TRANSPORT IN PLANTS
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2 types of transport tissue:
XYLEM & PHLOEM
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Vascular
Cambium
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creates new
xylem cells on
the inside, and
new phloem
cells on the
outside.
XYLEM & PHLOEM anatomy:
sieve cells
companion
cells
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plasmodesmata
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XYLEM~ moves water and minerals
• Acts like a bundle of
straws
• functions via.
negative pressure
• “sucking”
• http://www.youtube.
com/watch?v=Ir9bm
3fli90
XYLEM cells
(transport water & dissolved minerals)
~2 kinds of cells:
Tracheid = single cell,
elongated, tapered ends (wall
composed of cellulose)
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* Pits (holes) between and
surrounding these~
* Dead & hollow at maturity
* Conifers, have only these
XYLEM cells~ 2 kinds:
Vessel Element = numerous
elongated cells make up this
tube. (typically smaller in length
than tracheids, but wider in
diameter)
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* Have “pits” or openings on their
ends and are stacked to create
one long “pipe”
* Carry more water than tracheids
* Dead & hollow at maturity
* Conifers do not have these
XYLEM ~ water movement b/c
of 2 forces:
1) Transpirational
pull
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- via. negative
pressure as water is
evaporated out of the
stomata
http://www.youtube.com/watch?v=EKyH
banzYnI&feature=related
XYLEM ~ water movement b/c
of 2 forces:
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1) Transpirational
pull
- via. negative
pressure
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2) Root Pressure
- more negative water potential
in the root as compared to the
outside soil
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2) Root Pressure
- water moves from the soil… into
the root
PHLOEM~ has 2 types of cells:
companion
cells
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sieve tube
cells
PHLOEM
companion
cells:
nucleus
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sieve tube
cells:
no nucleus
both types are
alive
How do solutions move through
the
..
PHLOEM ~ Ernst Munch 1933
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PRESSURE
FLOW
HYPOTHESIS
How do solutions move through
the
..
PHLOEM ~ Ernst Munch 1933
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PRESSURE
FLOW
HYPOTHESIS
Solutes move through
plants as a result of
pressure gradients, not
negative pressure
(aka. pulling)
• SOURCE = area of
excess sugar supply
aka. leaf in the
summer
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• SINK = area of storage
aka. roots in the summer &
winter
• SINK = area of
storage aka. leaf are
in the spring
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• SOURCE = area of excess
sugar supply aka. roots in
the spring.
TRANSPORT OF SAP
(Photosynthesizing
cell)
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sugar
Phloem
companion
cell
source
1) @ SOURCE, sucrose moves from mesophyll cells
into companion cells via. active transport
TRANSPORT OF SAP
Photosynthesizing
cell
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sugar
Phloem
companion
cell
source
2) Companion cells move sap into SIEVE TUBE
CELLS via. active transport
Mineral absorption
• Proton pumps
– active transport (ATP) of H+ ions out of cell
• chemiosmosis
• H+ gradient
– creates membrane
potential
• difference in charge
• drives cation uptake
ATP
H+
H+
H+
H+
H+
H+
H+
H+
H+
– creates gradient
• cotransport of other
solutes against their
gradient
sugar
TRANSPORT OF SAP
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sugar
Phloem
companion
cell
source
3) Sugar accumulates in SIEVE TUBES, thus decreasing
(lowering) water potential (aka. making more negative)
TRANSPORT OF SAP
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H2O
sugar
Phloem
companion
cell
source
4) WATER then, moves in from the XYLEM (as a result)… thus
increasing turgor pressure in SIEVE TUBES -->
Maple
sugaring
TRANSPORT OF SAP
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H2O
sugar
Phloem
companion
cell
source
5) thus moving sugar to areas of lower pressure (the
sink).
TRANSPORT OF SAP
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sugar
Phloem
companion
cell
source
6) @ sink, sugar is unloaded from sieve tubes, thus
INCREASING water potential...
TRANSPORT OF SAP
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H2O
sugar
Phloem
companion
cell
source
7) Water moved back into xylem… and UP via.
negative pull
Pressure Flow Hypothesis
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