Vascular System in Plants

1
Chapter 36

• Vascular System in Plants

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Three ways water moves through root hairs

1. Apoplast water moves through cell walls and
   never enter cells
2. Symplast water moves from one cell to another
   through the cytoplasm
3. Transmembrane water repeatedly moves back and
   forth from cell wall, crossing the membrane, and
   through the cytoplasm

apoplastic
symplastic
3
Pathway of Water Movement

1. Root hairs use osmosis to soak up water
2. Water travels via apoplast or symplast through
   the cortex until it reaches the endodermis that
   lines the vascular cylinder (stele).
3. Endodermis has a casparian strip, a selectively
   waxy layer, which BLOCKS the apoplast pathway, so
   water MUST move into the stele via symplast (to
   regulate what minerals in the water can enter the
   stele)
4. Xylem within the stele transports water to shoots

4
Transpiration

• Definition the loss of water vapor from leaves
  and aerial parts of a plant via stomata
• Responsible for upward transport of water through
  the xylem due to
• pushing forces
• pulling forces

video
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Pushing Forces

• Endodermis accumlates ions in stele, causing
  these effects
• a) ?w ______________ in stele
• b) Water moves ____ stele, generating pressure
  forcing water up the xylem (root pressure)
• c) Guttation is a result of root pressure
• -low transpiration rate at night
• -more water moves into stele than transpired
• -water droplets forced out at end of leaves
  by morning

decreases
More solutes
into

• ?s
• ?w

?w 0
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Pulling Forces(transpiration, tension, cohesion
mechanism)

• Transpiration evaporation of water from stomata
  of leaves creates (-) pressure and develops
  tension
• Bulk Flow
• Water moves up the xylem to replace the (-)
  pressure

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Pulling Forces(transpiration, tension, cohesion
mechanism)

• Cohesion
• Water molecules are hydrogen bonded to each
  other due to its polarity
• Water, therefore, moves up the xylem acting as
  one large polymer-like substance

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How is transpiration controlled by plants?

• There must be a compromise between photosynthesis
  and transpiration

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The Great Debate!Pros and Cons of an Open Stomata

• Pros
• Transpiration can occur for PS
• Gas exchange for CR and PS can occur

• Cons
• Plant risks dessication from excessive
  transpiration, leading to wilting

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The Great Debate!Pros and Cons of a Closed
Stomata

• Pros
• Minimal dessication and minimal wilting

• Cons
• No transpiration for PS
• No PS or CR can occur due to lack of gas exchange

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Plants maintain a transpiration to PS ratio

• Ratio (in grams) water loss
• CO2 assimilated
  for PS
• C3 plants ? 6001 ratio
• C4 plants ?3001 ratio (greater CO2 assimilation
  rate)
• Goal maximize PS rate w/ minimal transpiration

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When do plants open and close their stomata?
Guard Cells
guard cells close
High temperatures ? CO2 is low ? Night ? Day
?
guard cells open
guard cells close
guard cells open
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How do guard cells open and close?
K is pumped out of guard cells ? ?w __________
inside, and water moves out of guard cells making
them flacid and close
K is pumped into guard cells ? ?w ________
inside, and water moves into guard cells making
them swell, turgid, and open
decreases
increases
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Translocation

• Definition the transport of carbohydrates (CHO)
  in plants from
• - the source (normally where CHO is produced)
• to
• - the sink (where CHO is used)
• storage organ (ex. bulb) can be a source or a
  sink

video
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Translocation occurs in 2 steps

1. Sugar Loading
2. Pressure Flow

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Sugar Loading
symplast

• CHO are moved from source to sieve-tube members
• Symplast pathway
• or
• Apoplast pathway
• - needs co-transport mechanism to move CHO
  from apoplast to cytoplasm of sieve tube

apoplast
H
H
H
H
Companion cell
H
ATP
ADP
ST members
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Pressure Flow

• CHO move from source to ST members by sugar
  loading mechanism
• ?__________ in the phloem (STM), causing water
  from xylem to flow into the phloem
• This increases pressure in the phloem. Increased
  pressure causes phloem sap to move to an area of
  less pressure
• Carbohydrates move out towards sink
• ? ____________ in phloem (STM) so water from
  phloem diffuses back out to xylem), decreasing
  pressure

Phloem (STmembers)
xylem
decreases
source
sink
increases