Mass Transport in plants

• Describe the function of Xylem tissue

  Transports water (and mineral ions) through the stem, up the plant to leaves of plant
• Suggest how xylem tissue is adapted for its 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 movement
• Explain the the cohesion-tension theory of water transport in the Xylem

  Leaf:

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

  2)Reducing water potential of mesophyll cells

  3)So water drawn out of xylem down a water potential gradient

  Xylem:

  4)Creating tension ('negative pressure') in xylem

  5)Hydrogen bonds result in cohesion between water molecules so water is pulled up as a continuous column

  6)Water also adheres to the walls of the xylem

  Root:

  7)Water enters roots via osmosis
• Describe how to setup a potometer

  1. Cut a shoot underwater at a slant -> prevent air entering xylem
  2. Assemble potometer with capillary tube end submerged in a beaker of water
  3. Insert shoot underwater
  4. Ensure apparatus is water/air tight
  5. Dry leaves and allow time for shoot to acclimatise
  6. Shut trap reservoir
  7. 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

  1)Record position of air bubble

  2)Record distance moved in a certain amount of time (e.g. 1 minute)

  3)Calculate volume of water uptake in a given time

  -Use radius of capillary tube to calculate cross-sectional area of water

  -Multiply this by distance moved by bubble

  3)Calculate rate of water uptake - Divide volume by time taken
• How does light intensity affect the rate of transpiration?

  Increases rate of transpiration

  • stomata open in light to let in CO2 for photosynthesis
  • Allowing more water to evaporate faster
  • Stomata close when it's dark so there is a low transpiration rate
• How does temperature affect the rate of transpiration?

  Increases rate of transpiration

  • Water molecules gain kinetic energy as temperature increases
  • So water evaporates faster
• How does wind intensity affect the 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 does humidity affect the 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 the Phloem

  Transports organic substances e.g. sucrose in plants
• Suggest how the phloem tissue is adapted for its function

  1)Sieve tube elements:

  • No nucleus/few organelles -> maximise space for/easier flow of organic substances
  • End walls between cells perforated (sieve plate)

  2)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 cell to sink cell by mass flow
• Explain the mass flow hypothesis for translocation in plants

  1)At source, sucrose is actively transported into phloem sieve tubes/cells

  2)By companion cells

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

  4)This increases hydrostatic pressure in sieve tube (at source)/creates a hydrostatic pressure gradient

  5)So mass flow occurs - movement from source to sink

  6)At sink, sucrose is removed by active transport to be used by respiring cells or stored in storage organs as starch
• Describe the use of tracer experiments to investigate transport in plants

  1)Leaf supplied with a radioactive tracer e.g. CO2 containing radioactive isotope 14C

  2)Radioactive carbon incorporated into organic substances during photosynthesis

  3)These move around the plant by translocation

  4)Movement tracked using autoradiography or a Geiger counter
• Describe the use of ringing experiments to investigate transport in plants

  1)Remove/kill phloem e.g. remove a ring of bark

  2)Bulge forms on source side of ring

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

  4)Tissues below ring die as can't get organic substances
• Suggest limitations in using a potometer to measure rate of transpiration:

  • Rate of water uptake might not be the same as rate of transpiration

  -Water used for turgidity and support

  -Water used in photosynthesis and produced in 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