Translocation Flashcards
What is translocation?
The movement of sucrose around the plant
Why is sucrose the main carbohydrate transported?
It is not used up in metabolism
How is sucrose moved?
From storage areas around the plant so can also travel up trunk
What are sources?
Sources of sucrose
What are sinks?
Where the sucrose goes
What are examples of sources and sinks?
Green leaves and stem - growing roots, fruits and seeds, storage organs - meristems that are actively dividing and food stores in seeds when they germinate - shoots and roots of germinating seeds and meristems
What is phloem loading?
How sucrose gets from the source into the phloem
How is phloem loaded?
Active transport or passively by diffusion
How is the phloem loaded by active transport?
From the apoplast
How is the phloem loaded by passive diffusion?
From the symplast
What is the symplast route for sucrose?
Sucrose moves from the source and into the sieve tube elements by diffusion through plasmodesmata
How does phloem loading in symplast affect water?
Water follows by osmosis as water potential goes down in sieve tube
How is sucrose moved along in the symplast route?
Hydrostatic pressure builds up and moves the sucrose along by mass flow as water moves from and area of high to low pressure
How is sucrose moved in the apoplast route?
- Sucrose is loaded into phloem side tube by an active process.
- Companion cells have proton pump which use ATP to actively transport H+ ions out of cytoplasm into surrounding tissue
- H+ diffusion gradient is set up.
- H+ ions diffuse back into companion cells via cotransporter proteins in the cell surface membrane, bringing sucrose across with them.
- Sucrose builds up, then diffuses across into sieve tube elements through plasmodesmata
What is the structure of companion cells?
They have many infoldings in cell membrane and many mitochondria
What is the function of many infoldings in cell membrane?
Increase the SA for proton pumps, cotransporter proteins and for sucrose loading into the phloem sieve tube
What is the function of many mitochondria?
Provide ATP for active transport for the proton pups
What is sucrose unloading?
Sucrose molecules diffuse from phloem sieve tube into active cells then is converted into starch or glucose
What is the effect of sucrose unloading on sucrose?
Sucrose concentration in the cells go down and sucrose concentration gradient is maintained
What is the effect of sucrose unloading on water?
Water potential in phloem sieve tube goes up and water follows the sucrose into cells by osmosis so hydrostatic pressure in phloem at sink is reduced
What is the hydrostatic pressure gradient?
It is a gradient set up between the sources and sink which produces a flow of water from source to sink
What is the effect of mass flow?
Sucrose is carried along
What is the direction of mass flow?
Either up or down plant, according to where sugar is needed
How do water molecules move as a result of mass flow?
Some of the water molecules are released from the sieve tube elements at the sink and joins the transpiration stream in the xylem
What is the impact of water molecules joining the transpiration stream?
It helps keep the movement in the phloem going
