Different Mechanisms Transport Substances over Long and Short Distances
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Transcript of Different Mechanisms Transport Substances over Long and Short Distances
Different Mechanisms Transport Substances over Long and Short
DistancesConcept 36.2
Mira Patel
Transport Continuums
APOplast• External to the plasma
membrane• Includes cell walls,
extracellular spaces, and the interior of dead cells such as vessel elements and tracheids
SYMplast• Cytosol • Includes the
plasmodesmata, and the cytoplasmic channels that interconnect them
Key
Symplast
Apoplast
The symplast is thecontinuum of
cytosol connectedby plasmodesmata.
The apoplast isthe continuumof cell walls andextracellularspaces.
Apoplast
Transmembrane route
Symplastic routeApoplastic route
Symplast
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
SHORT DISTANCE TRANSPORTTransport Across the Plasma Membrane
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
0.4 M sucrose solution:
Initial flaccid cell:
Plasmolyzed cellat osmotic equilibriumwith 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
LONG DISTANCE TRANSPORTTransport Across the Plasma Membrane
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