9.4 Translocation Flashcards
How is glucose transported around the plant?
Converted to sucrose for transport
What happens to glucose produced during photosynthesis?
Used for respiration Converted into starch for storage Add nitrate ions to form proteins Form cellulose cell walls Phosphorous is added to form fats Produces chlorophyll
What are sources in plants?
Regions of a plant that produce assimilates by photosynthesis or from storage materials (eg leaves, storage organs)
What are sinks in plants?
Regions of a plant that require assimilates to supply their metabolic needs (eg - roots and fruits)
What is translocation?
The movement of organic solutes around a plant in the phloem
Main sources of assimilates in a plant
Green leaves and green stems
Storage organs such as tubers and tap roots that are unloading stores at the beginning of a growth period
Food stores in seeds when they germinate
Main sinks in a plant
Roots that are growing or actively absorbing mineral ions
Meristems tht are actively dividing
Parts of a plant that are laying down food stores, like developing seeds, fruits or storage organs
Why is sucrose the main carbohydrate transported?
It is not used in metabolism as readily as glucose, so it is less likely to be metabolised during the transport process
2 main ways for loading assimilates into the phloem for transport
Apoplast route - active process
Symplast route - mostly passive process
Symplast route for phloem loading
Sucrose from the source moves through the cytoplasm of mesophyll cells and into sieve tubes by diffusion through the plasmodesmata
Sucrose ends uo in the sieve elements and water follows by osmosis
This creates a pressure of water which moves sucrose through the phloem by mass flow
The apoplast route for loading phloem
In the companion cells, sucrose is moved into the cytoplasm across the cell membrane in an active process
H+ ions are actively pumped out of the companion cell into the surrounding tissue using ATP
Hydrogen ions return to the companion cell down a concentration gradient via a cotransport protein
Sucrose is the molecule that is cotransported
This increases sucrose concentration in the companion cells and in the sieve elements through the many plasmodesmata between the 2 linked cells
How are companion cells adapted?
Many infoldings in the cell membranes to give an increased surface area for active transport of sucrose into the cell cytoplasm
Many mitochondria to supply the ATP needed for transport pumps
Water movement due to build up of sucrose in companion cells and sieve tube elements
Build up of sucrose in companion cells means water moves in (area of low WP)
This increases turgor pressure due to rigid cell walls
Water carrying assimilates moves into tubes of the sieve elements, reducing pressure in companion cells and moves up or down the plant by mass flow to areas of lower pressure (the sinks)
How do pressure differences cause translocation?
Solute accumulation in source phloem leads to an increase in turgor pressure that forces sap to regions of lower pressure in the sinks
This transports solutes and water rapidly around the plant
Unloading of phloem
Sucrose is unloaded from the phloem when cells require it
Diffusion of sucrose from the phloem to surrounding cells
Sucrose rapidly moves on into other cells by diffusion or is converted into another substance so that a concentration gradient is maintained between phloem and surrounding cells
What happens to water potential of phloem during unloading?
Loss of solutes from the phloem leads to a rise in the water potential of the phloem
Water moves out into the surrounding cells by osmosis
Some of the water that carries the solute to the sink is drawn into the transpiration stream in the xylem
Evidence for translocation
Microscopy allows us to see adaptations of companion cells for active transport
If mitochondria of companion cells are poisoned, translocation stops
Flow of sugars in phloem is much faster than just diffusion alone, suggesting an active process is driving the mass flow
Using aphids, it has been shown that there is a positive pressure in the phloem that forces out sap through the stylet
Pressure and flow rate in phloem is lower closer to the sink than the source and the concentration of sucrose in the phloem is also higher closer to the source than the sink
Questions about translocation
Not all solutes in the phloem move at the same rate
Sucrose always moves at the same rate regardless of concentration at the sink
The role of sieve plates in the process remains mostly unknown
