Mass transport in plants

  • Created by: gsemma
  • Created on: 14-02-19 09:45
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  • vascular tissues
    • vascular tissues are grouped in vascular bundles which may contain other tissues that can strengthen the bundles.
    • water and minerals are transported from the roots to the leaves of the plant in the xylem tissue.
    • Sugars produced during photosynthesis are transported around the whole plant in the phloem
    • Mass Transport in Plants
      • stem
        • non-woody plants- vascular bundles are separate and discrete.
        • woody plants- bundles form complete ring around he stem under the bark.
        • cambium- a layer of meristem found between the xylem and phleom
      • roots
        • endodermis- envelope around the vascular bundle
          • pericycle- layer underneath the endodermis
            • Medulla- all tissue in cortex (outer section of root that surrounds endodermis).
      • leaf
        • vascular bundles form the midrib and veins of a leaf ina  branching network
        • transpiration
          • transports minerals up the plant- maintains cell turgidity- evaporation for cooling- water is supplied for growth, division and photosynthesis.
          • light intensity
            • stomata can only open when light as photosynthesis can only occur when light
          • temperature
            • high temps. increase rate due to increases KE increasing evaporation from leaves- increasing rate od diffusion through stomata- decreasing water vapour potential of air making more water leave due to conc' grad.
          • Humidty
            • higher- decreases rate of transpiration due to smaller WP difference between air and leaf.
          • Air movement
            • decreases humidity therefore increases rate of transpiration.
          • Water availability
            • if there is not enough water stomata close so slows rate of transpiration.
          • Measure rate of transpiration using a potometer.
            • the distance the air bubble travels can be used to calculate the volume of water transpired per unit time.
          • evidence for transpiration
            • ringing experiment- tracer experiment- chemical tests and aphid test.
            • evidence against
              • not all solutes move at the same rate- the role of sieve plates is unclear.
      • translocation
        • phloem consists of companion cells and sieve tube elements.
          • sieve tube element- elongated tube aligned end to end. has no nucleus and very little cytoplasm. at each end the cells are separated by perforated sieve plates. five or six sided walls.
          • companion cells- large nucleus and dense cytoplasm many mitrocondria to create ATP for active transport. connected to the sieve tube element by pores called plasmodesmata.
        • 1. Sucrose is less reactive than glucose so the palisade mesophyll cells convert glucose from photosynthesis into sucrose. 
          • 2. Sucrose is actively pumped into the phloem via the compainon cell
            • 3. Hydrogen ions are actively pumped out of the companion cell. They diffuse back via co-transport protein, carrying with it a sucrose molecule.
              • 4. Water potential in phloem is lowered so water moves into the phloem (sieve tube element) from the xylem via osmosis
                • 5. Hydrostatic pressure increases so sucrose solution is forced downwards- mass flow.
          • Evidence for
            • radioactive markers of Carbon therefore sucrose- ringing a tree so it bulges when sucrose builds up- pH of companion cell is higher due to H+ ions- conc' of sucrose in source is higher than sink
          • evidence against
            • not all solutes move at same rate- sucrose is moved to parts of plant at same rate no matter that conc'- role of sieve plates is unclear.
        • active and passive process
          • sink- where carbs go
          • Source- where carbs come from
          • image
      • water transport in xylem
        • dead- lignin strengthens and waterproofs them- no lignin at bordered pits so osmosis can occur. narrow tube prevents air bubbles to maintain transpiration stream
          • Lignification kills cells.
        • Living parenchyma cells separate and support vessels.
        • root pressure- the loading of water into the xylem can push it up the stem a few metres.
          • transpiration pull- the loss of water that the leaves drags water up the xylem- water molecules are cohesive so they stick to each other.
            • capillairy action- the water also adheres t the walls and sue to the narrow nature of the xylem, the water pulls itself up
    • to identify them- plants are left in dyed water to stain the xylem
  • 2. Sucrose is actively pumped into the phloem via the compainon cell
    • 3. Hydrogen ions are actively pumped out of the companion cell. They diffuse back via co-transport protein, carrying with it a sucrose molecule.
      • 4. Water potential in phloem is lowered so water moves into the phloem (sieve tube element) from the xylem via osmosis
        • 5. Hydrostatic pressure increases so sucrose solution is forced downwards- mass flow.

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