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Water Movement Within a Plant Water Movement Within a Plant 

 AP BiologyUnit 5

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Review: Properties of Water 

• Water is a polar molecule

• Water molecules can form a“column”

 – due to hydrogen bonds between them

• Cohesion Water isattracted to other watermolecules

• !dhesion Water isattracted to other polar

molecules

OH

H

δ− δ−

δ+

δ+

δ+

δ+

"ydrogen bond

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Question…

• Why do plants wilt#

• When there is enoughwater it flows into

 plant cells cells are plump with water$turgid%

•&f there isn't enoughwater it flows out ofthe cell cells are limp$flaccid%

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Water Potential

• Pure ",) has + -

• Water potential is a combination

of the pressure from solutes

$+s%and the physical pressure$+ p%.

• /he more solutes there are in a

solution0 the more negative thewater potential will be.

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Water Potential

• Water always movesfrom areas of higherwater potential $less

solutes% to lowerwater potential $moresolutes%.

• (oves from lessnegative + to morenegative +.

No net

water

movement

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Pathways of Water

• !poplast Pathway water travels between

cells $outside of them%

• 1ymplast Pathway water travels through

cells $inside them to get from cell to cell%

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Water (ovement in Roots

• Water and ions are ableto freely diffuse up to a

 point in the root cells

• /o get into the stele$where the vasculartissue is located%0 thewater and ions must pass

through the cells of theendodermis $symplast

 pathway%.

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Casparian 1trip

• 2loc3s the water from

crossing the endodermis

through apoplast

 pathway

• control what enters the

vascular tissue

• Prevents water and

solutes from “lea3ing”out into the soil

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Water /ransport in Plant

• Water is transportedthrough the plant in the

4ylem

• Cells that ma3e up the4ylem:

 – /racheids– found in all

 plants – 5essel 6lements – found in

some plants

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7ormation of 4ylem

8. Cell dies.

,. Cell contents disintegrate.

9. Water can move through

these hollow cells with

little resistance.

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Capillary !ction

• movement of ",) up a very narrow tube

• oes account for a little water movement

up the 4ylem0 but not much $about ;- cm%

&mage obtained without permission from http:

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/ranspiration Pull

• !ccounts for most of thewater movement up the4ylem.

• !s ",) evaporates from thestomata $transpiration%0water from the 4ylem movesinto the leaf to ta3e its place.

• 2ecause of the hydrogen bonds between watermolecules0 water is “pulled”

up the 4ylem.

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/ension>Cohesion>6vaporation (odel

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Question…

• When you receive a bou*uet of flowers0

why is it important to cut them under water#

• /ranspiration is still continuing0 so if you

cut it under water0 it will ensure that water$not air% gets into the 4ylem.

• !ir bubble in 4ylem would disrupt the

column of water.

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/ranspiration Rate

• 7actors that affect transpiration

rate include: light0 humidity0

temperature.

• /he plant might also close itsstomata to limit transpiration

• We'll loo3 at this in more detail

in the upcoming dry lab.

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1tomata and ?uard Cells

• 1toma is ?ree3 for @mouth@ $plural

stomata%

• 7unction of 1toma to allow gas e4changewith the outside environment $C), and ),%

• ?uard cells regulate the opening of the

stomata

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1tomatal )pening and Closing

• When guard cells arefull of water $plump% stretch away from

each other stoma isopen

• When guard cells don't

have much water inthem $limp% don't pull away from eachother stoma is

closed

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Role of A B

• A B ions $potassium% controlthe opening of stoma by

changing the water content

in the guard cells. – K + in : ",- follows by

osmosis stoma open

 – K + out: ",- follows by

osmosis stoma closed

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Question…

• What conditions favor stomatal opening#

 – Plentiful water 

 – (oderate temperatures

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"ormonal Control of 1tomata

• !bscissic !cid $plant hormone% acts on

guard cells to close stomata.

 – “stress” hormone in plants

 – 7armers can use it as an anti>transpirant

$reduces water loss and the need for irrigation%.

 – /here are genetically engineered plants that

have a mutant era genethis ma3es themhighly sensitive to abscissic acid.