Water Movement Within a Plant

1
Water Movement Within a Plant
AP Biology Unit 5
2
Review Properties of Water

• Water is a polar molecule
• Water molecules can form a column
• due to hydrogen bonds between them
• Cohesion Water is attracted to other water
  molecules
• Adhesion Water is attracted to other polar
  molecules

3
Question

• Why do plants wilt?
• When there is enough water ? it flows into plant
  cells ? cells are plump with water (turgid)
• If there isnt enough water ? it flows out of the
  cell ? cells are limp (flaccid)

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Movement Into Roots

• Water is absorbed into the roots through osmosis
• Osmosis diffusion of water
• Osmosis is a passive process no energy required
• Water potential (?) determines the direction that
  osmosis will occur

5
Water Potential

• Pure H2O has ? 0
• Water potential is a combination of the pressure
  from solutes (?s)and the physical pressure (?p).
• The more solutes there are in a solution, the
  more negative the water potential will be.

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Water Potential

• Water always moves from areas of higher water
  potential (less solutes) to lower water
  potential (more solutes).
• Moves from less negative ? to more negative ?.

No net water movement
7
Pathways of Water

• Apoplast Pathway water travels between cells
  (outside of them)
• Symplast Pathway water travels through cells
  (inside them to get from cell to cell)

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Water Movement in Roots

• Water and ions are able to freely diffuse up to a
  point in the root cells
• To get into the stele (where the vascular tissue
  is located), the water and ions must pass through
  the cells of the endodermis (symplast pathway).

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Casparian Strip

• Blocks the water from crossing the endodermis
  through apoplast pathway
• control what enters the vascular tissue
• Prevents water and solutes from leaking out
  into the soil

10
Water Transport in Plant

• Water is transported through the plant in the
  xylem
• Cells that make up the xylem
• Tracheids found in all plants
• Vessel Elements found in some plants

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Formation of xylem

1. Cell dies.
2. Cell contents disintegrate.
3. Water can move through these hollow cells with
   little resistance.

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Capillary Action

• movement of H2O up a very narrow tube
• Does account for a little water movement up the
  xylem, but not much (about 40 cm)

Image obtained without permission from
http//ianrpubs.unl.edu/fieldcrops/graphics/soilh2
o5.gif
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Transpiration Pull

• Accounts for most of the water movement up the
  xylem.
• As H2O evaporates from the stomata
  (transpiration), water from the xylem moves
  into the leaf to take its place.
• Because of the hydrogen bonds between water
  molecules, water is pulled up the xylem.

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Tension-Cohesion-Evaporation Model
15
Question

• When you receive a bouquet of flowers, why is it
  important to cut them under water?
• Transpiration is still continuing, so if you cut
  it under water, it will ensure that water (not
  air) gets into the xylem.
• Air bubble in xylem would disrupt the column of
  water.

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Transpiration Rate

• Factors that affect transpiration rate include
  light, humidity, temperature.
• The plant might also close its stomata to limit
  transpiration
• Well look at this in more detail in the upcoming
  dry lab.

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Stomata and Guard Cells

• Stoma is Greek for "mouth" (plural stomata)
• Function of Stoma to allow gas exchange with
  the outside environment (CO2 and O2)
• Guard cells regulate the opening of the stomata

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Stomatal Opening and Closing

• When guard cells are full of water (plump) ?
  stretch away from each other ? stoma is open
• When guard cells dont have much water in them
  (limp) ? dont pull away from each other ? stoma
  is closed

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Role of K

• K ions (potassium) control the opening of stoma
  by changing the water content in the guard cells.
• K in H20 follows by osmosis ? stoma open
• K out H20 follows by osmosis ? stoma closed

20
Question

• What conditions favor stomatal opening?
• Plentiful water
• Moderate temperatures

21
Hormonal Control of Stomata

• Abscissic Acid (plant hormone) acts on guard
  cells to close stomata.
• stress hormone in plants
• Farmers can use it as an anti-transpirant
  (reduces water loss and the need for irrigation).
  
• There are genetically engineered plants that have
  a mutant era genethis makes them highly
  sensitive to abscissic acid.