Transport Pathways and Cohesion Tension Theory Flashcards
apoplastic transport
dead space
- paces outside of plasma membrane: through permeable cellulose cell wall and middle lamellae
symplastic transport
living space
- cytoplasmic continuum between cells, does not cross plasma membrane: through cytoplasm and plasmodesmata
transmembrane transport
living space
- movement through semi-permeable plasma membranes: through cytoplasm, plasma membrane, and cell wall
- diffusion, facilitated diffusion, active transport, endo/exocytosis, osmosis
symplastic transport: plasmodesmata
- tiny strands of cytoplasm that connect cells (symplast)
- water and solutes can pass through plasmodesmata
- only certain sized molecules can pass between cells; size exclusion limits
- do not have to pass through semi-permeable plasma membrane
root transport pathways
apoplastic: movement through permeable cell walls (dead space)
transmembrane support: must pass size-exclusion limit (living space)
symplastic: must pass through semi-permeable barrier (living space)
casparian strip/general root transport
- radial and transverse cell walls of exo/endodermis have casparian strips (cell wall and middle lamella reinforced with suberin and lignin): block apoplastic pathway, symplastic and transmembrane only
- plasmodesmata on front and back of endodermal cells facilitate transport into pericycle
- water and solutes enter xylem from pericycle via apoplastic pathway; tracheids and/or vessel members are dead at maturity thus no membrane
- in addition to casparian strips, exo/endodermis can also form lignified, secondary walls to further restrict apoplastic transport
- passage cells have no secondary wall thickening or casparian strips, to allow unrestricted transport at front and back of cell
transport in vascular cylinder
sap - fluid transported in xylem (tracheids and vessel members) or phloem (Sieve tube elements)
- made of water, nutrients, sugars
tracheids (vascular plants) vs vessel members (angiosperms)
tracheids: staggered stacking, closed and tapered at ends, narrow
vessel members: stacked on top of one another, perforation plates or open ended at top and bottom, continuous water flow, wide and short
both: dead at maturity, thick lignified secondary walls, provide structural support, pits on side of vessels allow lateral water and solute flow
bordered pits: tracheids
simple pits - pores formed from absence of secondary wall, primary wall only barrier
bordered pits - have secondary wall extending over opening and a torus that functions like a hydraulic valve
torus - helps block movement of gases and pathogenic microorganisms
the cohesion-tension theory
explains water transport in xylem
- adhesion: attraction between diff kinds of molecules
- cohesion: attraction between same kind of molecules
- tension: negative pressure on water or solutions, which sucks liquid up plant (transpiration)
cavitation
- tracheids and vessel elements have thick, lignified secondary walls to withstand tension generated from negative pressure
- greater tension increases risk of breakage of water column; tree trunks can contract from tension like straw
- formation of air bubble or ice crystals (embolisms) can break water column
- breakage occurs less in tracheids than vessel elements due to anatomical differences
vessel members vs tracheids: embolisms
- water is less supported by vessel member walls because they are wider, making formation of air bubbles more likely
- embolisms form separately in each tracheid; have to move through pits, lateral flow
- in vessel members, entire column fills with air or ice when cavitation occurs
embolisms block water flow
air bubble (embolism) in vasculature form when flower stems are cut, blocking water column
- can remove by cutting off lower 3cm of stem, underwater
