Phloem

• Translocation is the process by which products of photosynthesis are transported from a source to a sink
• Translocation is carried out by the phloem tissue
• a source is where the products of photosynthesis are made (usually leaves)
• A sink is where the product of photosynthesis are used or stored (roots, fruits, seeds etc.)
• Sucrose is the main substance that is translocated as it can be easily broken down into glucose and fructose when needed
• Phloem tissue is made up of sieve tube elements and their companion cells
• Sieve tubes are elongated living cells that have no nucleus or organelles, just a thin layer of cytoplasm around the edge of the cell
• Each sieve tube is supported by a companion cell which has the organelles including a nucleus and many mitochondria to produce ATP
• There are end walls between sieve tubes but they contain holes to let solutes through. They are known as sieve plates
• sources are nearly always the leaves as this is where the majority of photosynthesis takes place
• Some sinks will be above the sources and some will be below. This means that translocation has to be bidirectional. This is why translocation must be an active process - the solutes are pushed around under high pressure
• Sucrose is most often transported because it contains more energy and is less reactive and less likely to be oxidised and form products during transport
• At the source, a high sucrose concentration means sucrose is moved into sieve tubes by active loading. This results in a decreased water potential and means that water moves in by osmosis from the xylem
• When water moves into the sieve tubes, the volume increases and increases the turgor pressure. This results in a hydrostatic pressure gradient being created between the source and the sink and causes the solute to be pushed towards the sink
• sucrose moves out of the sieve tubes at the sink down a concentration gradient. a concentration gradient of sucrose is also created between the source and the sink as sucrose is constantly being used for respiration or converted for storage
• when sucrose moves out of the sieve tubes, there is an increased water potential and water moves back into the xylem. The reduction in volume decreases the turgor pressure
• Sucrose is actively loaded into companion cells at the source:

  • H+ ions are actively pumped out of the companion cell to source tissue, creating a concentration gradient
  • H+ diffuse back down the gradient via a cotransport protein that requires a sucrose molecule to be transported too.
  • The high sucrose concentration is the companion cells can then diffuse into the sieve tube
• When ringing a tree, tissue above the cut area swells as solutes build up. Plant above the cut remains healthy as it still gets sap and water. The plant below dies (so it eventually all dies)
• The sap removed via piercing the phloem contains organic compounds. The sap drains freely, showing that the phloem is under pressure
• 14C is a radioactive tracer that can be used to label organic compounds. Subsequent autoradiography shows movement of compounds
• All substances in the phloem are soluble in water
• It's more efficient to transport one molecule of sucrose than two molecules (glucose and fructose). Sucrose is also much less reactive than glucose
• Sieve plates are found between sieve tube elements
• Hydrostatic pressure is the pressure exerted by a fluid
• The movement of sucrose is an example of cotransport facilitated by active transport
• If substances are transported under pressure, they would be expected to travel at the same speed through the phloem. This isn't the case since sucrose travels faster than amino acids