Ch13: Transport Process
Concepts
Diffusion, Osmosis, and Active Transport
Water Potential
Short-Distance Intercellular Transport
Cells and water movement
Guard cells
Motor cells
Transfer cells
Long Distance Transport: Phloem
Long Distance Transport: Xylem
Properties of water
Water transport through xylem
Control of water transport by guard cells
Short-distance transport
long-distance transport
a few cell diameters or less
between cells that aren't close
necessary for survival of internal cells
transfer of basic nutrients
not essential
adaptive
almost everything transferred by plants is dissolved in water
isolation mechanisms: inhibit movement of substances
Diffusion: particles move from relatively high concentration areas to relatively low concentration areas
Osmosis: diffusion through a membrane
Membranes
freely permeable: allow all solutes to dissolve through them
completely impermeable: (isolation barriers) allow nothing to come through
Differentially/selectively permeable membranes: allow only certain substances to pass through
(all lipid/protein cell membranes are differentially permeable)
water passes through all membranes (goes faster if aquaporins are present) (channels on cell membrane)
molecular pumps: use energy of ATP to force molecules across the membrane (active transport)
the plasma membrane allows movement into/out of the cell
intracellular transport: vesicles migrate through the cytoplasm and fuse w/another organelle
"the chemical potential of water"
water can be heated, put under pressure, elevated (increases energy)
water can be cooled, lowered, or reduced pressure (lowers energy)
pronounced "sigh"
if the pressure potential increases, so does the water potential and vice versa
when water is under tension, pressure potential is a negative number
water potential is measured in units of pressure (megapascals or bars)
"sigh pi" is osmotic potential: the effect that solutes have on water potential
osmotic pressure is always negative
related to the number of particles present in solution
matric potential: (sigh m) water's adhesion to nondissolved structures (cell walls, membranes, soil particles)
matric potential is always negative
water moves whenever there is a difference in water potential within the mass of water
if the water potentials of two regions are equal, equilibrium, there is no net movement of water
water potentials must always be considered in pairs or groups
all protoplasm of one plant can be considered one continuous mass (symplast)
after transport, a molecule stays in the cell wall initially
most small molecules can move easily through both the wall and intercellular spaces (apoplast)
in glands, the apoplast is mostly intercellular space where molecules move easily
in nonglandular regions the apoplast is mostly cell walls
the opening/closing of stomatal pores are based on short distance intercellular transport
stomata closed at night (except CAM plants)
at night, guard cells are shrunk and have little internal pressure
they're in hydraulic equilibrium with other cells
K ions are actively transported to guard cells when they need to open
once inside, the K ions cannot leave
water potential in adjacent cells becomes less negative as a result
after the guard cells are open, potassium pumping stops, then the process is reversed
walls are smooth outside, but have many finger like outgrowths on the inner surface
plant "joints" similar to guard cells
can expel/take in K, thus adjusting their water potential and turgidity
room is available for many molecular pumps
found in areas where short distance transport is expected to occur
pressure flow hypothesis: has the most supporting evidence as to how water/nutrients are moved through phloem
sources: sites at which water/nutrients are transported
spring/summer: leaves are dominant sources
early spring: storage sites (tubers, corms, wood/bark parenchyma, taproots)
within sources of many species sugar is actively transported
in other species phloem is loaded by the polymer trap mechanism
as sugar accumulates in sieve tube elements, the protoplasm becomes more concentrated (osmotic and water potential becomes more negative)
water rushes into the sieve tube elements, protoplasm is squeezed through to other cells
phloem loading occurs along numerous vascular bundles
transport of sugar/other nutrients transferred by phloem per hour, mass transfer
sinks: sites that receive transported phloem sap
P-protein/P-protein plugs seal broken sieve elements to prevent sap loss
liquid water is cohesive (any force acting on one molecule acts on all neighboring ones)
since its molecules interact with many other substances, water is adhesive as well
DNA, sugars, cellulose, enzymes: have a water shell surrounding them
bulk transport of water through xylem is mostly influenced by water loss to the atmosphere
cohesion-tension hypothesis: water movement through xylem
when stomatal pores are open, they lose water
water molecules have a strong tendency to diffuse from intercellular spaces to the atmosphere
this water loss is called transstomatal transpiration
some water is lost directly through the cuticle: transcuticular transpiration
as a water molecule leaves the xylem, it doesn't leave a hole but instead draws other water molecules along with it
the pressure potential is a negative number as water moves into the leaf mesophyll (the xylem water potential becomes more negative
if leaf xylem water potential is more negative than root water potential, the water doesn't move
for every 10m of height leaf water potential must be 0.1 MPa more negative than root water potential
water is very adhesive, its molecules interact strongly with polymers of the cell walls of tracheids and vessel elements
transstomatal transpiration is more significant than water loss through cuticle
water loss is advantageous when the water in the soil is adequate in supply
if the leaf has adequate moisture content, light and CO2 are the normal controlling factors
for most healthy plants, light most often controls guard cell water relations
(stomata closed at night, open during day)
blue light is important
presence of light also leads to photosynthetic fixing of CO2, decrease of internal CO2 may also lead to stomatal opening
all of these mechanisms are overridden by a much more powerful mechanism triggered by water stress
abscisic acid is released when leaves are dehydrated
immediately causes guard cells to close stomatal openings