Chapter 36—Transport in
Plants
I. Plant Transport MechanismsTransport occurs at the cellular, tissue, and whole plant levels
H2O & minerals
� transport in xylem� transpiration
� evaporation, adhesion &
cohesion � negative pressure (pull)
Sugars
� transport in phloem� bulk flow
� Calvin cycle in leaves loads sucrose into phloem
� positive pressure (push)
Gas exchange
� photosynthesis� CO2 in; O2 out� stomata
� respiration
� O2 in; CO2 out � roots exchange gases
within air spaces in soil
Why does over-watering kill a plant?
Cellular Transport
� solutes are moved into plant cells via active transport� central role of proton pumps
� Chemiosmosis—using ATP → transmembrane proton gradient → energy to do work (i.e. uptake of ions)
(Uses ATP to pump H+ out of cell = stored energy)
Figure 36.2, pg. 750
Movement of water in plantsWater uptake/loss in plant cells is based
on water potential(potential energy that causes water to move)
� osmosis through aquaporins
(passive transport proteins)� water flows from high potential to
low potential (towards more negative)
Ψ= Ψs + Ψp
s = solute potential
(adding solute lowers Ψ)
p = pressure potential
(increasing pressure raises Ψ)
Now, a plant example:
Figure 36.4, pg. 752
Where
does th
e
pre
ssure
com
e fro
m?
Short Distance Lateral
(cell to cell) Transport
3 plant compartments:
� cell wall
� cytoplasm/cytosol
� vacuole
Transmembrane route� repeated crossing of plasma membranes� slowest route but offers more control
Symplastic route� move from cell to cell within cytosol� use plasmodesmata
Apoplastic route
� move through connected cell wallwithout crossing cell membrane
� fastest route but never enter cell
Long Distance Transport
Bulk flow
• movement of fluid driven by pressure– flow in xylem tracheids & vessels
• negative pressure– transpiration creates negative pressure pulling xylem
sap upwards from roots
– flow in phloem sieve tubes• positive pressure
– loading of sugar from photosynthetic leaf cells generates high positive pressure pushing phloem sap through tube
II. Absorption of water and minerals
by rootsMineral uptake by root hairs
� dilute solution in soil� active transport pumps
� this concentrates solutes (~100x) in root
cells
Water uptake by root hairs� flow from high H2O
potential to low H2O potential
� creates root pressure
Route to xylem can be apoplastic or symplastic
Hooray
for root
hairs!
Endodermis � controls the route of water into the
root � cell layer surrounding vascular
cylinder of root
� lined with impervious Casparianstrip (waxy)
� forces fluid through selective cell membrane & into symplast
Mycorrhizae increase absorption
Symbiotic relationship between fungi & plant
� greatly increases surface area for absorption of water & minerals� increases transport to host plant
Ahh, symbiosis…
III. Ascent of Xylem “Sap”Transpiration pull generated by water leaving leaf (transpiration)
Rise of Water in a
Tree
Transpiration pull
� adhesion & cohesion� H bonding
(brings water & minerals up to shoot)
Water potential (mostly due to Ψp)
� high in soil → low in leaves
What is the source of energy for transpiration?
IV. Control of Transpiration by
Stomata
Photosynthesis-Transpiration Compromise—
� 600 g water transpired : 1 g CO2 assimilated into organic material
How Stomata Open and CloseIon mechanism
� uptake of K+ ions by guard cells�proton pumps (chemiosmosis)� water enters by osmosis (due to lower ψ)� guard cells become turgid
� loss of K+ ions by guard cells
� water leaves by osmosis (due to higher ψ)� guard cells become flaccid
How Stomata Open and CloseMicrofibril mechanism
� guard cells attached at tips� microfibrils in cell walls
� osmosis in → turgid cell → increase in length > increase in width→ stomate opens
� and vice versa
Cues for Stomatal Opening
• Light (blue light receptors)
• Depletion of CO2 (from photosynthesis)
• Circadian rhythms (24 hour cycles)
Cues for Stomatal Closing
• Water deficiency
• High temperatures
• Circadian rhythms (24 hour cycles)
V. Transport of Sugars in PhloemLoading of sucrose into phloem
� flow through symplast via plasmodesmata or apoplast� active cotransport of sucrose with H+ protons
� proton pumps (chemiosmosis)
Pressure Flow in aSieve Tube
Water potential gradient
� “source to sink” flow� direction of transport in phloem is variable
� sucrose flows into phloem sieve tube decreasing H2O potential
� osmosis of H2O from xylem vessels� increase in pressure due to
increase in H2O causes flow� higher pressure at source than
at sink
Figure 36.17, pg. 764
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