transport in plants

• Transpiration


  The evaporation of water from a plant's surface, particularly through the stomata when they open to allow the entry of carbon dioxide for photosynthesis

  
• Factors affecting rate of transpiration

  • External environmental factors
  • Internal factors related to the structure of the plant
• Light


  During daylight, stomata open to allow carbon dioxide to enter for photosynthesis. This increases the rate of transpiration, as water evaporates from the mesophyll cells and diffuses out of the leaf.
• Temperature


  An increase in temperature increases the rate of transpiration by providing the water molecules with more kinetic energy allowing them to evaporate more readily
• Humidity


  Refers to the water vapour content of the air surrounding the plant. An increase in humidity of the air increases the water potential of the air. This will lead to a decrease in the rate of transpiration, as the water potential gradient for the diffusion of water decreases.

  
• Air movement


  Air movement removes water vapour from the leaf surface increasing the water potential gradient and the rate of transpiration. In still air, water vapour builds up around the leaf decreasing the water potential gradient and the rate of transpiration.

  
• Xerophytes


  Plants that possess a range of adaptations to limit water loss via transpiration

  
• Xerophyte adaptations


  • Thick cuticle providing a long diffusion pathway
  • Hairs on leaf surface trapping a layer of still air
  • Rolling up of leaves trapping a layer of still air
  • Reduced surface area to volume ratio of leaves
  • Stomata positioned in 'epidermal pits or grooves' beneath the leaf surface

  
• Cohesion-Tension Theory


  Explains the movement of water and mineral ions from roots to leaves in the xylem

  
• Structure of Xylem


  • The xylem tissue is dead, so there are no cell contents. This leaves hollow tubes so that there is minimal resistance to the flow of water and ions.
  • The cell wall has been strengthened by lignin; this makes the cell walls more rigid and provides support.
• Cohesion-Tension Theory


  1. Solar heat energy causes the evaporation or transpiration of water from leaves

  2. Water moves from cell to cell across the leaf by osmosis down a water potential gradient

  3. Water is drawn from the xylem creating a tension ('negative pressure') in the xylem vessels effectively "pulling up" the water and dissolved ions

  4. The water column is maintained in the xylem by cohesive forces and adhesive forces

  
• Potometer


  A device used to measure the rate of transpiration, which assumes the rate of water uptake is the same as the rate of transpiration

  
• Using a potometer


  1. Measure the distance the air bubble moves

  2. Measure the time taken for the air bubble to move that distance

  3. Measure the radius of the capillary tubing

  4. Calculate the volume of water taken up using the formula πr^2d

  
• For the potometer to work correctly, there must be no air bubbles and the rubber bung must provide an air-tight seal
• The potometer measures the rate of water uptake rather than the rate of transpiration

  
• Radioactive isotopes as tracers

  Used to provide evidence that the transport of mineral ions through a plant occurs mainly in the xylem
• Radioactive isotope experiment


  • Separate xylem and phloem using a wax cylinder
  • Supply roots with radioactively labelled potassium ions
  • Measure radioactivity in xylem and phloem tissues - greater radioactivity in xylem indicates transport occurs in this tissue
• Phloem


  Transports photosynthetic products such as sucrose, amino acids and fatty acids from the source (leaves) to the sinks (growing regions, roots, storage areas)

  
• Structure of Phloem


  • Consists of living cells including sieve elements and companion cells
  • Sieve elements are joined end to end to form sieve tubes with porous sieve plates
  • Mature sieve elements do not possess a nucleus and have little cytoplasm
  • Companion cells have dense cytoplasm and many mitochondria
• Mass Flow Hypothesis


  The main hypothesis for the method of translocation in phloem, involving mass flow of organic substances down a hydrostatic or turgor pressure gradient

  
• Some evidence supports the mass flow hypothesis, such as phloem sap being released when the stem is cut indicating hydrostatic pressure, and reduced translocation with temperature decrease or respiratory inhibitors

  
• Some evidence contradicts the mass flow hypothesis, such as the specific structure of sieve tubes and plates not being required, and substances moving in opposite directions in the same sieve tube