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Ascent of Water through Xylem

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Ascent of Water through Xylem

Postby srr427 » Thu Apr 20, 2006 9:15 pm

help please!

all of the following are factors contributing to the ascent of water through the xylem in plants except

- transpiration
- low water potential
- cohesion of water to the vessel walls
- adhesion of water to the vessel walls
- sources and sinks

I'm thinking it sources and sinks, but I have no clue

thanks
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Postby mith » Thu Apr 20, 2006 9:52 pm

sources and sinks sounds like something from the pressure flow model
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I remember that from my plant phys classes

Postby HELISA » Fri Apr 21, 2006 12:44 am

Ha! There is no such thing as cohesion of water to the vessel walls :-D
Cohesion refers to the tendency of water molecules to hydrogen bond to each other not to the vessel walls!
All the best!

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Postby AstusAleator » Fri Apr 21, 2006 3:20 am

Sources and Sinks is a term used pertaining to Island Biogeography and metapopulations, but I suppose it's general enough it could be used for other things. Quite a simple idea really... some areas have a net increase in a resource or product, others have a net loss, therefore that resource/product will move along that gradient.
With water, the source-sink model is probably a synonym for transpiration, in which the leaves are constantly losing water, and must be re-supplied by the roots which have (ideally) access to more water than they can use.

Anyhow, I agree with Helisa, your answer is probably cohesion. But from now on, DO YOUR OWN HOMEWORK. At least you made a guess. That's better than most people, who just ask the question and expect us to give the answer.
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Postby mindlessmusings » Wed Apr 26, 2006 11:35 pm

source and sinks refers to the flow of sugars down the phloem. sugar is transported through active transport from the source(leaves) into the phloem and then water follows by diffusion down the gradient. the water and sugar (in the form of sucrose) travel down the phloem by bulk flow and when it reaches the sink (roots in this case), the sugars are actively transported out and converted into starch. the water then also flows out down the gradient.

In the winter, when plants are doing little to no photosynthesis, the roots become the source and the stems become the sink.

Now, for water transport in the xylem. Transpiration in the leaves literaly pulls at the water. This pulling creates a negative pressure and puts tension onto the water. Since the water has cohesion, it all moves together and pulls at each next molecule as it goes up and out of the stomata. Root pressure and capillary action contribute a little to the transport of water, but it's mainly transpiration and the above which is called the Cohesion-Tension theory.
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Postby Linn » Thu Apr 27, 2006 2:07 pm

cohesion of water to the vessel walls

Is called the Cohesion-tension theory (Stephen Hales)

and is what is used to explain the pulling force from evaporation of water from stems and leaves. This is by the cohesion of water molecules in vessels and tracheids and the tension of the water columns from transpiration.
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Postby MrMistery » Thu Apr 27, 2006 8:28 pm

well, if you ask me it's neither cohesion of water to the vessel walls nor sources and sinks.
Cohesion, from what i know refers to the forces between water molecules themselves, it doesn't involve vessel walls.
Sources and sinks refers to the transport of xylem, as already mentioned.
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Postby Linn » Fri Apr 28, 2006 1:34 am

Water molecules adhere to the walls of vessels and xylem tracheids , and create tension when they cohere to each other.

The sources and sinks applies to substance movement in phloem.
Food solution is moved through along concentration gradients between sources and sinks.
so that is the most likely answer.>sourses and sinks :)
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Postby MrMistery » Fri Apr 28, 2006 5:24 pm

If this is a single answer question, then the answer is for sure like linn said. If you want to prove you know biology, than say they are both wrong and explain ;)
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Postby Linn » Fri Apr 28, 2006 9:24 pm

MrMistery wrote:If this is a single answer question, then the answer is for sure like linn said. If you want to prove you know biology, than say they are both wrong and explain ;)


?????muy?????? :?
I cant imagine why you would say that :?

only one is incorrect , as stated in the question.
so obviously its sinks and sources.
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Postby Linn » Fri Apr 28, 2006 9:43 pm

AH ha!!
I just googled sinks and sources and now I understand the confusion, as there are many differnet definitions for it in other sciences.
In botany however the description I gave would apply here to this problem, having to do with phloem and food solution.
Hope that clears it up. :) Actually there was not so many botanical descriptions, the first page talks about in peaches. :?

http://aolsearch.aol.com/aol/search?enc ... r&ie=UTF-8

If you want to know more about the botany description let me know.
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