Download APOplast

Different Mechanisms
Transport Substances over Long
and Short Distances
Concept 36.2
Mira Patel
Transport Continuums
APOplast
SYMplast
• External to the plasma
membrane
• Includes cell walls,
extracellular spaces, and
the interior of dead cells
such as vessel elements
and tracheids
• Cytosol
• Includes the
plasmodesmata, and the
cytoplasmic channels that
interconnect them
Key
Symplast
Apoplast
Transmembrane route
Apoplast
The symplast is the
continuum of
cytosol connected
by plasmodesmata.
Symplast
The apoplast is
the continuum
of cell walls and
extracellular
spaces.
Symplastic route
Apoplastic route
Transport routes between cells. At the tissue level, there are three passages:
the transmembrane, symplastic, and apoplastic routes.
Transport Routes
• Apoplastic Route
– Water and solutes move
along the continuum of cell
walls and extracellular
spaces
• Symplastic Route
– Water and solutes move
along continuum of the
cytosol
• Transmembrane Route
– water and solutes move out
of one cell, across the cell
wall, and into the
neighboring cell
Transport Across the Plasma Membrane
SHORT DISTANCE TRANSPORT
Osmosis
• Movement of water across the cell
• The diffusion of free water
• Water Potential (Ψ)
– Quantifies the ability of water to flow
• Indicates direction
• Takes into account the solute concentration and
physical pressure
– Water moves from an area of high water potential to
an area of low potential
– Water Potential
POTENTIAL Energy
Water Potential- Solute Potential
• Also called osmotic pressure
• Directly proportional to the molarity (M)
• An increase in solutes has a negative effect on
water potential
• Ψs is always a negative number
• As the solute concentration increases, the solute
potential becomes more negative
– Unless it is pure water, in which case the solute
potential is zero
Water Potential- Pressure Potential
• Physical pressure on a solution
• Can be either positive or negative
– Relative to the atmospheric pressure
• Pressure potential in a living cell is usually
positive
– Due to the osmotic uptake of water
Turgor Pressure
• The cell contents press the plasma membrane
against the cell wall
• The cell wall presses against the protoplast
– The living part of the cell
Turgor Pressure
• This internal pressure is critical for function
• Helps maintain the stiffness of the plant
• Serves as a driving force for plant elongation
Plasmolysis
• Flaccid cell placed inside a solution of higher
concentration (hypertonic)
• Results in plasmolysis
– The cell’s protoplast shrinks and pulls away from the
cell wall
– Causes wilting
Initial flaccid cell:
0.4 M sucrose solution:
Plasmolyzed cell
at osmotic equilibrium
with its surroundings
Facilitated Diffusion- Aquaporins
• Transport proteins specialized for water
• Helps avoid the hydrophobic tails of the bilayer
• Affect the rate at which water is transported
Transport Across the Plasma Membrane
LONG DISTANCE TRANSPORT
Bulk Flow
• Movement of liquid in response to a pressure
gradient
• From high concentration to low concentration
• Independent of solute concentration
• Occur within the tracheids and vessel elements of
the xylem and within the sieve-tube elements of
the phloem
The Beauty of it All
• Diffusion, active transport, and bulk flow work
harmoniously to transport through the whole plant