Translocation Flashcards
(20 cards)
What is translocation
Process by which sucrose is transported from a source (leaves) to a sink (tissue that needs them/storage)
Is it passive or active
Active
Properties for phloem
Tube formed from elongated living cells called sieve tubes
No nucleus or organelles, just a thin layer of cytoplasm around the edge to maximise the space for mass flow
Each sieve tube supported by a companion cell which has organelles (including nucleus, increased mitochondria to produce ATP + help sieve tubes carry out living functions
There are end walls between cells that contain holes to let solutes pass through =sieve plates
Examples of sinks
Roots carrying out active transport of mineral ions
Any actively growing/dividing tissue - the meristem of roots and shoots
Where storage of nutrients is occurring eg seeds fruits or tubers
Mass flow hypothesis (1)
Source- releases sucros into phloem
High sucrose concentration, sucrose moved into sieve tube. by active loading. Decreasing WP so that water moves in by osmosis from xylem. Increase in volume increases turgor pressure
Mass flow hypothesis (2)
Sink removes sucrose from phloem
-Low sucrose concentration as sucrose moves out of sieve tubes down a concentration gradient as cells use it or convert to glucose/starch.
- increase WP in sieve tubes so water moves back to xylem by osmosis, reduction in volume decreases turgor pressure
Mass flow hypothesis (3)
Mass flow of solutes
- turgor pressure gradient created between source and sink causes solutes to be pushed in that direction
- concentration gradient of sucrose between source and sink created as sucrose constantly being used for respiration or converted for storage
How does water move from the xylem to the phloem and how does sucrose move out of the companion cell
Water= due to high solute conc in phloem
Sucrose= down conc gradient
How is sugar forced down the phloem
High pressure forces movement of sugar down the phloem down the pressure gradient
Where does water move at the sink
Moves in both directions out of the phloem into the xylem and the companion cell
How does sugar move from the phloem to the sink
Diffusion down the concentration gradient
What are actively pumped out of the companion cell and where too
What does it create
H+ ions using ATP
Into the cells of the source tissue
It creates a hydrogen concentration gradient across the companion cell membrane
How do hydrogen ions return to the companion cell
What does this do
Diffuse back down the gradient through transport proteins that require a sucrose molecule to be contransported
Increases concentration of sucrose in the companion cell which can then diffuse into the sieve tube
What are the two experiments scientists can use to provide evidence for mass flow hypothesis
Ringing- the removal of a ring of bark which includes the phloem but leaves the xylem intact
Tracing- these rely on the plant taking up a substance that has a radioactive label
What occurs after ringing
The section above swells due to accumulation of sugars
The section below has no sugars
Benefits of ringing
-Growers and farmers use to break flow of nutrients from leaves to shoots for a short amount of time
- reduces length of new branches
- enhances fruit growth + increases flower bud formation (due to more nutrients available to upper parts of the than the roots
How do radioactive tracers work
Plants supplied with CO2 made with carbon-14 to the leaves. Would then be used in photosynthesis to produce sugars which can then be traced as they are translocated
How can you use radioactive material in the xylem
Radioactive mineral ions eg phosphorus dissolved in water and taken up from the roots
What is auto radiography
Take images to see where radioactive sample appears
How does auto radiography work
Plant killed by freezing with liquid nitrogen or drying at 100 degrees so all water is evaporated and plant doesn’t burn
Plant placed on photographic film
Areas of radioactivity will change colour amd turn film black (xylem will not show up as no sugars)
