plant roots are responsible for the uptake of water and mineral ions and can have root hairs to increase the surface area for absorption of the substances
  • The uptake of water is a passive process and occurs by osmosis
  • The uptake of minerals can be passive or active and occurs by diffusion or active transport respectively
• • Plants must take in a constant supply of water and dissolved minerals to compensate for the continuous loss of water via transpiration in the leaves, and so that they can photosynthesise and produce proteins
  • There are two pathways that water (and the dissolved solutes) can take to move across the cortex (and molecules can change between routes at any time):
  • Apoplast (also known as apoplastic)
  • Symplast (also known as symplastic)
• Apoplast pathway

  The series of spaces running through the cellulose cell walls, dead cells, and the hollow tubes of the xylem
• Most water travels via the apoplast pathway when transpiration rates are high
• Water movement in the apoplast pathway

  • By diffusion (as it is not crossing a partially permeable membrane)
  • From cell wall to cell wall directly or through the spaces between cells
• Water movement through the apoplast pathway

  Occurs more rapidly than the symplast pathway
• Endodermis


  • Presence of a thick, waterproof, waxy band of suberin within the cell wall blocks the apoplast pathway
  • This band is called the Casparian strip and forms an impassable barrier for the water
• Water and dissolved minerals reaching the Casparian strip

  Must take the symplast pathway
• Symplast pathway

  • A smaller volume of water travels via the symplast pathway, which involves the cytoplasm and plasmodesmata, and vacuole of the cells
  • The water moves by osmosis into the cell (across the partially permeable cell surface membrane), possibly into the vacuole (through the tonoplast by osmosis) and between cells through the plasmodesmata
• • Transpiration is important to the plant in the following ways
  • It provides a means of cooling the plant via evaporative cooling
  • The transpiration stream is helpful in the uptake of mineral ions
  • The turgor pressure of the cells (due to the presence of water as it moves up the plant) provides support to leaves (enabling an increased surface area of the leaf blade) and the stem of non-woody plants
• Movement of water through a plant's xylem

  1. Evaporation of water vapour from the leaves

  2. Cohesive and adhesive properties exhibited by water molecules
• Water potential gradient


  The driving force behind the movement of water from the soil (high water potential) to the atmosphere (low water potential), via the plant's cells
• Plant water movement

  1. Taking water in at the roots

  2. Losing water via the stomata (in the leaves)

  3. Maintaining the water potential gradient between the roots and the leaves
• Transpiration

  The loss of water vapour via the stomata by diffusion
• Water vapour lost by transpiration

  Lowers the water potential in the air spaces surrounding the mesophyll cells
• Transpiration pull

  1. Water within the mesophyll cell walls evaporates into these air spaces

  2. Results in water moving through the mesophyll cell walls or out of the mesophyll cytoplasm
• Apoplast/Apoplastic pathway


  Movement of water through the cell walls of a plant
• Symplast/Symplastic pathway

  Movement of water from the cytoplasm of a cell
• Transpiration stream

  1. Pull from the water moving through the mesophyll cells results in water leaving the xylem vessels through pits (non-lignified areas)

  2. Causes water to move up the xylem vessels to replace this lost water (due to the cohesive and adhesive properties of the water)
• High rates of transpiration


  Walls of the xylem are pulled inwards by the faster flow of water
• The role of the stomata

  • Transpiration is mainly controlled by the pairs of guard cells that surround stomata (singular stoma)
  • Guard cells open the stomata when they are turgid and close the stomata when they lose water
  • When the stomata are open there is a greater rate of transpiration and of gaseous exchange
  • When the stomata close, transpiration and gaseous exchange decrease
  • As stomata allow gaseous exchange (CO2 in and O2 out) they are generally open during the day
• label the diagram

  A) waxy cuticle

  B) upper epidermis

  C) palisade mesophyll

  D) xylem

  E) spongy mesophyll

  F) vascular bundle

  G) lower epidermis

  H) guard cell

  I) stoma

  J) phloem

  
• • Transpiration is the loss of water vapour from the leaves or stem
• transpiration stream is the movement of water through the xylem tissue and mesophyll cells
• Method of using a potometer

  1. Cut a shoot underwater

  2. Place the shoot in the tube

  3. Set up the apparatus as shown in the diagram

  4. Make sure it is airtight, using vaseline to seal any gaps

  5. Dry the leaves of the shoot

  6. Remove the capillary tube from the beaker of water to allow a single air bubble to form and place the tube back into the water

  7. Set up the environmental factor you are investigating

  8. Allow the plant to adapt to the new environment for 5 minutes

  9. Record the starting location of the air bubble

  10. Leave for a set period of time

  11. Record the end location of the air bubble

  12. Change the light intensity or wind speed or level of humidity or temperature (only one - whichever factor is being investigated)

  13. Reset the bubble by opening the tap below the reservoir

  14. Repeat the experiment
• Cutting a shoot underwater


  To prevent air from entering the xylem
• Air enters the apparatus

  Readings will be affected
• Moisture on the leaves

  Will affect the rate of transpiration
• The further the bubble travels in the same time period, the faster transpiration is occurring and vice versa
• Translocation within phloem tissue

  The transport of assimilates from source to sink and requires the input of metabolic energy (ATP)
• Phloem sap


  The liquid that is being transported (found within phloem sieve tubes)
• Composition of phloem sap

  • Sugars (mainly sucrose)
  • Water
  • Amino acids
  • Hormones
  • Minerals
• Sources of assimilates


  • Green leaves and green stem (photosynthesis produces glucose which is transported as sucrose)
  • Storage organs eg. tubers and tap roots (unloading their stored substances at the beginning of a growth period)
  • Food stores in seeds (which are germinating)
• Sinks (where the assimilates are required)

  • Meristems (apical or lateral) that are actively dividing
  • Roots that are growing and / or actively absorbing mineral ions
  • Any part of the plant where the assimilates are being stored (eg. developing seeds, fruits or storage organs)