can someone explain the cohesion tension theory and root pressure and its exact impact on water?

  • 1 vote

I am resitting module 2 biology

Posted Fri 18th May, 2012 @ 18:53 by claire hopkins

14 Answers

  • 6 votes

Ok cool, so am I! Monday then! :D Good luck!

Ok so a water molecule actually looks like this... click here  (hope this works)

Background knowledge:-

(Water is polar due to its partial positive charge on the Hydrogen and partial negative charge on the Oxygen...

As you can see from the image, each water molecule is attracted to another molecule of water due to hydrogen bonding, which is attracted to another molecule of water, which is attracted to another molecule of water, which is attracted to another molecule of water....... AND SO ON....

So in theory I guess you can say that they bind to each other due to the positive-negative forces attracting them [study the image]. They 'stick'... This is called cohesion)

(I'm going to try and simplify this as much as I can, but please also refer to your text book for more detail - just getting the basics straight)

The cohesion-tension theory therefore is:-

  • We know that water evaporates from the leaves AS A RESULT OF A process called transpiration...
  • The water forms a continuous pathway from the mesophyll cells and down the xylem.
  • The reason why water forms this continuous pathway is due to the cohesion between water molecules. (as discussed above)


  • When water evaporates from the MESOPHYLL cells in the leaf through the holes called STOMATA...
  • MORE Water is pulled up behind the evaporating water due to the COHESION
  • Therefore: Water is constantly being pulled UP the xylem AS A RESULT OF TRANSPIRATION (evaporation of water through leaves... try picturing it in your head)
  • The Water being pulled (as a RESULT OF TRANSPIRATION) up is called the TRANSPIRATION PULL
  • This TRANSPIRATION PULL puts the xylem under TENSION (because it's forcing water up the xylem)

-------> this is why it's called the cohesion-tension theory....

I really hope this has helped you understand the Cohesion-tension theory... If not then please tell me and I'll explain a bit more if need be....


Answered Sat 19th May, 2012 @ 18:17 by Lil' A
  • 1 vote

Root pressure:

  • Mineral ions move up the xylem by ENDODERMAL CELLS by using ACTIVE TRANSPORT.
  • These minerals lower the water potential in the XYLEM.
  • Because the water potential is lower in the xyelm (knowing that osmosis occurs down a concentration gradient ie. High to low - like diffusion except we're dealing with WATER MOLECULES).....
  • Water now moves into the xylem by osmosis (due to the lower water potential in the xylem).
  • This water potential gradient occurs as a result of the ACTIVE TRANSPORT of SALTS and MINERALS into the Xylem from endodermal cells - (remember the salts and minerals ie. the solute lowers the water potential as it is no longer PURE WATER)
  • This creates a force that helps water move up the plant (through XYLEM).
  • Force = root pressure

(Hope this helps...)


Answered Sat 19th May, 2012 @ 18:34 by Lil' A
  • 1 vote

ahh thankyou again! you are wonderful :)

good luck to you too! Is yours a resit or first time? I'm banking on all the first timers not doing aswell so the grade boundaries are lower :\

my only question is-

Is the tension created just an effect, or does it actually help move with up the xylem?

Answered Sat 19th May, 2012 @ 19:51 by claire hopkins
  • 1 vote


Answered Sat 19th May, 2012 @ 22:11 by claire hopkins
  • 0 votes

no I dont :)

Answered Sat 19th May, 2012 @ 16:55 by claire hopkins
  • 0 votes

Oops... sorry about the delete, but oh well...

It's ok... it's just that if I explained it to you I'd be able to  give you the chemistry behind it as well... But do you know what a polar molecule is?


Answered Sat 19th May, 2012 @ 17:00 by Lil' A
  • 0 votes

Also, which exam board are you doing?


Answered Sat 19th May, 2012 @ 17:10 by Lil' A
  • 0 votes

AQA , and polar.. is that to do with the cohesive properties of water?

Answered Sat 19th May, 2012 @ 17:29 by claire hopkins
  • 0 votes

No worries at all...

You're welcome :)

And thaankkkss... :D

I'm in my first year, so yep first time... I hope it goes well... pretty nervous...

Hmmm, good question...

Ok so: There are 2 main reasons for the movement of water up the xylem and they are:-

  • root pressure
  • cohesion-tension

So I guess it's both but:-

  1. Know what root pressure is
  2. Know what cohesion-tension is

The above two help the water move up the xylem.

I'd say they are both: due to water potential(Osmosis - root pressure) and cohesion of water molecules (cohesion-tension)...

So: it is an effect which make it move up the xylem... (Does that make sense? :S)

Just practice 478 billion past-papers (not literally lol), memorize the mark scheme and practice many questions.



Answered Sat 19th May, 2012 @ 20:09 by Lil' A
  • 0 votes

I just did a quiz you produced...

There was this question:-

  1. Which description is wrong

    The correct answer is: Adenine pairs with thymine by 3 bonds

    • Cytosine and thymine are pyrimadines
    • Adenine and Thymine are purines
    • Cysotine pairs with guanine by 3 bonds
    • Adenine pairs with thymine by 3 bonds
    • adenine and thymine are joined by a hydrogen bond
  2. I picked the one in bold, because:-

A and T aren't purines.

A & G = Purines (double rings)

T & C = Pyramidines (single rings)

Look at this:-

                      A [2-H2 bonds] T

                      G [3-H2 bonds] C

   A & G = Purine                 T & C = Pyramidines

Try picture that arrangement in your head and it should make the differentiation easier.

To remember the number of H bonds in this arrangement think of it as ascending downwards from 2-3...


Answered Sat 19th May, 2012 @ 20:33 by Lil' A
  • 0 votes

oh im so sorry it definetly sdhould have said guanine!

I best change that!

I wish I had resat in January, I did the paper and got full UMS :(

Yeah I got a list of frequently asked questions from someone on here, helpful :)

I need as close to full marks as possible because im doing module 5 soon, and Im worried im not going to get an A which I need for uni!


Answered Sat 19th May, 2012 @ 20:40 by claire hopkins
  • 0 votes

It's fine, just thought I'd let you know in case that was what you genuinely thought it was.


Awww, don't worry, I'm sure you'll do well.

Just don't stress, stress is a real killer. Keep calm and don't let nerves get to you.


Are the frequently asked questions on a topic thread on this website?


Answered Sat 19th May, 2012 @ 21:40 by Lil' A
  • 0 votes

Yes they are.. I'll have a look for them so i can give you the link :)

Answered Sat 19th May, 2012 @ 22:10 by claire hopkins
  • 0 votes

Thank you ever so much, Claire! That's fantastic.

You're awesome!



Answered Sat 19th May, 2012 @ 23:48 by Lil' A