(not mock) phloem and translocation

• phloem's function is purely transport --> made of sieve tube elements and companion cells
• sieve tube elements:

  • living cells that form the tube for transporting solutes
  • joined end to end to form sieve tubes
  • the end walls (sieve plate) have many holes to allow solutes to pass through
  • they have no nucleus, a very thin layer of cytoplasm and few organelles
  • the cytoplasm of adjacent cells is connected through the holes in the sieve plates
  • thin cell wall is made of cellulose
• companion cells:

  • the lack of nucleus and other organelles in sieve tube elements means they can't survive on their own --> there's a companion cell for every sieve tube element
  • companion cells carry out the living functions for both themselves and their sieve cells --> the many mitochondria provides the energy for the active transport of solutes
• method to dissect plant stems:

  1. use a razor blade to cut a cross-section of the stem --> thin so can see in microscope
  2. use tweezers to place sections in water --> to prevent drying out
  3. place sections into a dish containing a stain e.g. toluidine blue O (TBO) and leave for 1 min --> TBO stains the lignin blue-green - so can see the position of the xylem vessels and examine their structure
  4. rinse off the sections in water and mount onto a slide to view under the microscope
• adaptations of root hair cells:

  • small = can penetrate between soil particles
  • thousands on each root = large SA:V ratio
  • thin surface layer --> just cell wall and cell surface membrane = quick osmosis and diffusion
  • high solute concentration = maintains water potential gradient
• what is translocation?

  transport of assimilates e.g. sucrose and amino acids from source to sink in a plant
• translocation can occur in either direction, so is bidirectional and is an active process
• what do sources do?

  provide assimilates to the plants from reactions or storage for transport e.g. leaves for transport and food stores in seeds --> an area of high concentration of substance
• what do sinks do?

  use assimilates for various processes: --> an area of low concentration of substance

  • growing roots
  • meristem cells dividing
  • developing stores e.g. seeds, fruits or storage organs
• sucrose makes up the largest percentage of phloem sap --> 20-30%
• what role do enzymes play in sources and sinks?

  they maintain a concentration gradient from the source to the sink by changing the dissolves substances at the sink (e.g. by breaking them down) --> ensures there's always a lower concentration at the sink than at the source

  example:

  in potatoes, sucrose is converted to starch in the sink areas so there's always a lower concentration of sucrose at the sink than inside the phloem --> ensures a constant supply of new sucrose reaches the sink from the phloem
• assimilates are moved into the phloem by active processes. The 2 routes are: symplast and apoplast
• symplast route in phloem loading:

  • assimilates are stored in the vacuoles of cells
  • these are moved through the cytoplasm of mesophyll cells into the sieve tubes across connecting plasmodesmata
  • this is a largely passive process
  • the assimilates are moved by changes in the water potential of cells
• apoplast route in phloem loading:

  • assimilates diffuse through the cell wall and intermembrane spaces
  • when they reach the companion cells, they are actively transported across the membrane into sieve cells cytoplasm
  • hydrogen ions act as co-transporters and actively move assimilates across the membrane
• active loading is used to move substances into the companion cells from surrounding tissues and from companion cells into sieve tubes, against a concentration gradient --> sucrose is moved using active transport and co-transporter proteins
• phloem loading by the apoplast pathway requires energy, which is released in the form of ATP by respiration in mitochondria
• active loading:

  1. in the companion cell, ATP is used to actively transport hydrogen ions out of the cell and into surrounding tissues = concentration gradient
  2. hydrogen ions bind to co-transport proteins in the companion cell membrane and re-enters the cell
  3. a sucrose molecule binds to the co-transport protein at the same time. The movement of hydrogen ions is used to move the sucrose molecule into the cell, against the concentration gradient
  4. sucrose molecules are then transported out of the companion cells and into the sieve tubes by the same process
• mass flow in the phloem:

  at the source:

  1. sugars are actively moved into the sieve cytoplasm, this decreases the water potential
  2. water moves into the sieve cells by osmosis
  3. this increases the hydrostatic pressure inside the phloem, water moves to decrease the pressure

  at the sink:

  1. assimilates are removed from the phloem to be used up
  2. this increases the water potential, so water moves out by osmosis, decreasing the hydrostatic pressure

  this results in a pressure gradient from the source end to the sink end, this gradient pushes solutes along
• evidence for mass flow:

  1. adaptations of companion cells for active transport
  2. if the mitochondria in companion cells are poisoned, transpiration stops
  3. the flow of sugars is about 10000 times faster than diffusion alone, suggesting an active process is driving mass flow
  4. when aphids feed from the phloem it's shown that there's a positive pressure in the phloem that forces sap out through the stylet (part of aphid)
  5. ringing a tree causes a buldge of sugars to collect above the ring