3.4.2.2 Phloem And Translocation Flashcards
What is the phloem and what does it do
Transports sucrose and amino acids
Living therefore has no lignin
Transport away from source (site of photosynthesis) towards all other parts of plants where they’ll be used or stored (sinks)
Describe the structure of phloem tissue
Companion cell, cell wall, cell membrane, cytoplasm, vacuole, nucleus, sieve tube cell
What is the structure and function of the sieve tube element in phloem tissue
Stacked Elongated cells to form a series of tubes
Perforated end walls - sieve plates - allowing cytoplasmic connections
No nucleus/organelles
Function = transport of sucrose and amino acids
Describe the structure and function of the companion cells in phloem tissue
Directly next to every sieve tube element
Large nucleus and have all their organelles
Plasmodesmata in cell wall allows movement of substances between companion cell and sieve tube element
Function = synthesis of ATP and proteins to sieve tube elements, loading/unloading of sucrose from sieve tube element
What is function of fibre and parenchyma in phloem tissue
Fibre provides mechanical support for tissue
Parenchyma is Packing cell between vessel elements
Name the process by which organic materials are transported around the plant and what hypothesis is used to explain this
Translocation
Via Münch’s mass flow hypothesis
What does Münch’s mass flow hypothesis suggest
A passive flow of sucrose from source to sink, moving down a hydrostatic pressure gradient
How does sucrose in leaf load into phloem
Hydrogen ions are actively pumped out of companion cells by ATP via carrier protein into source tissue creating a hydrogen ion gradient
Passive diffusion of hydrogen ions back into companion cells through a symport down conc grad, also taking sucrose in with it - indirect active transport as sucrose is moving against its own gradient
Sucrose builds up in companion cell creating sucrose conc grad
Sucrose diffuses passively into sieve tube element down grad
How do phloem vessels transport sucrose around the plant according to mass flow hypothesis
As sucrose moves into tube elements, water potential inside phloem decreases creating water potential gradient, water moves from xylem into phloem down this gradient, creating hydrostatic pressure in phloem.
Sucrose moves down hydrostatic pressure gradient as there is lower hs pressure at bottom of phloem.
Sucrose moves into sink cell passively via f diffusion, decreasing hydrostatic pressure and water potential is higher in phloem here than in xylem.
Active transport of mineral ions and osmosis of water up xylem vessel. Water moves down water potential gradient from phloem to xylem, water moves up xylem vessel due to cohesion tension theory.
Repeat cycle
Give evidence against Münch’s mass flow hypothesis
Sucrose and amino acids translocate at different rates in same tissue (in Mmfh they’d move all together)
Sucrose and amino acids translocate in different directions (in Mmfh, wouldn’t be possible)
Companion cells contain high number of mitochondria - so translocation is not passive
Phloem tissue has high oxygen consumption - so translocation is not passive
Rs inhibitors added to phloem tissue results in reduced rate of translocation so not passive
How can ringing experiments be used to investigate transport in plants
Bark and phloem of tree are removed in a ring, leaving behind xylem
Eventually, tissues above missing ring sweeps due to accumulation of sucrose as the tissue below ring begins to die, therefore sucrose must be transported in the phloem
How can tracing experiments be used to investigate transport in plants
1 leaf exposed to radioactive CO2
Carbon is taken up by plant during photosynthesis
After varying exposure times, plant is frozen in liquid N, killing it and stopping and reactions from occurring so all molecules stay in same place
Autoradiograph is taken to show location of any compounds containing the carbon
This experiment shows translocation is bidirectional, as carbon moves upwards and downwards
How can aphids be used to investigate translocation
They have specialised mouth parts called stylets - can pierce stem and get sap from phloem. Aphids produce an enzyme which stops coagulation of sap (sap dries quickly so would get stuck in needle and syringe).
They are put on different points of stem, killed whilst eating, stylets stay in stem to be used as sampling tubes to take sap out stem
