Translocation

Phloem

• living cells

  • sieve tube elements

  • companion cells

• movement of organic molecules up and down

  • source → sink

• 

Translocation (mass flow theory)

• source

  • cells that make products of photosynthesis known as assimilates

    • green leaves and stems

    • storage organs at the beginning of the season

    • seeds when they germinate

• sink

  • cells that need these molecules

    • roots

    • meristem tissue

    • developing seeds fruits or storage organs

• active process

  • needs energy

• glucose → sucrose before this is transported around the plant by translocation

  • sucrose is less likely to be metabolised during transportation compared to glucose

• 

Phloem loading

• symplast route

  • sucrose diffuses from the source cells into the sieve tubes by diffusion through the cytoplasm and plasmodesmata

  • sucrose enters the sieve tubes and water follows by osmosis

  • this creates a pressure of water that moves the sucrose through the phloem by mass flow

    • passive process

• apoplast route

  • sucrose moves from the source cells into the companion cells by diffusion through the cell walls and intercellular spaces

  • sucrose is moved into the cytoplasm of the companion cells across the cell membrane

    • active process

    • transport proteins also needed

  • the concentration of sucrose in the companion cell will then increase

  • it then moves into the sieve tubes via the plasmodesmata

  • water follows the increase in sucrose concentration in the phloem

  • this again creates a pressure of water that moves the sucrose through the phloem by mass flow

Mass flow

• hydrostatic pressure is higher in the phloem where loading has taken place

• it is lower in the source

• water moves down the hydrostatic pressure gradient from source to sink

Phloem unloading

• when the contents of the phloem reaches cells that need sucrose, sucrose diffuses from the phloem into the surrounding cells

  • sucrose then diffuses to other cells or is converted to other products eg glucose

  • this maintains a sucrose concentration gradient

Water potential in the phloem

• solute is lost so water potential increases

• water moves out of the phloem by osmosis into surrounding cells and into the transpiration stream in the xylem

  • also reduces hydrostatic pressure at the sink which increases mass flow

Evidence for mass flow

• adaptations of the companion cells for active transport can be seen under the microscope

• if the mitochondria stop working translocation stops

• flow of sugar is much faster than if it was just due to diffusion which suggests active process

• aphids can be used to show the positive pressure in the xylem