3.4.2 mass transport in plants

• function of xylem tissue

  transports water (and mineral ions) through the stem, up the plant to leaves of plants
• how xylem tissue is adapted for it's function

  cells joined with no end walls forming a long continuous tube → water flows as a continuous column

  cells contain no cytoplasm / nucleus → easier water flow / no obstructions

  thick cell walls with lignin → provides support / withstand tension / prevents water loss

  pits in side walls → allow lateral water movements
• explain the cohesion-tension theory of water transport in the xylem

  water lost from leaf by transpiration - water evaporates from mesophyll cells into air spaces and water vapour diffuses through (open) stomata

  reducing water potential of mesophyll cells

  so water drawn out of xylem down a water potential gradient

  creating tension (‘negative pressure’ or ‘pull’) in xylem

  hydrogen bonds result in cohesion between water molecules (stick

  together) so water is pulled up as a continuous column

  water also adheres (sticks to) to walls of xylem

  water enters roots via osmosis
• describe how to set up a potometer

  cut a shoot underwater at a slant → prevent air entering xylem

  assemble potometer with capillary tube end submerged in a beaker of water

  insert shoot underwater

  ensure apparatus is watertight / airtight

  dry leaves and allow time for shoot to acclimatise

  shut tap to reservoir

  form an air bubble - quickly remove end of capillary tube from water
• describe how a potometer can be used to measure the rate of transpiration

  record position of air bubble

  record distance moved in a certain amount of time (eg. 1 minute)

  calculate volume of water uptake in a given time ->

  -> use radius of capillary tube to calculate cross-sectional area of water (πr2)

  -> multiply this by distance moved by bubble

  calculate rate of water uptake - divide volume by time taken
• potometer

  potometer estimates transpiration rate by measuring water uptake
• describe how a potometer can be used to investigate the effect of a named environmental variable on the rate of transpiration

  carry out the above, change one variable at a time (wind, humidity, light or temperature)

  eg set up a fan or spray water in a plastic bag and wrap around the plant or change distance of a light source or change temperature of room

  keep all other variables constant
• suggest limitations in using a potometer to measure rate of transpiration

  rate of water uptake might not be same as rate of transpiration

  water used for support / turgidity

  water used in photosynthesis and produced during respiration

  rate of movement through shoot in potometer may not be same as rate of movement through shoot of whole plant

  shoot in potometer has no roots whereas a plant does

  xylem cells very narrow
• how light intensity affects rate of transpiration

  increases rate of transpiration

  stomata open in light to let in carbon dioxide for photosynthesis

  allowing more water to evaporate faster

  stomata close when it’s dark so there is a low transpiration rate
• how temperature affects rate of transpiration

  increases rate of transpiration

  water molecules gain kinetic energy as temperature increases

  so water evaporates faster
• how wind intensity affects rate of transpiration

  increases rate of transpiration

  wind blows away water molecules from around stomata

  decreasing water potential of air around stomata

  increasing water potential gradient so water evaporates faster
• how humidity affects rate of transpiration

  decreases rate of transpiration

  more water in air so it has a higher water potential

  decreasing water potential gradient from leaf to air

  water evaporates slower
• describe the function of phloem tissue

  transports organic substances eg. sucrose in plants
• suggest how phloem tissue is adapted for its function

  sieve tube elements

  no nucleus / few organelles → maximise space for / easier flow of organic substances

  end walls between cells perforated (sieve plate)

  companion cells

  many mitochondria → high rate of respiration to make ATP for active transport of solutes
• what is translocation 


  movement of assimilates / solutes such as sucrose

  from source cells (where made, eg. leaves) to sink cells (where used / stored, eg roots) by mass flow
• the mass flow hypothesis for translocation in plants

  at source, sucrose is actively transported into phloem sieve tubes / cells

  by companion cells

  this lowers water potential in sieve tubes so water enters (from xylem) by osmosis

  this increases hydrostatic pressure in sieve tubes (at source) / creates a hydrostatic pressure gradient

  so mass flow occurs - movement from source to sink

  at sink, sucrose is removed by active transport to be used by respiring cells or stored in storage organs
• describe the use of tracer experiments to investigate transport in plants

  leaf supplied with a radioactive tracer eg carbon dioxide containing radioactive isotope 14C

  radioactive carbon incorporated into organic substances during photosynthesis

  these move around plant by translocation

  movement tracked using autoradiography or a Geiger counter
• describe the use of ringing experiments to investigate transport in plants

  remove / kill phloem eg. remove a ring of bark

  bulge forms on source side of ring

  fluid from bulge has higher conc. of sugars than below - shows sugar is transported in phloem

  tissues below ring die as cannot get organic substances