Transport in Plants

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Transport in Plants
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Coast redwoods (Sequoia sempervirens)
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Introduction to Transport

• Three kinds of transport occur in plants
• 1. Uptake of water and solutes by individual
  cells, such as a root hair
• 2. Short-distance transport from cell to cell at
  the level of tissues and organs
• 3. Long-distance transport in xylem and phloem at
  the level of the whole plant

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An overview of transport in a vascular plant
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Osmosis

• The uptake of water across cell membranes occurs
  through osmosis, the passive movement of water
  across a membrane along a concentration gradient
• The water potential (?)?is defined as the
  combined effects of solute concentration (?s) and
  the pressure that the cell wall contributes (?p).
  The water potential equation is ?????s????p
• Another way to describe osmosis is the movement
  of water from an area of high water potential to
  an area of low water potential

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Turgor

• Turgor pressure is produced by the plasma
  membrane exerting force against the cell wall.
• A walled cell that has a greater solute
  concentration (greater water potential) than its
  surroundings has a high turgor.
• Loss of turgor results in wilting.

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A watered Impatiens plant regains its turgor
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Uptake of Water

• Most water absorption occurs near the root tips
  through the root hairs
• The roots of many types of plants have symbiotic
  relationships with fungi. This is called
  mycorrhizae, and leads to increased surface area
  that aids the absorption and transport of water
  and certain minerals deep into the plant

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Mycorrhizae, symbiotic associations of fungi and
roots
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Intercellular Transport of Water

• Aquaporins are the transport proteins (channels)
  in the plant cell plasma membrane specifically
  designed for the transport of water across the
  membrane
• The cytosol of neighboring cells are connected
  through plasmodesmata. This allows for exchange
  of materials between cells.
• The symplast is the continuum of the cytoplasm.
• The apoplast is the continuum of cell walls and
  the extracellular spaces.
• Water moves from a higher to lower water
  potential through both the symplast and the
  apoplast.

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Intercellular Transport of Water

• Water and minerals from the soil enter the plant
  through the root epidermis, cross the cortex,
  pass into the vascular cylinder (xylem) and then
  flow up the tracheids and vessels to the shoot
  system
• The endodermis, the innermost layer of cells in
  the root cortex, surrounds the vascular cylinder
  and functions as a last checkpoint for the
  selective passage of minerals from the cortex
  into the vascular cylinder.
• The endodermis has a waterproof Casparian strip
  that forces water to enter the vascular tissue
  through the symplast

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Water Flow Into the Xylem
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Transpiration

• Transpiration is the loss of water vapor from the
  leaves and other parts of the plant that are in
  contact with air
• There are two mechanisms that influence how water
  is pulled up through the plant
• Root pressure occurs when water diffusing in from
  the root cortex generates a positive pressure
  that forces fluid up through the xylem (this can
  be thought of as a push on the water)
• In the transpiration-cohesion-tension mechanism,
  water is lost through transpiration through the
  leaves of the plant due to the lower water
  potential of the air. The cohesion of water due
  to hydrogen bonding plus the adhesion of water to
  the plant cell walls enables the water to form a
  column, which is drawn up through the xylem as
  water evaporates from the leaves (this can be
  considered a pull on the water).

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Transpiration Rates

• The rate at which water is transpired from plants
  varies with environmental conditions you will
  explore these in one of the two labs for this
  unit
• Light
• Heat
• Wind
• Humidity

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Water Loss Through Stomata

• Turgor changes in guard cells control the size of
  the openings in the stomata. When the stomata are
  open, the exchange of carbon dioxide and oxygen
  takes place. Water also exits through the open
  stomata due to transpiration
• Guard cells control the size of the stomata
  opening by changing shape, widening or closing or
  closing the gap between them. Taking up water
  causes the guard cells to swell and buckle,
  increasing the size of the pore between them.
  When the guard cells lose water, the cells become
  less bowed and the pore closes.

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Open stomata (left) and closed stomata (SEM)
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Transport of Organic Nutrients

• Phloem transports organic products of
  photosynthesis from leaves throughout the plant
• Sieve tubes always carry sugar from a sugar
  source (e.g., leaves) to a sugar sink (an organ
  that is a net consumer or storage site for sugar)

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Summary of the Pressure Flow Model of Phloem
Translocation

• 1. Active transport (loading) of sucrose into
  sieve tubes at source (e.g., leaves) .
• 2. This increase in solute concentration in sieve
  tube decreases water potential within the sieve
  tubes.
• 3. Water flows into sieve tube by osmosis (high
  to low water potential).
• 4. This flow of water into the sieve tube causes
  the turgor pressure at source to increase. This
  pushes sucrose down the sieve tube.
• 5. Sucrose actively transported out of sieve tube
  at sink.
• 6. Water moves out of sieve tubes at sink. Turgor
  pressure decreases.
• 7. Solutes are translocated down a pressure
  gradient.

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Phloem Xylem

• Conduits are living cells called sieve cells in
  gymnosperms, sieve tube members in angiosperms
• Used for transport of organic compounds
• Bidirectional movement (up or down, can change
  seasonally)
• Slow - maximum flow rate of 1 m/h

• Conduits are dead cells called tracheids and
  vessel elements
• Used for transport of water and minerals
• Unidirectional movement (up)
• Fast - maximum flow rate of 15 m/hr