Mass Transport in Plants

• Types of tissue involved in transport in plants

  1. xylem
  2. phloem
• Xylem

  Tissue that transports water and dissolved mineral ions from the roots, up the stem, and to the leaves
• Phloem

  Tissue that carries organic substance (e.g. sucrose) from the source to the sink in plants.
• Xylem structure

  • Long hollow tubes
  • Formed from dead cells, joined together
  • One-way only
  • No end walls between cells
  • Walls contain lignin for strength -> provides structural support for the plant
• Cohesion

  Water molecules are attracted to other water molecules by hydrogen bonding
• Transpiration stream

  The movement of water and dissolved mineral ions through xylem vessels from roots to the leaves
• Transpiration

  The evaporation of water from a plant's surface, especially the leaves.
• Cohesion-tension theory

  1. Water transpired from leaves, reducing water potential in leaf cells. Water is drawn out of xylem by osmosis.
  2. This creates tension (negative pressure), which draws more water up the xylem. Water molecules are cohesive due to hydrogen bonding, so move up the xylem as a continuous column.
  3. This allows more water to enter the roots by osmosis, as water potential in the root cells decreases.
• Factors affecting rate of transpiration

  1. Light intensity
  2. Humidity
  3. Temperature
  4. Wind
• Investigating rate of transpiration

  • Potometer can be used to estimate
  • Assumes uptake of water is equal to rate of transpiration
  • Not the case because:
  • Some water is used in photosynthesis and hydrolysis reactions
  • Maintains turgidity of stem
  • Some water produced by respiration
• Source

  Where sucrose molecules are formed

  • Photosynthesising leaf cell
  • Storage organ (bulbs/tubules)
• Sink

  Where sucrose is needed (the rest of the plant)
• Phloem structure

  • Sieve tube elements- elongated living cells with no nucleus and few organelles, which join end to end to form long tubes, through which dissolved organic substances can move.
  • Companion cells- carry out living functions for sieve tube elements (provide energy for active transport of solutes)
• How is the concentration gradient between the source and the sink maintained?

  Enzymes regulate the concentration of solutes at the sink by hydrolysing them.
• Mass flow hypothesis

  1. Sucrose is actively transported from the cells in the source into the phloem sieve tube elements (ATP provided by companion cells).
  2. Lowers water potential of the phloem.
  3. Water enters the phloem by osmosis, increasing the hydrostatic pressure as volume of water increases.
  4. At the sink, sucrose is actively transported out of the phloem into sink cells
  5. Water potential of sink cells decreases and water potential of phloem increases-> water follows sucrose into sink cells by osmosis or returns to xylem
  6. Lowers hydrostatic pressure at sink end
  7. Creates a pressure gradient so solutes are pushed towards the sink.
• Evidence for mass flow hypothesis: ringing experiments

  1. Ringing experiments

  • Ring of bark (contains phloem, not xylem) removed from woody stem
  • Bulge forms above ring as pressure is applied from above- sucrose accumulates in phloem
  • Conc. of sucrose above bulge > conc. of sucrose below bulge.
  • Sucrose moves down phloem
• Evidence for the mass flow hypothesis: tracing experiments

  1. Radioactive tracers

  • Plant supplied with CO2 with radioactive 14C atom.
  • Radioactive carbon incorporated into glucose when the plant photosynthesises, which is converted into sucrose.
  • Sucrose moved around plant by translocation
  • Movement can be traced by a radioactive tracer
• Evidence AGAINST mass flow hypothesis

  • Sap moves up and down
  • Sucrose moves at a standard speed which doesn't change according to concentration gradient