Translocation Flashcards
What vessel transports organic substances around the plant?
The phloem vessel
Structure of the phloem vessel?
Composed of the sieve tubes: contains sieve plates and has no end walls
Next to companion cells which carry out living functions for the sieve tubes as they have few organelles, specifically respiration
Process of transporting organic substances around a plant name?
Translocation
What organic substances are transported in translocation
Amino acids and sugars
Assimilates
The organic substances (amino acids and sugars, esp sucrose) which are transported in translocation
Source
Where assimilates are produced in a plant such as in a leaf in photosynthesis
At a high concentration here
Sink
Where assimilates are used up in a plant such as storage organs or meristems in the roots and stem and leaves (needed for metabolic processes)
They are at a lower concentration here due to being broken down or converted to something else for these processes
The mass flow hypothesis
The idea of how translocation occurs in a plant to transport organic substances: movement of these from source to sink
Still contradicting evidence so is a hypothesis
Mass flow hypothesis
The theory of how assimilates in the source are transported to the sink in a plant using a hydrostatic pressure gradient
What happens at the source in mass transport?
Active transport of solutes into sieve tubes over companion cells from the source
Decreases water potential in sieve tubes near source as a result
So water moves into sieve tubes from xylem (from area or high water potential to lower one) by osmosis = high hydrostatic pressure at the source
What happens at the sink in mass flow?
The solutes are removed from sieve tubes by moving into the sink over companion cells and used up (by active transport)
Increases water potential at sink end in the sieve tubes as a result of
So water moves by osmosis down its water potential gradient from sieve tubes into the xylem
Causes lower hydrostatic pressure at sink end of sieve tubes
So as a result, how is the fluid moving from source to sink?
Down the hydrostatic pressure gradient to carry assimilates where they are needed
By MASS FLOW
Evidence against mass flow hypothesis
Could be gravity
Sieve plates
Multiple different sinks present in a plant
How do sieve plates contradict mass flow hypothesis?
Because they would be a barrier to mass flow so a lot of pressure would be needed for solutes to flow at reasonable rate
Evidence to support mass flow
Ringing experiments
Metabolic inhibitors effect
Aphids
Ringing experiments
Remove bark from a cutting thus remove phloem in a ring
Forms bulge above rung which has a high conc of sucrose due to accumulation after phloem was cut thus shows they are transported by phloem (because they can’t move into phloem below)
Decrease in water potential above = moves into cells
Metabolic inhibitors as evidence for mass flow
Stops ATP production to release energy: no more active transport of substances into the sieve tubes from source or out into sink
Using aphids as support for mass flow
Cut their body off their mouth piece when it uses it to feed on sugars from phloem
So the fluid continues to run
The fluid runs faster (higher hydrostatic pressure) at source than the sink (lower hydrostatic pressure)
Xylem + phloem =
Vascular bundle
Xylem + phloem =
Vascular bundle
Adaptations of sieve tubes
End walls (sieve plates) have pores for solution carrying assimilates to flow through
Lost nucleus and cytoplasm to allow for uninterrupted flow of solution
Is the phloem alive?
Yes because it consists of companions cells
What do companion cells do?
Control activity of sieve tube
Adaptations of companion cells
Nucleus and rough endoplasmic reticulum for synthesis of carrier proteins (for active transport of assimilates into/out of sieve)
Large number of mitochondria for more aerobic respiration to produce a lot of ATP for this active transport
