Transport in plants

• Transport systems

  Plants need them to obtain water, sugars, and get rid of waste
• Plants can't obtain these things by simple diffusion because they are multicellular and have a large surface area: volume ratio
• Vascular bundles

  Contain vessels that transport water, sugars, and provide structural support
• Components of vascular bundles

  • Xylem vessels
  • Phloem vessels
  • Sclerenchyma fibres
• Xylem vessels

  • Made of dead, hollow cells with no end cell walls
  • Cell walls contain lignin for strength and support
• Phloem

  Transports dissolved substances like sucrose and amino acids from sources to sinks
• Phloem vessels

  • Made of sieve tube elements and companion cells
  • Sieve tube elements are living cells joined end-to-end with sieve plates
  • Companion cells contain a nucleus and mitochondria to provide energy
• Dissecting plant stems to view vascular bundles

  1. Cut thin section

  2. Place in water

  3. Stain with toluidine blue O

  4. Rinse and mount on slide
• Water movement into roots

  By osmosis from soil into root hair cells, then through root cortex and endodermis to xylem
• Pathways for water to reach xylem

  • Symplast pathway (cell-to-cell through cytoplasm)
  • Apoplast pathway (through cell walls)
• Casparian strip

  Waxy strip in cell walls that forces water to go through cell membranes
• Movement of water through xylem

  Driven by tension (transpiration) and cohesion of water molecules
• Transpiration


  Loss of water vapour through stomata in leaves
• Factors affecting transpiration rate

  • Light intensity
  • Temperature
  • Wind
  • Humidity
• Measuring transpiration rate using a potometer

  1. Cut shoot underwater

  2. Insert into potometer

  3. Acclimatise

  4. Measure air bubble movement in capillary tube
• Xerophytes

  • Waxy epidermis
  • Sunken stomata
  • Hairs
  • Spines
  • Rolled leaves
  • Stomatal closure
• Hydrophytes


  • Stomata on upper leaf surface
  • Air spaces
  • Flexible leaves and stems
• Translocation


  Movement of dissolved substances like sucrose and amino acids from sources to sinks
• Mass flow hypothesis for translocation in phloem


  1. Sucrose moves into sieve tube elements

  2. Reduces water potential

  3. Water moves in by osmosis, increasing pressure

  4. Pressure gradient drives mass flow to sinks

  5. Solutes removed at sinks, maintaining pressure gradient
• Active loading of sucrose into phloem


  1. Companion cells actively transport H+ out

  2. H+ move back in through co-transporter, bringing sucrose with them