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plant transportchapter 37
• Transport systems in plants• Three levels of transport occur in plants:
• Uptake of water and solutes by individual cells.• cell-to-cell transport (lateral transport)• long-distance transport
transport
O2
CO2water and minerals
Lightwater
O2CO2
uptake by cellsCYTOPLASM EXTRACELLULAR FLUID
ATP
H+
H+
H+H+
H+
H+
H+H+
H+
Proton pump generates membrane potential and gradient.
+
+
+
+
+
_
_
_
_
_
• Transport at the cellular level• Passive transport
• transport proteins• Carrier proteins • Selective channels
• active transport• proton pump • cations pulled into cell
CATIONS can now diffuse into the cell
• Water potential and osmosis• osmotic potential• hyperosmotic conditions -- plasmolysis • hypoosmotic conditions -- turgor pressure
• other aspects of water potential• osmotic, pressure, matrix • pure water --> ψ = 0
• solutes --> negative• pressure --> positive• negative pressure?
• water moves from high potential to low potential
lateral transport• mechanisms/pathways
• transmembrane route • symplast route• apoplast route
Cell wall
Cytosol
Vacuole
Plasmodesma
Vacuolar membrane
Plasma membrane
Transmembrane route
Apoplast
Symplast
Symplastic route
Apoplastic route
lateral transport• anatomical considerations --
• Root hairs• mycorrhizae • large surface area of cortical cells
• pathway of water:• soil - epidermis - root cortex - xylem
lateral transport• Soil to epidermis• Epidermis to root
cortex• routes:• apoplastic route• symplastic route --
• diffusion not enough -- active transport
• Root cortex to Xylem• Casparian strip
• suberin• tracheids and xylem
vessels = apoplast
Pathway along apoplast
Casparian strip
Endodermal cell
Pathwaythroughsymplast
Casparian strip
Apoplasticroute
Symplasticroute
Roothair
EPIDERMIS CORTEX ENDODERMIS
Vessels(xylem)
Stele(vascular cylinder)
long distance transport• Bulk flow -- long-distance
transport• vertical axis of the plant• diffusion?• transports water and solutes
• Cohesion and adhesion of water • transmission of negative
pressure• Cohesion • adhesion (matrix pressure)• small diameter of vessels
Xylem sap
Mesophyll cells
Stoma
Water molecule
Atmosphere
Adhesion by hydrogen bonding
Cellwall
Xylemcells
Cohesion by hydrogen bonding
Water molecule
Root hair
Soil particle
Water
Wa
ter
po
ten
tia
l gr
ad
ien
t
TRANSPIRATION
COHESION AND ADHESION IN THE
XYLEM
WATER UPTAKEFROM SOIL
• control of transpiration• Guard cells
• turgid -- guard cells "buckle" • flaccid -- guard cells sag
stomata• open at dawn in response to three cues:
• light • decrease of CO2 in leaf air spaces• internal clock of guard cells
• stomata may close during daytime if:• water deficiency resulting in flaccid cells• mesophyll production of abscisic acid• high temperature
stomata
• photosynthesis-transpiration compromise• anatomical considerations related to transpiration
• large interior surface areas• stomata more concentrated on bottom of leaves• waxy cuticle on surface of leaves
• Benefits of transpiration• pulls water• Assists in mineral transfer from roots to shoots• evaporative cooling
• Wilting?
transpiration xylem sap ascension• Xylem sap
• provides water to all cells of the plant -- provides nutrients (minerals)
• transpiration • Pushing xylem sap: root pressure
• Root pressure --> guttation
• Pulling xylem sap• transpiration-cohesion-tension mechanism
• Transpirational pull
xylem sap ascension
Cuticle Xylem
Upperepidermis
Mesophyll
Lower epidermis
Cuticle
Air space
Microfibrils in cell wall ofmesophyll cell
Stoma
Microfibril (cross section)
Water film
Air-water interface
• PHLOEM SAP BULK FLOW• translocation
• sieve-tube members• form long sieve tubes.• sieve plates
• phloem sap • sucrose• minerals• amino acids • hormones
phloem sap
phloem sap• Source-to-sink transport
• Source • Sink • depend on season.
• tubers? • Direction of flow in phloem can change
1
2
34
Vessel(xylem)
Sieve tube(phloem)
SOURCE CELL (LEAF)
SINK CELL (STORAGE ROOT)
Sucrose
H2O
H2O
Bu
lk fl
ow
by
neg
ati
ve p
ress
ure
H2O
Sucrose
Bu
lk fl
ow
by
po
siti
ve p
ress
ure
1Loading of sugar
2Uptake of water
3Unloading of sugar
4Water recycled
phloem sap• Phloem loading and unloading
• loading• symplast • usually apoplastic routes• active transport
• unloading• sometimes active transport• sometimes diffusion
• Bulk flow from sap• pressure-flow mechanism
1
2
34
Vessel(xylem)
Sieve tube(phloem)
SOURCE CELL (LEAF)
SINK CELL (STORAGE ROOT)
Sucrose
H2O
H2O
Bu
lk fl
ow
by
neg
ati
ve p
ress
ure
H2O
Sucrose
Bu
lk fl
ow
by
po
siti
ve p
ress
ure
1Loading of sugar
2Uptake of water
3Unloading of sugar
4Water recycled
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