firefly
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hello all you biologically informed people!

Having a bit of a problem understanding the cohesion-tension theory in plants.
If anyone could explain this to me I would be very very grateful!

Thanx
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Zwitter Ion
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The cohesion-tension theory suggests that water is obtained in plants from soil due to the fact that water is cohesive and so is able to be drawn up from soil particles into the root hairs via osmosis. Water is then able to move through to neighbouring cortex cells by osmosis until it reaches the xylem. The reason water is able to travel up the xylem is due to the tension created. When water leaves the plant due to transpiration water is drawn up from the root to keep a constant supply in the plant at all times.
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kitty123
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Isn't there something about adhesion of water molecules to the xylem vessels as well which helps?
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Zwitter Ion
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Yes thats to do with the fact that water molecules are polar so their adesive which enables them to withdraw water and have a constant supply
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Rosie18
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Water is "pulled" up the xylem by the water lost through transpiration. The colum of water doesn't break because of the cohesive forces between the water molecules. Hydrogen bonds between indiviual water molecules is the force of attraction. The pulling action of transpiration stretches the water column in the xylem so that it is under tension.

So yeah, thats just a simpler version of what Zwitter Ion said really, but typing it helps my revision too!
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Holsy
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(Original post by Rosie18)
Water is "pulled" up the xylem by the water lost through transpiration. The colum of water doesn't break because of the cohesive forces between the water molecules. Hydrogen bonds between indiviual water molecules is the force of attraction. The pulling action of transpiration stretches the water column in the xylem so that it is under tension.

So yeah, thats just a simpler version of what Zwitter Ion said really, but typing it helps my revision too!
Yep, I think that's a great explanation. The adhesive and cohesive forces also help the water move in a continuous column of water called the transpiration stream because of the difference in hydrostatic pressure.
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Rosie18
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(Original post by Holsy)
The adhesive and cohesive forces also help the water move in a continuous column of water called the transpiration stream because of the difference in hydrostatic pressure.
Thanks, I'll add that sentence to my notes!
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Holsy
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(Original post by Rosie18)
Thanks, I'll add that sentence to my notes!
You're welcome.
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Rosie18
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While we're on this topic could anyone please explain to me the evidence for the "Root Pressure" theory of transpiration? I only seem to have the limitations in my notes and it came up in a mock exam.
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shungun
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is it cohesion-tension

i learnt it as cohesion-adhesion...:rolleyes:
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dancinbb
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(Original post by Rosie18)
While we're on this topic could anyone please explain to me the evidence for the "Root Pressure" theory of transpiration? I only seem to have the limitations in my notes and it came up in a mock exam.
from my knowledge XD
mineral ions in the soil are actively pumped into the roots, therefore decrease the water potential in the root
water will move in by osmosis, increasing hydrostatic pressure and this pressure causes water to move upwards to leaves,etc.
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Rosie18
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(Original post by dancinbb)
mineral ions in the soil are actively pumped into the roots, therefore decrease the water potential in the root
water will move in by osmosis, increasing hydrostatic pressure and this pressure causes water to move upwards to leaves,etc.
Surely that's the theory behind Root pressure, not the evidence supporting it?
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Skipper
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(Original post by Rosie18)
Surely that's the theory behind Root pressure, not the evidence supporting it?
Isn;t it something along the lines of when you cut the root, sap seeps out because of the pressure? It might not be right as I did my exams last year...
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Rosie18
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(Original post by Skipper)
Isn;t it something along the lines of when you cut the root, sap seeps out because of the pressure? It might not be right as I did my exams last year...
Ah that sounds more like it. Thank you
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Skipper
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(Original post by Rosie18)
Ah that sounds more like it. Thank you
You're welcome
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hobbs82
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I am still a bit unsure. What evidence supports the cohesion theory?
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AdamStott64
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(Original post by hobbs82)
I am still a bit unsure. What evidence supports the cohesion theory?
Lol I'm 4 years late, you've probably got a job and a husband/wife by now. If you're still interested evidence is that if you 'puncture' a tree water doesn't leak out due to the tension, sort of sucking the water in.
also the diameter of the tree decreases in hot weather and fluctuates between day and night (more evaporation of water therefore more transpiration)

Sorry for the delay but I was 13 when you asked.
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tara009tara
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funny thing is that you're 21 now
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Curious_boy
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#19
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Xylem vessels are non living, they work just like any capillary, the adhesion force is between the walls of capillary and water molecule and the cohesion is between water molecules. Further the surface tension helps in transport of water.
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eden3
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#20
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wow, 9 year old thread!
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