mass transport in plants

• Xylem


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


  Tissue that carries organic substances such as dissolved sucrose from the source to the sink in plants
• Cohesion


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


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


  Evaporation of water from a plant surface, especially the leaves
• Cohesion-tension theory


  1. Water transpires from the leaves, reducing water potential in leaf cells

  2. This causes water to be drawn out of the xylem by osmosis, creating tension/negative pressure

  3. More water is drawn up through the xylem as water molecules are cohesive due to hydrogen bonding

  4. This allows more water to enter the roots by osmosis as water potential in root hair cells decreases
• Factors affecting rate of transpiration

  • Light intensity (higher intensity increases rate)
  • Humidity (higher humidity decreases rate)
  • Temperature (higher temperature increases rate)
  • Wind (more wind increases rate)
• Using a potometer


  1. Shoot draws up water from potometer

  2. Air bubble in capillary tube moves as water is drawn up

  3. Amount of water drawn up is measured and assumed to equal amount lost in transpiration
• Amount of water taken up by capillary tube not exactly equal to amount lost in transpiration
• Source


  Where sucrose molecules are formed, e.g. photosynthesizing leaf cell or storage organ
• Sink


  Where sucrose is needed, e.g. rest of plant especially roots
• Phloem structure


  • Sieve tube elements (living cells that form long tubes for transport of dissolved organic substances)
  • Companion cells (carry out living functions for sieve cells)
• Mass flow hypothesis


  1. Sucrose actively transported into phloem at source, lowering water potential and causing water to enter by osmosis

  2. Increased hydrostatic pressure at source end

  3. Sucrose actively transported out of phloem at sink, decreasing water potential and causing water to follow by osmosis

  4. Decreased hydrostatic pressure at sink end

  5. Pressure gradient pushes solutes from source to sink
• Evidence for mass flow hypothesis

  • Ringing experiments (sucrose accumulates above ring in phloem)
  • Radioactive tracers (movement of radioactive sucrose can be traced)
• Evidence against mass flow hypothesis
• Translocation refers to the process by which sugars are moved within the plant's phloem tissue.
• Translocation refers to the process by which sugars are moved from source to sink tissues within a plant.
• Xylem is responsible for transporting water and minerals from the roots to the rest of the plant.
• The xylem is responsible for the upward movement of water from roots to leaves.
• Phloem is responsible for transporting sugars produced during photosynthesis from the leaves to other parts of the plant.
• Xylem consists of tracheids and vessel elements, while phloem is made up of sieve tubes and companion cells.
• Water moves through transpiration pull, which occurs when water evaporates from leaf surfaces due to high temperatures or low humidity levels.
• The movement of sugars is driven by a concentration gradient, with higher concentrations of sugar in the source tissue and lower concentrations in the sink tissue.
• Water moves through transpiration pull, which occurs when water evaporates from leaf surfaces due to high temperatures or low humidity levels.
• The movement of sugars occurs through a process called transpiration stream, where water moves upwards along with dissolved sugars due to differences in water potential between different parts of the plant.
• Phloem transports organic compounds such as glucose and amino acids throughout the plant.
• Phloem transports organic compounds such as glucose, amino acids, and vitamins throughout the plant.
• Phloem transports organic compounds such as glucose and amino acids throughout the plant.
• Active transport mechanisms are involved in loading sugars into the phloem at the source, while passive diffusion occurs during unloading at the sink.
• In the transpiration stream, water enters the xylem vessels through root hair cells and travels upward towards the leaves, carrying dissolved minerals and nutrients with it.
• Water moves through transpiration stream in xylem vessels due to cohesion-tension theory.
• Sugars move through the phloem via transpiration pull, where evaporation of water from the leaves creates negative pressure that pulls solutes upwards.
• Sugars move through pressure flow mechanism in phloem sieve tubes.
• Water moves through transpiration stream in xylem vessels due to cohesion-tension theory.
• Sugars move through pressure flow mechanism in phloem sieve tubes.
• Sugars produced during photosynthesis move through the phloem to other parts of the plant where they are used for growth or storage.
• The mass flow theory proposes that transpiration creates a negative pressure gradient along the length of the phloem, driving the movement of sugars through active transport.
• Sugars move through pressure flow mechanism in phloem sieve tubes.
• The movement of substances through xylem occurs via cohesion-tension theory, where water molecules adhere together due to surface tension and are pulled upwards by negative pressure created during evapotranspiration.
• As water evaporates from the stomata on the underside of the leaf, it creates negative pressure or tension in the xylem vessels, pulling more water upwards.