Translocation

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  • Created by: LivviR123
  • Created on: 08-04-18 18:01

Translocation

- transport of organic compounds in the phloem from sources to sinks. 

  • An active process
  • Requires energy to take place and substances can be transported up or down the plant 
  • Assimilates is the name for products of photosynthesis that are transported up or down the plant 

Main sources of assimilates in a plant are:

  • green leaves and green stems
  • storage organs e.g. tubers, rap roots
  • food stores in seeds when they germinate

Main sinks in a plant:

  • growing roots/ roots that are actively absorbing minerals
  • meristems that are actively dividing
  • any parts of the plant that are laying down food stores, such as developing seeds
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Phloem loading

There are two ways in which plants load assimilates into the phloem for transport- one is largely passive and the other is active. 

Sucrose is the main carbohydrate transported

The symplast route:

  • Sucrose from the source moves through the cytoplasm of the mesophyll cells and on into the sieve tubes by diffusion through plasmosdesmata (symplast route) 
  • This process is largely passive.
  • Sucrose ends up in the sieve elements and water follows by osmosis- this creates a pressure of water that moves the sucrose through the phloem by mass flow. 
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The Apoplast Route

  • Sucrose travels through the cell walls and inter-cellular spaces to the companion cells and sieve elements (apoplast route) by diffusion down a concentration gradient, maintained by removal of sucrose into the phloem vessels. 
  • In the companion cells sucrose is moved into the cytoplasm across the cell membrane actively
  • Hydrogen ions are actively pumped out of the companion cells into the surrounding tissue using ATP
  • Hydrogen ions return to the companion cell down a concentration gradient via a co-transport protein- sucrose is the molecule co-transported
  • This increases the sucrose concentration in the companion cells and in the sieve tube elements through the many plasmosdesmata between the two linked cells. 
  • CCs have many infoldings in their cell membranes to give increased surface area for the active transport of sucrose into the cell cytoplasm and many mitochondria to supply the ATP needed for the transport pumps 
  • build up of sucrose in the companion cell and sieve tube element means water also moves in by osmosis, leading to turgor pressure due to the rigid cell walls- water carrying assimilates moves into tubers of sieve elements, reducing the pressure in companion cells, and moves up or down the plant by mass flow to areas of lower pressure. 
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Phloem Unloading

Sucrose in unloaded from the phloem at any point to cells that need it- main mechanism of phloem unloading is by diffusion of sucrose form the phloem to surrounding cells. 

Sucrose rapidly moves into other cells by diffusion or is converted into another substance so that a concentration gradient of sucrose is maintained between the contents of the phloem and surrouding cells. 

Loss of solutes from phloem leads to a rise in the water potential- water moves out into the surrounding cells by osmosis- some of the water carried the solute to the sink is drawn into the transpiration stream in the xylem. 

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Evidence

  • Advances in microscopy allow us to see the adaptations of companion cells for active transport
  • If the mitochondria of the companion cells are poisoned, translocation stops
  • Flow of sugars in the phloem is a lot faster than it would be by diffusion alone, suggesting an active process is involved, driving the mass flow
  • Aphids can be used to demonstrate the translocation of organic solutes in the phloem- it has been shown that there is a psotivie pressure in the phloem that forces the sap out through the stylet- the pressure and flow rate in the phloem is lower closer to the sink than it is near the source, the concentration of sucrose in the phloem sap is also higher near the source than near the sink 

However:

  • not all solutes move at the same rate
  • what is the role of the sieve plates in the process
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