Translocation Flashcards
definition of translocation
movement of dissolved substances (eg sugars like sucrose, and amino acids), to where they’re required in the plant
What are ‘assimilates’?
dissolved substances
In which vessel does translocation occur in?
Does it require energy?
occurs in phloem and requires energy
What is the term for the area that substances are moved from?
the ‘source’
What happens in the ‘source’?
Give an example
-where a substance is made and is in high concentration
-eg. leaves are source for sucrose (photosynthesis)
What is the term for the area that substances are moved to?
the ‘sink’
What happens in the ‘sink’? Give an example of a sink
-where a substance is used up and is in low concentration
-eg other parts of plant such as food storage organs and meristems
How is the concentration gradient ensured?
-enzymes change/break down dissolved substances at the sink to ensure there is always a lower sucrose concentration there
-always a concentration gradient between sink and source
What enzyme breaks down sucrose in potatoes?
starch
Which parts of a plant can act as both sinks and sources?
roots and leaves
What is active loading?
process used to move substances from surrounding tissues, to companion cells, into sieve tubes, all against a concentration gradient
What process and biological feature are used in active loading?
-active transport
-co transporter proteins
Why are co transporter proteins and active transport needed?
there’s usually a higher sucrose concentration in companion cells, and even higher in sieve tube cells
What is ATP and why is it necessary in active loading?
-one of the products of respiration
-breakdown of ATP supplies initial energy needed for active transport of H+ ions
1st step in active loading
in the companion cell, ATP is used to actively transport H+ ions out of the cell and into surrounding tissue
2nd step in active loading
this means there are more H+ ions in surrounding tissue than in companion cells, which creates concentration gradient
3rd step in active loading
H+ ions re-enter cell, down a concentration gradient, once they’ve bound to a co-transport protein which in turn is bound to a sucrose molecule, in the companion cell membrane
What is the result of H+ ions re-entering the cell, down a concentration gradient?
sucrose molecules are moved into the cell, against the concentration gradient
4th step in active loading
sucrose molecules transported from companion cells, into sieve tubes by same process
What is the mass flow hypothesis?
process of how translocation happens
What happens in the source end of the phloem?
-solutes actively loaded into sieve tubes via active transport
-water potential is therefore lowered so water also moves into sieve tubes via osmosis, from xylem and companion cells
What is the pressure and water potential like in the source end?
-high pressure created in sieve tubes
-low water potential
What happens in the sink end of the phloem?
-solutes removed from phloem at sink end, to be used up
-water potential inside sieve tubes increases, so water leaves via osmosis
What is the pressure and water potential like in the sink end?
-pressure is lowered inside sieve tubes
-high water potential
What is the pressure gradient in the mass flow hypothesis, and why is it necessary?
-gradient from source end to sink end created
-gradient pushes solutes along sieve tubes to where they’re needed
