translocation

• what are assimilates?

  the products of photosynthesis
• what is the main assimilate transported in a plant?

  sucrose
• what are sources and sinks?

  • sources- where assimilates are produced
  • sinks- where assimilates are used up, eg growing parts of the plant
• describe and summarise the process of translocation/mass flow

  • phloem loading- soluble products of photosynthesis are moved into the phloem by active transport- not used in respiration so unlikely to be metabolised like glucose (phloem loading and translocation are active processes)
  • sucrose loaded into phloem- through plasmodesmata from mesophyll cells into sieve tube elements (symplast) or by active proton pumping of protons out of the phloem, which is co-transported back into the phloem with sucrose, using a co-transporter protein (apoplast pathway). Water follows by osmosis
  • build up of turgor pressure in phloem because water moves in, so water carrying assimilates move up and down the plant by mass flow, carrying said assimilates to sinks, reducing the pressure in the phloem
  • solutes are translocated either up/down the plant, depending on the source
  • 2 main ways in which assimilates are loaded- symplast and apoplast path

  
• describe the process of phloem unloading

  • sucrose is unloaded from the phloem at any point to the cells that need it
  • diffusion of sucrose from the phloem into surrounding cells
  • sucrose rapidly moves onto other cells by diffusion or is converted into another substance like glucose for respiration to maintain a concentration gradient between the phloem and the cells
  • because sucrose is moving out, the WP of the phloem increases. This means that water moves out of the phloem by osmosis to surrounding cells
  • happens at the sink
  • This process is passive
• describe the symplast and apoplast route of the phloem

  SYMPLAST-

  • sucrose from the source moves through the cytoplasm of cells and into sieve tubes by diffusion through plasmodesmata
  • this route is largely passive
  • sucrose ends up in the sieve elements and water follows by osmosis
  • creates a pressure of water that moves through the phloem by mass flow

  APOPLAST-

  • sucrose from the source travels through cell walls and intercellular spaces to companion cells and sieve tube elements by diffusion, maintained by the removal of sucrose into phloem vessels
  • in the companion cells sucrose is moved from these cell walls and intercellular spaces into the cytoplasm of the phloem
  • using a proton pump- hydrogen ions pumped out using ATP then co-transported in with sucrose, water follows by osmosis
• how are companion cells adapted to actively transport sucrose?

  • many infoldings in cell membrane to increase surface area for active transport of sucrose into the cell cytoplasm
  • many mitochondria which supply ATP needed for transport pumps