L4 - mechanism of translocation

• Companion cells:

  -transport products of photosynthesis from cells in leaves to sieve tube elements trough plasmodesmata

  -contain many mitochondria to provide the cellular energy required for active transport
• Translocation is the transport of assimilates (sugars and other plant made chemicals) through the plant in the phloem

  -the most common transported sugar is sucrose (a disaccharide made of glucose and fructose)
• Phloem transports sucrose from a “source” to a “sink”

  -source being any region that releases sucrose

  -sink being any region that removes sucrose
• A strong sink has a high requirement for assimilates
• In source tissues, sugars are moved from photosynthetic cells and loaded into companion cells and sieve tube elements. This happens by two main ways:

  -symplast route

  -apoplast route
• The symplast route:

  -sucrose from the source moves through the mesophyll cells and into the sieve tubes by diffusion

  -this is passive

  -water moves into the sieve tubes containing the sucrose by osmosis, this sets up a hydrostatic pressure gradient that moves the sucrose though the phloem by mass flow
• Apoplast route:

  -sucrose from the source travels through the cell walls to the companion cells by diffusion

  -once in the companion cell, sucrose is actively moved into the cytoplasm
• Apoplast translocation pathway:

  -active and uses ATP

  -high $H^+$concentration gradient set up

  -via a cotransporter protein, this is facilitated diffusion

  -sucrose moves through the plasmodesmata

  
• 5 step mechanism companion cells load sucrose into phloem:

  -ATP used by companion cells to actively pump hydrogen ions out of their cytoplasm into neighbouring cells

  -diffusion gradient of hydrogen ions is set up

  -hydrogen ions diffuse back through cotransporter proteins, hydrogen ions linked with sucrose to transport into the cell

  -sucrose concentration then becomes high in the companion cell so dissolved in water and diffuse into sieve tubes through plasmodesmata

  -solution of assimilates in phloem moves up and down the phloem by mass flow
• Mass flow is caused by the raised hydrostatic pressure at the source
• Sucrose movement in the phloem (source):

  -sucrose enters sieve tube and reduces water potential inside

  -water moves in by osmosis (from xylem)

  -lots of fluid increases the hydrostatic pressure
• Sucrose movement in the phloem (sink):

  -leaves sieve tubes by diffusion (or active transport)

  -neighbouring cells use the sucrose in respiration or store it as starch, helping to maintain the sucrose concentration gradient

  -water potential is greater in the sieve tube as the sucrose has moved out, so water diffuses into neighbouring cells

  -less fluid is present so hydrostatic pressure is lowered
• Evidence for mechanism of translocation:

  -puncture experiment

  -ringing experiment

  -radioactive tracer experiment

  -aphids (greenfly) specialised mouthparts
• Puncture experiment:

  -if phloem is punctured with a hollow tube then the sap oozes out, showing there is high pressure inside the phloem

  -if xylem is punctured then air is sucked in, showing there is low pressure (tension) inside the xylem

  -this illustrates the main difference between transport in xylem and phloem; pulled up in xylem, pushed down in phloem
• Ringing experiment:

  -since the phloem vessels are outside the xylem vessels, they can be selectively removed by cutting a ring in a stem just deep enough to cut the phloem but not the xylem

  -after a week there is a swelling above the ring and reduced growth below the ring and the leaves are unaffected (early evidence sugars were transported downwards in the phloem)
• Radioactive tracer experiment:

  -radioactive isotopes can be used to trace precisely where different compounds are being transported from and to, as well as the transport rate

  -radioactivity can be traced using photographic film

  -a plant grown with one leaf exposed to radioactive $CO_2$ for a short time showed that it will be taken up by photosynthesis and then incorporated into glucose and then sucrose
• Aphids:

  -have specialised mouthparts called stylets, which are used to penetrate phloem tubes and sup out the sugary sap. if aphids are anaesthetised with carbon dioxide and cut off, the stylet remains in the phloem so pure sap can be collected through the styles for analysis

  -this is more accurate than a human with a syringe and the aphid’s enzymes ensure the stylet doesn’t get blocked