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: TRANSPIRATION- COHESION-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 H- bonds 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 CO 2 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 H 2 O is lost stomata close *when K + enters cell, s decreases H 2 O 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 CO 2 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 CO 2 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)