Module 3.1.3- Transport in plants

• The need for plant transport systems


  metabolic demands

  increasing size

  surface area : volume ratio
• Dicotyledenous plants (dicots)


  plants that produce seeds containing two cotyledons, which act as food stores for the developing embryo and form the first leaves when the seed germinates.
• Vascular system


  collection of specialized tissues in some plants that transports mineral nutrients up from the roots and brings sugars down from the leaves
• Vascular bundle


  plant stem structure that contains xylem and phloem tissue

  
• Stem of a young herbaceous plant


  vascular bundles are around the edge to give strength and support

  
• Root of a young herbaceous plant


  vascular bundles are in the middle to help the plant withstand the tugging strains that result as the stems and leaves are blown in the wind

  
• Dicot leaf


  vascular bundle is in the midrib

  helps to support structure of the leaf

  
• Function of xylem


  transport water and minerals from roots, up the plant, to the leaves and support
• Structure of xylem


  long continuous hollow tube (no resistance to water flow)narrow lumen wall made out of lignin

  lignin: strong, waterproof, adhesive

  wall contains pits/pores (water and minerals can leave)

  
• Lignin


  substance in vascular plants that makes cell walls rigid

  so xylem vessels do not collapse under the transpiration pull

  
• Xylem parenchyma


  Thick walled tissue that packs around the xylem vessels, storing food and containing tannin deposits - a bitter chemical that deters Herbivore attacks.
• Xylem fibres


  long cells with lignified secondary walls that provide extra mechanical strength but do not transport water

  Lignin can be laid in rings, spirals or relatively solid tubes.
• Function of the phloem


  transport organic material (e.g. Sucrose) up and down a plant
• Structure of the phloem

  companion cells (Many mitochondria for ATP production for active loading)sieve tube elements (Little cell contents for ease of flow)sieve plates formed by cells being perforated(connect the elements together to allow ease of flow)

  
• Importance of water in plants


  turgor pressure

  turgor drives cell expansion

  loss of water by evaporation keeps plants cool

  substances transported in aqueous solutions

  water is a raw material for photosynthesis
• Root hair cell adaptations


  Large surface area to volume ratio

  microscopic size means they can penetrate easily between soil particles

  concentration of solutes in the cytoplasm of root hair cells maintains a water potential gradient between the soil water and the cell

  
• Symplast pathway


  The route taken by water through the cytoplasm of cells in a plant.

  
• Apoplast pathway


  The route taken by water between the cells or through the cell walls in a plant

  goes around the casparian strip and joins symplast pathway

  
• Casparian strip


  A band of waxy material called suberin that runs around each of the endodermal cells forming a waterproof layer forcing water in apoplast pathway to join the symplast pathway
• How do endodermal cells create a lower water potential to increase rate of osmosis into the xylem


  move mineral ions into the xylem by active transport to decrease water potential
• Root pressure


  The upward push of water within the stele of vascular plants, caused by active pumping of minerals into the xylem by root cells
• Pathway of water transport


  soil

  root hair cell

  root cortex cell

  endodermal cell (with casparian strip)

  xylem vessel

  
• Evidence for the role of active transport in root pressure


  poisons affecting mitochondria cause root pressure to disappear

  root pressure increases with rise in temperature suggesting chemical reactions

  if levels of oxygen etc. fall, root pressure falls

  xylem sap may exude from the cut end of stems at certain times
• Stomata


  Small openings on the underside of a leaf where gaseous exchange can occur. can be opened and closed by guard cells
• Transpiration


  loss of water vapour from plant leaves by evaporation of water at the surfaces of the mesophyll cells followed by diffusion of water vapour through the stomata
• Transpiration stream


  the movement of water through a plant from the roots until it is lost by evaporation from the leaves.
• Movement of water across a leaf


  loss of water vapour from mesophyll cell lowers water potential of cell so water moves into cell from an adjacent cell by osmosis and this is repeated across the leaf to the xylem

  
• Adhesion


  An attraction between molecules of different substances
• Cohesion


  Attraction between molecules of the same substance
• Combined effects of cohesion and adhesion results in water...


  ...exhibiting capillary action where water rises up a narrow tube against the force of gravity in a continuous stream to replace water lost by evaporation. This is the transpiration pull.

  
• Evidence for the cohesion-tension theory


  Change in diameter of trees. during the day tree diameter decreases since tension is at its highest (transpiration is highest) and during the night diameter increases (vice versa)

  broken xylem vessels take up air rather than letting water outbroken xylem vessels can't move water because the continuous stream has broken

  
• Stomata- controlling the rate of transpiration


  turgor-driven process

  inner wall of the guard cell is less flexible than the outer wall, the cells become bean-shaped and open the pore

  turgor loss closes the stomatal pore conserving water

  
• Factors affecting transpiration- light


  increasing light intensity increases number of open stomata, increasing rate of water vapour diffusing out therefore transpiration rate increases
• Factors affecting transpiration- humidity


  a very high relative humidity will lower the rate of transpiration because of the reduced water vapour potential gradient
• Factors affecting transpiration- temperature


  increasing temp: increases KE of water molecules and therefore rate of evaporation

  increases concentration of water vapour that external air can hold before it becomes saturated decreasing humidity
• Factors affecting transpiration- air movement


  air movement or wind increases the rate of transpiration since it increases the diffusion gradient
• Factors affecting transpiration- soil-water availability


  if the soil is very dry, the plant will be under water stress and the rate of transpiration will be reduced
• Translocation


  when plants transport organic compounds in the phloem from sources to sinks
• Assimilates


  the products of photosynthesis that are transported around a plant, e.g., sucrose
• Source in plants


  site where photosynthesis primarily takes place to produce plant food in the form of sucrose