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