Transcript Ch.36: Transport in Plants (part 1)

NOTES: CH 36
- Transport in
Plants
 Recall that transport across the cell membrane of
plant cells occurs by:
-diffusion
-facilitated diffusion
-osmosis (diffusion of water)
-active transport (done by transport proteins)
PROTON PUMPS:
● pump out H+ ions,
producing a proton
gradient (more H+
outside cell) and a
membrane potential
(inside is negative
relative to outside)
• this “stored” energy is used to transport
other molecules across the membrane:
-K+ ions pulled into cell
-sugar molecules are loaded into
companion cells via COTRANSPORT
**These are all examples of
CHEMIOSMOSIS
WATER POTENTIAL (): predicts the
direction water will flow
● combines solute concentration (osmotic
potential, s) with differences in pressure
(pressure potential, p)
WATER POTENTIAL ()
● as solute conc. increases, s decreases
● as pressure (hydrostatic) increases, p
increases
 = s + p
● water flows from HIGH water potential to
LOW water potential!
● PLASMOLYSIS:
when a plant cell
loses water by
osmosis; protoplast
pulls away from cell
wall
● TURGOR
PRESSURE:
develops when a
plant cell gains water
by osmosis
Recall the 3 major parts of a plant cell:
1) cell wall
2) cytosol / cytoplasm
3) vacuole (surrounded by TONOPLAST)
● SYMPLAST: continuum of cytoplasmic
compartments of neighboring cells; connected
by PLASMODESMATA 
● APOPLAST: continuum of adjacent cell walls
and intercellular spaces
Lateral / short-distance transport
can occur:
1) across cell membranes (trans-membrane
route)
2) via the SYMPLASTIC ROUTE (molecules
travel through the plasmodesmata)
3) via the APOPLASTIC ROUTE (molecules
don’t enter cells)
Vertical / long-distance transport occurs by:
1) BULK FLOW: movement due to pressure
differences (substances move from regions
of higher to lower pressure)
2) TRANSPIRATION: creates tension which
“pulls” sap up through the xylem from the
roots
3) HYDROSTATIC PRESSURE: builds up at
one end of phloem vessels; forces sap to the
other end of the tube
Absorption of Water & Minerals by Roots:
● Transport pathway:
*soil  epidermis  root cortex  xylem
-minerals moving through symplastic route move
directly into vascular tissues
-minerals & water moving
through apoplastic route
are blocked at the
endodermis by the
CASPARIAN STRIP (a
ring of waxy substance,
SUBERIN) and must
enter an endodermal cell
**this ensures that all
minerals entering the
STELE pass through at
least one selectively
permeable membrane.
Transport of Xylem Sap
● xylem sap flows upward at 15 m per hour
● is it pushed from below or pulled from
above?
● root pressure builds up when transpiration is
slow (i.e. at night); this causes GUTTATION
● this only accounts for small amt. of xylem
transport
Transport of Xylem Sap
● most xylem sap moves
via the mechanism of:
TRANSPIRATIONCOHESION-TENSION
TRANSPIRATION-COHESION-TENSION:
● TRANSPIRATION: (loss of water from leaf
cells through stomata) creates negative
pressure
TRANSPIRATION-COHESION-TENSION:
● neg. pressure pulls
water from the xylem
● transpiration pull on
xylem sap is transmitted
from one water molecule
to another through
COHESION (due to Hbonds between water
molecules)
THE CONTROL OF TRANSPIRATION
 stomata provide openings in leaf tissue
for the transpiration of water (out of leaf)
and the diffusion of CO2 into the leaf for
photosynthesis
 GUARD CELLS surrounding the stomata
regulate the requirements for
photosynthesis with the
need to conserve water
Adaptations to reduce water loss:
 more stomata on bottom of leaves
 waxy leaf cuticle on rest of leaf surface
Benefits of transpiration:
 assists in mineral transfer from roots 
shoots
 evaporative cooling
Stomatal Opening / Closing:
 GUARD CELLS: cells that flank the
stomata and control stomatal diameter by
changing shape:
-when TURGID, guard cells
“buckle” and stomata
open
-when FLACCID, guard
cells sag and
stomata close
 a change in turgor pressure in guard cells
results from the reversible uptake of K+
*when K+ leaves cell,
 s increases 
H2O is lost 
stomata close
*when K+ enters cell,
 s decreases  H2O
is taken up 
stomata open
 Studies show that K+ fluxes across guard cell
membrane are likely coupled to membrane
potentials created by PROTON PUMPS
Stomata open at dawn in response to:
1) Light: induces K+ uptake; activates a
blue-light receptor which drives
photosynthesis  ATP
2) Decrease of CO2 in air spaces due to
photosynthesis
3) internal clock in guard cells
(CIRCADIAN RHYTHM = 24 hour
cycle)
Guard cells may close during daytime if:
1) there is a water deficiency
 flaccid guard cells
2) production of abscisic acid (hormone); in
response to water deficiency; signals guard
cells to close
3) high temperature increases CO2 in air
spaces due to increased respiration
Xerophytes have special adaptations:
 small, thick leaves
 thick cuticle
 stomata are in depressions
on underside of leaves
 some shed leaves
during driest time of year
 cacti store water in stems
during wet season
Translocation of Phloem Sap
 TRANSLOCATION = transport of products
of photosynthesis by phloem to rest of plant
 PHLOEM SAP = sucrose, minerals, amino
acids, hormones
 phloem sap moves through sieve tubes from
a SUGAR SOURCE (production area) to a
SUGAR SINK (use or storage area)
 Sugars move into sieve tubes via
symplastic and/or apoplastic routes
 Sucrose is “loaded” into cells at the source
end by active transport (COTRANSPORT)
 Sucrose is “unloaded” at the sink end of
sieve tubes
Pressure Flow / Bulk Flow of Phloem Sap
 pressure builds up at source end (phloem
loading  s decreases  water enters
tubes  hydrostatic pressure)
 pressure is released at sink end (phloem is
unloaded  s outside tube decreases 
water leaves  release of hydrostatic
pressure)