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Transport Processes (Short-Distance Intercellular Transport (Guard Cells…
Transport Processes
Short-Distance Intercellular Transport
Guard Cells
Stomata is closed at night
guard cells are shrunken
have little internal pressure
When guard cells open..
during the day
(K+) are actively transported from surrounding cells into guard cells
Guard cells & adjacent cells are in equilibrium when stomata are fully open & fully closed
Motor Cells
cells at these "joints"
can either accumulate or expel potassium
Transfer Cells
walls are smooth on the outer surface
have numerous finger-like & ridge-like outgrowths on inner surface
distances of a few cell diameters or less
Symplast
All the protoplasm of one plant can be considered 1 continuous mass
Apoplast
molecules can move easily through both the wall and the intercellular space
the 2 together = apoplast
Long-Distance Transport: Phloem
between cells that are not close neighbors
Pressure flow hypothesis
Most evidence supports this
Sources
sites where water & nutrients are transported
polymer trap mechanism
Conducting-cell plasma membranes are permeable to monosaccharides & disaccharides
not to polysaccharides
amount of sugars and other nutrients (excluding water) transported by phloem per hour
Mass Transfer
Sinks
sites that receive transported phloem sap
extremely diverse
Long-Distance Transport: Xylem
Properties of Water
molecules interact strongly w other molecules
behaving as if weakly bound together
when frozen
molecules become strongly bound together
liquid water = cohesive
molecules interact with many other substances
it is adhesive
it is heavy
Water Transport Through Xylem
cohesion-tension hypothesis
most widely accepted model of the process
stomatal pores are open, they allow water loss
transstomatal transpiration
water loss
At 50% relative humidity, warm air can have a water potential as negative as -50.0 MPa
transcuticular transpiration
some water is lost directly through the cuticle
Control of Water Transport by Guard Cells
Whenever water supply in the soil is adequate, water loss is actually advantageous
water movement is the primary means of carrying minerals upward from roots to shoots
if the soil is too dry to supply water
transpiration represents an immediate, potentially lethal threat due to desiccation
If the leaf has an adequate moisture content
light & carbon dioxide are the normal controlling factors
between cells that are not close neighbors
Diffusion, Osmosis, and Active Transport
Diffusion
Simplest method
random movement of particles in solution
causes them to move from areas
from high to low concentration
Osmosis
Diffusion through a membrane
3 types of membrane:
Freely permeable
allow all solutes to diffuse through them
have little biological significance
Completely impermeable
do not allow anything to pass through
isolation barriers
Differentially/Selectively permeable
allow only certain substances to pass through
Molecular pumps
use energy of ATP to force molecules across membrane
Water Potential
Cells & Water Movement
Water molecules do move between cell & solution
but approx. equal numbers move in each direction every second
Lysis
Animal cells often burst if placed in pure water in Lysis
but plant cells can never burst
rising pressure potential or osmotic potential
causes cell’s water potential to rise & reach equilibrium w/ the surrounding cells,
stopping the net inflow of water
incipient plasmolysis
protoplast lost just enough water to pull slightly away from the wall
waters chemical potential
ψ is the symbol
3 components:
ψ = ψπ + ψp + ψm
ψp = pressure potential
potential measured in units of pressure
usually megapascals (MPa)
ψπ = osmotic potential
effect that solutes have on water potential
ψm = matric potential
water’s adhesion to nondissolved structures
