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root pressure and cohesion tension

Mr power

How do these occur? Mainly root pressure which I'm stuck on.

Reply 1

rahmed

Transpiration causes water to be drawn up the xylem vessels...

Reply 2

Prometheus

Root pressure is the idea that because the roots take in ions by active transport, water is absorbed by osmosis due do the change in water potential (the roots are more "salty" [DONT EVER USE THAT TERM IN AN EXAM] because of all the ions they've absorbed). This means that water is constantly being drawn in. This water moves with the ions (to keep a constant osmotic potential) and moves up the plants stalk to wherever the ions are going, once they are used up the water moves by osmosis to anywhere in the plant that has a low osmotic potential.

Evidence to support this is the fact that if you make a cut low in a plant stalk water comes out, but continues to come out (i.e. its not just a release of pressure, but something is actively pushing water up from the roots).

The water itself moves by cohesion because of hydrogen bonding which makes it stick to itself an means that it can form water columns (like when you draw water up in a straw). Because of this water can be "sucked up" a plant stalk.

A lot of water is also drawn by the transpiration stream (as mentioned above) which is where water evaporates from the stomata in the leaves, so water moves from the roots up to the leaves (a high osmotic potential to a low osmotic potential). This then means there is less water in roots, so water is drawn into the roots by osmosis.

Reply 3

Mr power

OP

i get you to a certain detail but if you could put the necessary points in order for each it would be helpful. Thanks for help so far...

Reply 4

Stranded

11

potter

i get you to a certain detail but if you could put the necessary points in order for each it would be helpful. Thanks for help so far...

It is in order :s:

Reply 5

Mr power

OP

i meant like number 1,2,3 and 4 :smile:

Reply 6

moleo

1. Active transport of salts into xylem;
2. Endodermis / Casparian strip;
3. Prevents leakage / water / ions must use symplast pathway;
4. Lower water potential inside xylem;
5. Water (enters xylem) down WP gradient / by osmosis;
6. Upward water movement by root pressure is relatively low;
From a markscheme

Reply 7

Mr power

OP

ok I think ive got it, does it go like this:
1. Water enters the xylem in the roots by osmosis
2. Once in the xylem the water molecules have hyrodgen bonding with each other forming a continous string of H20 Molecules up the leaft
3. water is lost at the top of the xylem by transpiration
4. When 1 water molecule is lost another is pulled along due to tension

Is this right? :confused:

Reply 8

iceman_jondoe

18

Heres a better version: :biggrin:

1. Water enters the root by osmosis through the root hair cells.
2. Water cross the root cortex via the apoplast and symplast pathways until it encounters a barrier to the apoplast pathway (due to the casparian strip) so it travels via the symplast pathway into the xylem vessels.
3. Tanspiration causes water vapour to be lost from the leaf, this creates a low pressure at the top of the stem where the leaves are and a high pressure at the bottom. IN addition you should note that the uptake of ions also helps to create a hydrostatic pressure which allows more water to be "pushed" up the xylem vessel.
4. Water travels down the pressure gradient up the stem. Water forms a continuous column due to the attractions between water molecule dipoles. THis attraction is called cohesion. Water is also attracted to the thick lignified walls of the xylem vessel elements, this is known as adhesion.

So in essence water flows up the stem from the roots to the leaves due to transpiration. When water vapour is lost from the leaves (specifically from the spongy mesophyll layer). It "pulls" up the column of water behind it.

Reply 9

Mr power

OP

thanks for that, thats cohesion tension right so what about root pressure dude?

Reply 10

yukinko

1

potter

thanks for that, thats cohesion tension right so what about root pressure dude?

iceman_jondoe

Heres a better version: :biggrin:

3. Tanspiration causes water vapour to be lost from the leaf, this creates a low pressure at the top of the stem where the leaves are and a high pressure at the bottom. IN addition you should note that the uptake of ions also helps to create a hydrostatic pressure which allows more water to be "pushed" up the xylem vessel.
4. Water travels down the pressure gradient up the stem. Water forms a continuous column due to the attractions between water molecule dipoles. THis attraction is called cohesion. Water is also attracted to the thick lignified walls of the xylem vessel elements, this is known as adhesion.

.

^ water goes from a high pressure(the roots) to a low pressure (leaf)

Reply 11

tomtheone

2

Heres a better option, DONT DO BIOLOGY!!!

Reply 12

DartsOfPleasure

10

Instead of saying high pressure, could you say high water potential?

Reply 13

7589200

18

stick to hydrostatic pressures...

Reply 14

Mr power

OP

came up on the exam and guess wat.... I NAILED IT!!EASY MARKS

Reply 15

stratomaster

1. Secretion of ions into xylem
2. by active transport
3. from endodermal cells
4. water movement into xylem down water potential gradient
5. by osmosis
6. Casparian strip prevents return of water

Reply 16

Mr power

OP

y u posting now strato la

Reply 17

Tim Street

Prometheus

Evidence to support this is the fact that if you make a cut low in a plant stalk water comes out, but continues to come out (i.e. its not just a release of pressure, but something is actively pushing water up from the roots).

This is wrong. Infact, if you cut a plant stalk very little water comes out, proving that, although the root does provide a slight amount of pressure, water is pulled up a plant..by the cohesion tension hypothesis.

root pressure and cohesion tension

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