The Sucrose Loading Mechanism
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Carbohydrates are generally transported in plants in the form of sucrose because
It allows for efficient energy transfer and increased energy storage (sucrose is a disaccharide and therefore contains more energy) It is less reactive than glucose as it is a non-reducing sugar and therefore no intermediate reactions occur as it is being transported
Loading of assimilates (eg. sucrose) The molecules may move by the:
symplastic pathway (through the cytoplasm and plasmodesmata) which is a passive process as the sucrose molecules move by diffusion apoplastic pathway (through the cell walls) which is an active process
If the sucrose molecules are taking the apoplastic pathway then modified companion cells (called ………..) pump hydrogen ions out of the cytoplasm via a proton pump and into their cell walls. This is an ………..process and therefore requires ATP as an energy source
transfer cells active
The large concentration of hydrogen ions in the cell wall of the companion cell results in the
hydrogen ions moving down the concentration gradient back to the cytoplasm of the companion cell
The hydrogen ions move through a ………………protein. While transporting the hydrogen ions this protein also carries….
cotransporter sucrose molecules into the companion cell against the concentration gradient for sucrose
The hydrogen ions move through a cotransporter protein. While transporting the hydrogen ions this protein also carries sucrose molecules into the companion cell against the concentration gradient for sucrose The sucrose molecules then move INTO VIA ………
into the sieve tubes via the plasmodesmata from the companion cells
Companion cells have infoldings in their cell surface membrane to increase
the available surface area for the active transport of solutes and many mitochondria to provide the energy for the proton pump
This mechanism permits some plants to build up the sucrose in the phloem to up to three times the concentration of that in the
mesophyll
A high concentration of sucrose decreases the water potential in the which causes
phloem and water enters by osmosis
phloem and water enters by osmosis The entry of water results in a ………….(pressure gradient) which enables………..
high pressure the mass flow of sugars towards sink tissues
At the sink tissues, sugars are
unloaded