Source to Sink Theory

[jesshika]

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When organic materials from photosynthesis are loaded into the phloem vessel from the source, why is it going against the concentration gradient?
Wouldn't there be more materials in the source than in the phloem ? so why would it require actice transport?

Also can someone please explain the osmosis part of it ...
like how the hydrostatic pressure forces the sap down the vessel.

thanxx~~ >.<" i got confusedd

[queenie]

Originally posted by jesshika
why would it require actice transport?

its like the kidney man... why does the kidney use active transport?

[t-i-m-m-y]

the sugars are dissolved into the water that comes in from adjacent xylem

hmm; heres something about models u ppl!

[~TeLEpAtHeTiC~]

The pressure-flow mechanism is a model for phloem transport now widely accepted.
The model has the following steps.
Step 1: Sugar is loaded into the phloem tube from the sugar source, e.g. the leaf (active transport)
Step 2: Water enters by osmosis due to a high solute concentration in the phloem tube. Water pressure is now raised at this end of the tube.
Step 3: At the sugar sink, where sugar is taken to be used or stored, it leaves the phloem tube. Water follows the sugar, leaving by osmosis and thus the water pressure in the tube drops.
The building up of pressure at the source end, and the reduction of pressure at the sink end, causes water to flow from source to sink. As sugar is dissolved in the water, it flows at the same rate as the water. Sieve tubes between phloem cells allow the movement of the phloem sap to continue relatively unimpeded.


essentially all source to sink invovles is when food builds up in one end, the presure builds to a point where it gets pushed through the stem, when the flow comes to a low concentration area it flows into it to fill it up and continues further. hmmm its difficlt to describe in simple terms but once you get your head around it you'll hit yourself for not getting it sooner, good luck

[t-i-m-m-y]

yep teles right on it

i couldn't be bothered writing all that

for this, a diagram is invaluable

[jesshika]

hehehe thankzzz telepathic and timmy ~~ keke

i kinda got a grasp of the concept ~~

[babydoll_]

how many times do i have to tell everyone

POST IN THE RELEVANT SUBFORUM

[t-i-m-m-y]

* i made a thread tellin' ppl off hehe and u closed it