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Transport Processes (Short-Distance Intercellular Transport (Guard Cells,…
Transport Processes
Short-Distance Intercellular Transport
all cells connected by plasmodesmata
allows for communication, passing of water, sugars, minerals, and hormones
symplast
all protoplasm of one plant=one continuous mass
osmosis, molecular pumps, or vesicles also used
a molecule initially resides in a cell wall when across membrane
diffuses across and into intercellular spaces
spaces+wall=apoplast
Guard Cells
stomata at night= guard cells shrunken and little internal pressure
cells in hydraulic equilibrium with surrounding cells
guard cells open when K+ ions are actively transported within
water follows potassium and causes swelling
guard cells open
K+ pumping stops
close when potassium is pumped out of guard cells into surrounding cells
H2O follows
Motor Cells
Cells at "joints" of certain species
similar to guard cells
expel or accumulate potassium
causes surrounding cells to adjust their water potential and turgidity
midrib of Venus' flytraps are a example
Transfer Cells
rate at which materials can be actively transported depends on the number of molecular pumps
larger the membrane=larger number of pumps
transfer cells used to increase surface area
can be found in glands, areas that pass nutrients to embryos, and where sugar is loaded to phloem
walls have numerous finger-like and ridge-like outgrowths
Long Distance Transport: Phloem
Pressure flow hypothesis
membrane bound molecular pumps and active transport= driving forces
Sources
sites where water and nutrients are transported
leaves during spring and summer
before new leaves, tubers, corms, bark and wood parenchyma
sugars are actively transported into sieve elements
Phloem
loaded by polymer trap mechanism
sugars diffused into conducting cells and polymerized into polysaccharides
companion cells used to load
companion + conducting = STM/CC complex
sugars cant diffuse back out
in sources, phloem loading occurs across vascular bundles
massive loading occurs causing great pressure build up
sugars/nutrients transported by phloem per hour = mass transfer
specific mass transfer = cross section area of phloem
Sinks
sites that receive transported phloem sap
sugars actively unloaded from sieve elements to surrounding cells
Repairing vascular bundles
can be caused by chewing insects or animals
if cut, causes bleeding of sap from phloem
Two mechanisms used to seal
P-protein
carried by sap creating a tangled mass
forms P-protein plug
Callose
carried along with P-protein
contributes to plug
Long Distance Transport: Xylem
based on properties of water and solutions
water
interacts strongly with other water molecules
cohesive
force acting on one acts on all surrounding
molecules interact with other substances
adhesive
except lipids
adheres to soil particles
absorbed by roots
heavy molecule so lifting it upwards in tree requires lots of energy
Water Transport through Xylem
governed by water principles
cohesion-tension hypothesis
stomal pores opened allowing water loss through intercellular spaces to the atmosphere
=transstomatal transpiration
water also lost through the cuticle
transcuticular transpiration
these water losses from epidermal and mesophyll cells to have a more negative water potential
brought back to equilibrium via diffusion of water back into cells from the cells deeper inside the leaf
forms a gradient of water potentials that hits a tracheid or vessel element
water cohesive property begins to play a role
water leaves xylem and drags more water with it
pulls water upward from the roots
water adhesion to neighboring cells causes water to remain fixed to walls of tracheids and vessel elements
1 more item...
transpiration = loss of water
leaf cells have a more negative water potential drawing water from tracheid
tension pulls on water in xylem
uppermost tracheids more negative then lower
friction and gravity are overcame causing water to move upward
lower most xylem pulls water from root cortex, and cortex draws water from epidermis of root
2 more items...
cavitation
hydrogen breaking of water columns
molecules above cavitation are drawn up
molecules below cavitation rush downward
movement of molecules forms an embolism
air bubble in tracheid or vessel element
never able to conduct water again
Control of water transport by guard cells
water movement through xylem is influenced by and powered by water loss to atmosphere
stomata open for CO2 absorption and water loss
adequate moisture in soil allows nutrients to be carried up from roots to the shoots and reducing stress on the leaves by cooling them
if soil is dry though it can pose a danger to the plant
dessication of plant can occur
mechanisms for stomatal opening and closings have been formed to prevent this
mechanism keyed into certain environmental factors
Diffusion, Osmosis, and Active Transport
Diffusion
moves from material in a solution across a membrane
through a membrane= osmosis
three types of membranes
freely permeable
completely permeable
differentially or selectively permeable
aquaporins
in the membrane and allow H2O molecules to diffuse faster
membranes also have molecular pumps that use ATP to move molecules across membrane
=active transport
proton pumping in respiration and photosynthesis
all membranes are crucial in governing the flow of materials
intracellular transport
ER membrane and dictyosome membrane
active or osmosis
ER forms vesicles
they migrate through cytoplasm to fuse with other organelles
area of high concentration to low concentration
Water Potential
water has free energy
its importance earned it to be referred as water potential
can be increased in several ways
heated
pressure
elevated
can be decreased in several ways
cooling it
reducing pressure
lowering it
adhesion
water molecules form hydrogen bonds to a substance
not as free for diffusion
capacity to do work has decreased
three components
pressure potential
effect of pressure
measured in MPa
osmotic potential
effect of solutes on H2O
always negative
pure H2O= 0.0 MPa
related to number of particles in a solution
matric potential
water's adhesion to nondisolved structures
adhesion only decreases water's free energy
always negative
Cells and Water Movement
equilibrium
occurs when the cell and the solution have the same water potential
movement of water molecules still occurs
no shrinkage or nor swelling
if placed in a higher concentration causes the water in the cell to move towards the solution
solutes in protoplasm become more dilute
causes osmotic and pressure potential to become more negative
water potential becomes more negative in turn as well
allows for equilibrium to be reached
cell with a more negative water potential placed in pure water
causes water to move into cell to dilute solutes
osmotic becomes less negative
swelling of protoplast
causes increase of pressure potential
lysis of some animal cells can occur if placed in pure water
not plant cells however
cells only increase in size
