Mass transport of Plants

• The xylem transports water and mineral ions from the roots to the leaves, in one direction.
• Xylem structure:

  • long, hollow tubes made up of dead cells with no end walls
  • lignin in cell walls: spiral-shaped, impermeable, provides structural support
  • pits: allow water to move sideways between vessels
• Mineral ions move into root hair cells by active transport, lowering its water potential. Water moves in by osmosis, down a potential gradient.
• Symplast pathway:

  • water moves through the cytoplasm of cortex cells
  • cytoplasm between cells linked by plasmodesmata
  • water moves down a potential gradient
• Apoplast pathway:

  • water moves along the cell walls or intercellular spaces of cortex cells
  • caused by a transpiration pull
  • blocked by a layer of suberin at the cell wall of the endodermis (called the Casparian strip), so water joins the symplast pathway
• Root pressure: active transport by endodermis pumps mineral ions into the xylem, lowering its water potential. Water moves in by osmosis, down a potential gradient.
• Cohension-tension theory:

  • evaporation of water from leaves lower its water potential
  • water is pulled up the xylem, creating tension
  • water is cohesive and stick together by hydrogen bonds
  • adhesion of water to the walls of xylem
  • forms a continuous column of water
• Evidence to support cohesion-tension theory:

  • when xylem vessels break: air is drawn in -> xylem vessel is under tension
  • when xylem vessels break: water movement stops -> air prevents cohesion between water molecules
  • tree trunk diameter reduces when transpiration is at its maximum: transpiration pull generates tension, pulling xylem walls inwards
• A potometer estimates the rate of transpiration by measuring the water uptake of a plant. The more water uptake, the quicker the rate of transpiration.
• The rate of transpiration may not be the same as the rate of water uptake

  • water used in photosynthesis
  • water used in hydrolysis
  • water used to provide turgidity and support
  • water produced during respiration
• Potometer method:

  1. Cut a shoot underwater to prevent air from entering the xylem, at a slant to increase surface area
  2. Assemble the potometer underwater
  3. Make sure the apparatus is airtight, use petroleum jelly to seal any gaps
  4. Dry the leaves, allow time for the shoot to acclimatise then shut the tap
  5. Remove the capillary tube from the beaker of water to allow one air bubble to form, then put it back into the water
  6. Record the distance moved by the bubble in a time period
  7. Repeat by opening the tap below the reservoir
• The phloem transports assimilates from sources to sinks, in both directions.
• Sources of plant: photosynthesising leaves, storage organs such as tubers

  Sinks of plant: meristems in roots/stems, storage organs
• Phloem structure:

  • sieve tube elements: living cells with no nucleus and few organelles; end walls perforated to form sieve plates; thin layer of cytoplasm reduces resistance to sap flow
  • companion cell: provides ATP needed for the active transport of solutes; linked to sieve tube elements by plasmodesmata
• Mass flow hypothesis:

  • At source: sucrose is actively transported into the phloem by companion cells, lowering the water potential of sieve tubes
  • Water from xylem moves in by osmosis, increasing hydrostatic pressure inside sieve tube elements
  • This causes a mass flow of solutes towards the sink, down a pressure gradient
  • At sink: sucrose is converted to glucose for use in respiration, or to starch for storage
  • Water moves out of sieve tube elements into the xylem, reducing pressure
• Evidence for mass flow hypothesis:

  • when plant stem is cut, sap is released: there is pressure in sieve tube elements
  • if mitochondria in companion cells is inhibited, translocation stops: active transport is involved as energy from ATP is needed
  • sucrose concentration is higher in leaves (source) than roots (sink)
  • rate of flow of sucrose in phloem is faster than diffusion alone
• Evidence against mass flow hypothesis:

  • solutes don't all move at the same speed
  • sieve plates appear to hinder mass flow by creating a barrier
  • sucrose is delivered at the same rate to all regions, instead of fastest to regions with lowest sucrose concentrations
• Ringing experiments:

  • remove a ring of bark and phloem from the stem, leaving the xylem intact
  • tissues above the ring would swell with high sucrose concentration, while tissues below the ring would die
  • this shows sucrose is transported in phloem
• Tracing experiments:

  • plants are grown in an environment containing radioactively labelled 14CO2
  • radioactive carbon is incorporated into sugars produced in photosynthesis
  • the movement of sugars by translocation are traced using autoradiography
  • areas exposed to radiation produced by 14C in sugars will appear black
  • blackened areas correspond to phloem tissue, which shows that sucrose is transported in the phloem