3.3.9 Translocation Flashcards
What is translocation?
translocation within phloem tissue can be defined as the transport of assimilates from source to sink and requires the input of metabolic energy (ATP)
What is the liquid that is being transported called?
The liquid that is being transported (found within phloem sieve tubes) is called phloem sap
What does the phloem sap consist of?
This phloem sap consists not only of sugars (mainly sucrose) but also of water and other dissolved substances such as amino acids, hormones and minerals
What could the source of the assimilates be?
Green leaves and green stem (photosynthesis produces glucose which is transported as sucrose, as sucrose has less of an osmotic effect than glucose)
Storage organs eg. tubers and tap roots (unloading their stored substances at the beginning of a growth period)
Food stores in seeds (which are germinating)
What could the sinks of the assimilates be?
Meristems (apical or lateral) that are actively dividing
Roots that are growing and / or actively absorbing mineral ions
Any part of the plant where the assimilates are being stored (eg. developing seeds, fruits or storage organs)
What type of process is the loading and unloading of the sucrose from the source to the phloem and from the phloem to the sink?
An active process
How is the loading or unloading of sucrose slowed down or stopped?
It can be slowed down or even stopped at high temperatures or by respiratory inhibitors
How is translocation of assimilates understood- what studies are there?
Collecting and studying the sap from plants with ‘clotting’ sap (eg. castor oil plants)
Using aphids to collect the sap – after the aphid inserts its stylet (tubular mouthpart) scientists remove the aphids head and collect the sap that continues to flow
Using radioactively labelled metabolites (eg. Carbon-14 labelled sugars) which can be traced during translocation
Advances in microscopes enabling the adaptations of companion cells to be seen
Observations about the importance of mitochondria to the process of translocation
Why are carbohydrates normally transported in plants in the form of sucrose?
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
Which pathway do the sucrose molecules that travel to the sieve tubes take?
The pathway that sucrose molecules use to travel to the sieve tubes is not fully understood yet. 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
What happens if the sucrose molecules are taking the apoplastic pathway?
If the sucrose molecules are taking the apoplastic pathway then modified companion cells (called transfer cells) pump hydrogen ions out of the cytoplasm via a proton pump and into their cell walls. This is an active process and therefore requires ATP as an energy source
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 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 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
Where does the unloading of assimilates occur?
At the sinks
How does the process of unloading sucrose happen?
Scientists believe that the unloading of sucrose is similar to the loading of sucrose, with the sucrose being actively transported out of the companion cells and then moving out of the phloem tissue via apoplastic or symplastic pathways
How is a concentration gradient maintained in the sink tissue?
To maintain a concentration gradient in the sink tissue, sucrose is converted into other molecules. This is a metabolic reaction so requires enzymes (eg. invertase which hydrolyses sucrose into glucose and fructose)
