1. NOTES: CH 36 - Transport in Plants

2.  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)

4. PROTON PUMPS:
● pump out H+ ions, producing a proton gradient (more H+ outside cell) and a membrane potential (inside is negative relative to outside)

5. • 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

8. WATER POTENTIAL (): predicts the direction water will flow
● combines solute concentration (osmotic potential, s) with differences in pressure (pressure potential, p)

9. 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!

11. ● 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

12. Recall the 3 major parts of a plant cell:
1) cell wall
2) cytosol / cytoplasm
3) vacuole (surrounded by TONOPLAST)

13. ● SYMPLAST: continuum of cytoplasmic compartments of neighboring cells; connected by PLASMODESMATA 
● APOPLAST: continuum of adjacent cell walls and intercellular spaces

15. 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)

17. 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

19. Absorption of Water & Minerals by Roots:
● Transport pathway:
*soil  epidermis  root cortex  xylem
-minerals moving through symplastic route move directly into vascular tissues

21. -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.

22. 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

24. Transport of Xylem Sap
● most xylem sap moves via the mechanism of: TRANSPIRATION-COHESION-TENSION

25. TRANSPIRATION-COHESION-TENSION:
● TRANSPIRATION: (loss of water from leaf cells through stomata) creates negative pressure

26. 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)

30. 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

32. Adaptations to reduce water loss:
 more stomata on bottom of leaves
 waxy leaf cuticle on rest of leaf surface

33. Benefits of transpiration:
 assists in mineral transfer from roots  shoots
 evaporative cooling

34. 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

36. *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

37.    Studies show that K+ fluxes across guard cell membrane are likely coupled to membrane potentials created by PROTON PUMPS

39. 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)

40. 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

43. 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

44. Translocation of Phloem Sap
 TRANSLOCATION = transport of products of photosynthesis by phloem to rest of plant
 PHLOEM SAP = sucrose, minerals, amino acids, hormones

45.  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

47. 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)