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 ()  = s + p ● 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 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

*when K+ leaves cell, s increases  H2O is lost  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)