Transport In plants

• Translocation

  1. Movement of solutes (e.g. sugars like sucrose, and amino acids) to where they're needed in a plant

  2. An energy-requiring process that happens in the phloem
• Mass Flow Hypothesis


  • Best explains phloem transport
  • Active transport is used to load solutes from companion cells into the sieve tubes of the phloem at the source
  • Translocation moves solutes from 'sources' to 'sinks'
  • Enzymes maintain a concentration gradient from the source to the sink by changing the solutes at the sink
• Phloem


  • It's all about moving dissolved organic substances around the plant
  • Phloem tissue is adapted for transporting solutes, mainly sugars like sucrose, round plants
  • Phloem is formed from cells arranged in tubes
  • Sieve tube elements and companion cells are important cell types in phloem tissue
  • Sieve tube elements are living cells that form the tube for transporting solutes and have no nucleus and few organelles
  • Companion cells provide the energy needed for the active transport of solutes in sieve tube elements
• Translocation

  • In potatoes, sucrose is converted to starch in the sink areas, ensuring a constant supply of new sucrose reaches the sink from the phloem
• The mass flow hypothesis proposes a different theory for translocation
• Enzymes maintain a concentration gradient from the source to the sink


  Changing the solutes at the sink (e.g. by breaking them down or making them into something else) ensures a lower concentration at the sink than at the source
• If a metabolic inhibitor is applied, this is evidence for the translocation process
• The higher the concentration of sucrose at the source, the higher the rate of translocation
• Experiments have shown that some sucrose is transported through the cell walls of the phloem
• Phloem transport process

  1. Lowers the water potential inside the sieve tubes, so water enters the tubes by osmosis from the xylem and companion cells

  2. Creates a high pressure inside the sieve tubes at the source end of the phloem

  3. Results in a pressure gradient from the source end to the sink end, pushing solutes along the sieve tubes towards the sink

  4. Increases the water potential inside the sieve tubes at the sink end, causing water to leave the tubes by osmosis

  5. Lowers the pressure inside the sieve tubes

  6. Solutes are removed from the phloem at the sink end to be used up
• Sugar travels to the highest water potential