Biology transport in plants 3.1.3

• Mod


  Xylem and Phloem
• Multicellular Plants Need Transport Systems

  • Plants need substances like water, minerals and sugars to live
  • Plants need to get rid of waste substances
  • Plants are small cells with a small surface area to volume ratio
  • Plants are relatively big with a relatively high metabolic rate
  • Exchanging substances by direct diffusion on the outer surface to the cell would be too slow to meet the metabolic need
• Plants need transport systems to move substances to and from individual cells
• Xylem


  Transports water and mineral ions in solution, moving up the plant to the leaves
• Phloem


  Mainly transports sugars in solution both up and down the plant
• Xylem and phloem make up a plant's vascular system, found throughout a plant
• Location of xylem and phloem in different plant parts

  • In roots, xylem is in the centre surrounded by phloem to provide support
  • In stems, xylem and phloem are near the outside to provide scaffolding that reduces bending
  • In leaves, xylem and phloem make up a network of veins which support the thin leaves
• Xylem Vessels


  • Very long tube-like structures formed from cells (vessel elements) joined end to end
  • No end walls, allowing uninterrupted tube for water to pass up
  • Cells are dead, containing no cytoplasm
  • Walls thickened with lignin to support vessels and prevent collapse
  • Amount of lignin increases as cell gets older
  • Water and ions move in/out through small pits in walls where there's no lignin
• Phloem Tissue


  • Transports solutes (dissolved substances, mainly sugars) around the plant
  • Contains phloem fibres, phloem parenchyma, sieve tube elements and companion cells
  • Sieve tube elements and companion cells are most important for transport
• Sieve Tube Elements


  • Living cells that form the tube for transporting solutes
  • Joined end to end to form sieve tubes
  • Have 'sieve' end walls with holes to allow solutes to pass through
  • Unusually for living cells, have no nucleus, very thin cytoplasm and few organelles
  • Cytoplasm of adjacent cells connected through holes in sieve plates
• Companion Cells


  • Provide living functions for sieve tube elements, e.g. energy for active solute transport
• Active transport of solutes requires energy
• Dissecting plant stems


  1. Cut thin cross-section or longitudinal section using scalpel/razor

  2. Place sections in water to prevent drying out

  3. Stain with toluidine blue O to highlight xylem vessels

  4. Rinse and mount on slide
• Multicellular plants need transport systems to move substances to and from individual cells
• Lignin


  Woody substance that thickens the walls of xylem vessels
• Water enters a plant through its root hair cells
• Water movement into and through the root

  1. Water moves down a water potential gradient from soil (high) to root (low)

  2. Water can take the symplast pathway (through cell cytoplasm) or apoplast pathway (through cell walls)

  3. At the endodermis, the Casparian strip blocks the apoplast pathway, forcing water to go through cell membranes
• Water evaporates from leaves


  Creates tension (suction) that pulls more water into the leaf
• Water molecules are cohesive


  When some are pulled into the leaf, others follow, moving the whole column of water upwards
• Water molecules are adhesive


  Attracted to walls of xylem vessels, helping water rise up
• Transpiration is the loss of water from a plant's surface, especially the leaves
• Transpiration is a consequence of gas exchange needed for photosynthesis
• Factors affecting transpiration rate

  • Light (higher light = higher rate)
  • Temperature (higher temp = higher rate)
  • Humidity (lower humidity = higher rate)
  • Wind (higher wind = higher rate)
• Transpiration is the evaporation of water from a plant's surface, especially the leaves
• Transpiration is a result of gas exchange needed for photosynthesis
• Four Main Factors Affect Transpiration Rate

  • Temperature
  • Humidity
  • Wind
  • Light
• As light intensity increases

  The transpiration rate increases
• When it's dark


  The stomata are usually closed, so there's little transpiration
• When it's light


  The stomata open, allowing CO2 to diffuse into the leaf for photosynthesis
• Higher temperature


  Increases the transpiration rate
• Higher humidity


  Decreases the transpiration rate
• Stronger wind


  Increases the transpiration rate
• Potometer


  A special piece of apparatus used to estimate transpiration rates by measuring water uptake by a plant
• Using a potometer to estimate transpiration rate

  1. Cut a shoot underwater

  2. Assemble the potometer in water and insert the shoot

  3. Remove the apparatus from the water but keep the end of the capillary tube submerged

  4. Check the apparatus is watertight

  5. Dry the leaves, allow time for the shoot to acclimatise and then shut the tap

  6. Remove the end of the capillary tube from the beaker of water until one air bubble has formed, then put the end of the tube back into the water

  7. Record the starting position of the air bubble

  8. Start a stopwatch and record the distance moved by the bubble per unit time

  9. Only change one variable at a time
• Xerophytic plants


  • Adapted to live in dry climates
  • Have stomata sunk in pits to shelter from wind
  • Have a layer of hairs on the epidermis to trap moist air
  • Can roll their leaves to trap moist air and reduce exposed surface area
  • Have a thick, waxy layer on the epidermis to reduce water loss by evaporation
  • Have spines instead of leaves to reduce surface area for water loss
  • Close their stomata at the hottest times of day
• Hydrophytic plants


  • Have air spaces in their tissues to help them float and store oxygen
  • Have stomata only on the upper surface of floating leaves
  • Have flexible leaves and stems to avoid damage from water currents
• Translocation


  The movement of dissolved substances, such as sugars and amino acids, to where they are needed in a plant
• Source


  The area where a substance is made and at a high concentration
• Sink


  The area where a substance is used up and at a lower concentration
• The mass flow hypothesis for phloem transport

  1. Active transport is used to load solutes into the sieve tubes at the source

  2. This lowers the water potential inside the sieve tubes, so water enters by osmosis

  3. This creates high pressure at the source end of the phloem

  4. At the sink end, solutes are removed from the phloem, increasing the water potential and lowering the pressure

  5. The pressure gradient pushes the solutes along the sieve tubes to where they are needed