transport in plants

• what do xylem vessels transport

  Water and mineral ions
• in what direction do xylem vessels transport water and ions

  from roots to leaves
• what is the structure of a xylem vessel

  Hollow and tube-like with no end walls.
• what does the lignin coating the xylem walls do

  - strengthens and thickens the wall - prevents xylem collapsing under low pressure

  - also provides waterproofing to reduce water loss
• why doe the cell walls of the xylem contain pits


  allow liquids to move sideways , in and out between vessels as required
• how does the transpiration steam work

  1. water evaporates from leaves

  2. lowers water potential in mesophyll cells in leaf

  3. this creates tension which pulls more water into the leaf by osmosis

  4. water molecules are cohesive so the whole collumn of water in the xylem moves upwards

  5. adhesion to xylem walls helps water move in collumn

  6. water enters stem through roots (entered roots via osmosis)
• what is transpiration

  evaporation of water from plant leaves
• how does light affect transpiration

  Light affects transpiration by opening the stomata; the more intense the light the more it widens, this allows more water vapour to diffuse out of the stomata.
• how does temperature affect transpiration

  The warmer it is, the faster the rate

  The hotter it gets, the more kinetic energy the water molecules gain so evaporate faster
• how does humidity affect transpiration

  The drier the air around a leaf, the faster the rate

  Humid air contains lots of water, so the lower the humidity, the faster diffusion will take place
• how does wind affect transpiration

  Transpiration is faster in windy conditions

  Water vapour is removed quickly by air movement, increasing the concentration gradient
• potometer

  Apparatus used to measure water uptake in a leafy shoot and so to estimate rate of transpiration.
• how to use a potometer
• what do phloem vessels transport

  sugars - mainly sucrose
• what is another name for these solutes

  assimilates
• in what direction does the phloem move substances

  both directions - to wherever the sugars are needed

  from source to sink
• what are the two types of cell in the phloem

  sieve tube elements and companion cells
• what are sieve tube elements

  they are living cells that form the tube for transporting solutes, they have no nucleus and few organelles
• what are companion cells

  cells that surround sieve tube elements and carry out living functions e.g. provide energy needed for active transport of solutes
• where is the source for sucrose usually

  the leaves - made by photosynthesis
• where is the sink for sucrose usually

  other parts of plant e.g. food storage organs and meristems (roots shoots fruits)
• how is it ensured there is always a lower concentration of sucrose at the sink

  enzymes will convert the sucrose to starch in sink areas- constant concentration gradient
• explain the mass flow hypothesis

  1. sucrose enters companion cells through active loading

  (companion cells use ATP to transport H+ ions into the surrounding tissue, creates a diffusion gradient, causes the H+ ions to diffuse back into the companion cells)

  2. as the H+ ions diffuse (facilitated) back into companion cells they attach to a sucrose molecule - increasing the concentration of sucrose molecules in the companion cells

  3. sucrose diffuses down concentration gradient into sieve tube elements through plasmodesmata

  4.water potential in phloem tube decreases so water moves into phloem from xylem by osmosis - increases hydrostatic pressure in sieve tube element

  5. water moves down sieve tube element from high HSP to low HSP

  6. sucrose moves into sink cell by diffusion or active transport
• how does removing a ring of bark support mass flow hypothesis

  the ring of bark contains phloem vessels but not xylem vessels

  when removed a bulge of fluid with high sugar concentration forms above the ring- showing downward movement of sugars

  below the ring the roots die as they don't have any sugars

  - must be phloem vessels that transport sugars
• how can radioactive tracers prove mass flow hypothesis

  radioactive C14 that has become radioactive 14CO2 enters plant and is used in photosynthesis

  take cross section of plant, place on X-ray film- the sugars produced by the 14CO2 will appear blackened- these regions correspond with where phloem is so phloem must be responsible for translocation

  - image produces is an autoradiograph
• how are aphids used to prove mass flow hypothesis

  mouthpart penetrate phloem

  their bodies are removed leaving the mouth part

  sap flows out

  sap flows out quicker near leaves than further down stem - evidence of pressure gradient
• how can a metabolic inhibitor show how translocation works

  will stop at production - translocation stops so evidence active transport is involved
• what are two objections to mass flow

  1. model suggests sugar travels to many different sinks, not just one with highest water potential

  2. sieve plates would create barrier to mass flow - a lot of pressure would be needed for the solutes to get through at a reasonable rate