9.4 translocation Flashcards
What is translocation
Movement of organic solutes in phloem from sources to sinks.
What are assimilates
Products of photosynthesis that are transported around plant
- Glucose is made during photosynthesis but the main assimilate is sucrose
Cell sap sucrose level - 0.5%
Phloem sap sucrose level: 20-30%
Examples of sinks
Where the assimilates are needed
Roots that are growing and/or actively absorbing mineral ions
Actively dividing meristems
Any part of plants that are laying down food stores such as developing seeds, fruit stores in organs
Examples of sources of assimilates
Green leaves and green stem
Storage organs (in tubers and tap roots) - they unload stores at beginning of growth period
Food stores in seeds when they germinate
Two ways of phloem loading
Passive route
Active route (uses apoplast route): Sucrose travels from source, through cell walls and inter-cell spaces to companion cells & sieve-tube-elements
- By diffusion down conc. gradient - gradient maintained by removal of sucrose from phloem vessels.
What’s a proton pump
Intrinsic protein that creates a proton (H+) gradient across a biological membrane
Process of active loading into the phloem
H+ ions actively pumped (uses ATP, ATP + H2O > ADP + Pi + energy) out of companion cell into surrounding cell (creating conc. gradient)
Co-transporter transports H+ ions and sucrose molecules into the companion cells down the concentration gradient into the sieve-tube-elements.
(Sucrose moves through plasmodesmata into STE)
Since solute conc. increases, water potential decreases, so water moves into the STE by osmosis increasing turgor pressure
- Turgor pressure moves water carrying assimilates up and down plant.
Adaptions of companion cells
Many infoldings in cell membrane to increase SA for active transport of sucrose
Contains many mitochondria to provide ATP for transport pump
Evidence for translocation
Microscopy allows us to see adaptations of companion cells
If mitochondria in companion cells are poisoned, translocation is stopped
Flow of sugars in phloem is 10,000x faster than diffusion, suggesting it is an active process
Aphids feed off of plant tissue, they insert their stylet (mouth), if aphids are anaesthetised and removed, sap continues to move out of phloem.
Process of phloem unloading
Sucrose unloaded to sink via diffusion down conc. gradient
Sucrose moves to other cells to maintain the conc. gradient between cells and phloem
Increases water potential in STE, H2O then flows into sink, or into xylem and subsequently the transpiration stream.
What’s the effect of an increase in turgor pressure
Creates pressure difference between phloem and sink, allowing rapid transportation of solutes and water up and down plant.
