Download WATER TRANSPORT in PLANTS

TRANSPORT in PLANTS
What must be transported in plants?

H2O & minerals

Sugars

Gas Exchange
Transport of Water & Minerals



Occurs in the xylem
H2O is moved from
root to leaves
Transpiration  loss of
H2O from leaves (thru
stomata)

Processes


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Evaporation
Cohesion
Adhesion
Negative Pressure
Transport of Sugar


Occurs in the phloem
Bulk Flow



Calvin Cycle (Dark
Rxns) in leaves loads
sugar into the
phloem
Positive Pressure
Movement
Source (where sugar
is made) to Sink
(where sugar is
stored/consumed)
Gas Exchange

Photosynthesis



CO2 in
O2 out
Transport occurs through
stomata

Surrounded by guard
cells

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Control opening & closing
of stomata
Respiration



O2 in
CO2 out
Roots exchange gases w/
air spaces in the soil
Why can over-watering kill a
plant?
Transport in Plants

Three main physical forces that fuel
transport in plants:


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Cellular
 Gases from the environment into plant cells
 H2O & minerals into root hairs
Short-Distance Transport
 Cell to cell
 Moving sugar from leaves into phloem
Long-Distance Transport
 Moving substances through the xylem &
phloem of a whole plant
Cellular Transport

Passive


Diffusion down a concentration gradient
Occurs faster w/ proteins

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Carrier Proteins (facilitated diffusion)
Active

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Requires energy
Proton Pump
Pumps H+ out of a cell
 Creates a proton gradient (stored energy)
 Generates a membrane potential


Used to transport many solutes
Cellular Transport –Active Transport
Cellular Transport -Water Potential


Combined effects of solute concentration
& physical pressure
Moves from high H2O potential to a low
H2O potential



Inversely proportional to solute concentration
 Adding solutes – Lowers water potential
Directly proportional to pressure
 Raising pressure- Raises water potential
Negative pressure (tension) decreases water
potential
Cellular TransportWater Potential

H2O potential =
pressure potential +
solute potential
A) adding solutes reduces
H2O potential
B & C) adding pressure,
increases H2O
potential
D) negative pressure
decreases H2O potential
Short-Distance Transport

Movement from cell to
cell by…

Transmembrane




Crosses membranes &
cell walls
Slow, but controlled
Called the apoplastic
route
Cytosol (cytoplasm)


Plasmodesmata
junctions connect the
cytosol of
neighboring cells
Called the symplast
route
Long-Distance Transport

Bulk Flow



Movement of a fluid driven by pressure
Xylem: tracheids & vessel elements
 Negative pressure
 Transpiration creates negative pressure by
pulling xylem up from the roots
Phloem: Sieve tubes
 Positive pressure
 Loading of sugar at the leaves generates a
high positive pressure, which pushes phloem
sap thru the sieve tubes
Four Basic Transport Functions
1)
2)
3)
4)
Water & Mineral Absorption of
Roots
Transport of Xylem Sap
Control of Transpiration
Translocation of Phloem Sap
Water & Mineral Absorption

Root Hairs


Mineral Uptake by Root Hairs



Increase surface area
Dilute solution in the soil
Active Transport Pumps
 May concentrate solutes up to 100X in the
root cells
Water Uptake by Root Hairs


From high H2O potential to low H2O potential
Creates root pressure
Water and Mineral Absorption –
Root Structure
DICOT ROOT
MONOCOT ROOT
Water and Mineral Absorption –
Water Transport in Roots
Apoplastic or
symplastic
Until the
endodermis
Is reached!!
Water and Mineral Absorption –
Control of Water & Minerals in the Root

Endodermis

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Surrounds the stele
Selective passage of
minerals
Freely enters via the
symplastic route
Dead end via the
apoplastic route
 Casparian Strip


Waxy material
Allows for the
preferential transport
of certain minerals into
the xylem
Water & Mineral Absorption &
Mycorrhizae
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Symbiotic
relationship b/w
fungi & plant
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Symbiotic fungi
increase surface area
for absorption of
water & minerals
Increases volume of
soil reached by the
plant
Increases transport of
water & minerals to
host plant
Transport of Xylem Sap: Pulling

TRANSPIRATION-COHESION-TENSION MECHANISM

Transpirational Pull

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Cohesion

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b/w H2O molecules
causes H2O to form a continuous column
Adhesion


Drying air makes H2O evaporate from the stomata
of the leaves
H2O molecules adhere to the side of the xylem
Tension

As H2O evaporates from the leaves, it moves into
roots by osmosis
Transport of Xylem Sap: Pushing

Root Pressure –
pushes H2O up xylem

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Due to the flow of H2O
from soil to root cells
at night when
transpiration is low
Positive pressure
pushes xylem sap into
the shoot system
 More H2O enters
leaves than exits (is
transpired) at night
Guttation - H2O on
morning leaves
Transport of Xylem SapAscent of H2O in Xylem: Bulk Flow
Due to three main
mechanisms:
 Transpirational Pull

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Water potential


Adhesion & cohesion
High in soil  low in
leaves
Root pressure


Upward push of
xylem sap
Due to flow of H2 O
from soil to root cells
Control of Transpiration:
Gas Exchange

Stomate Function

Compromise b/w photosynthesis &
transpiration

Amount of transpiration (H2O loss) must be
balanced with the plant’s need for
photosynthesis

OPEN
STOMATA
Leaf may transpire more than its weight in water
every day!
CLOSED
STOMATA
Control of TranspirationLeaf Structure
Control of Transpiration Photosynthesis vs. Transpiration

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Open stomata allow for CO2 needed for
photosynthesis to enter
There is a trade-off…..


Plant is losing water at a rapid rate
Regulation of the stomata allow a plant to
balance CO2 uptake with H2O loss
What types of environmental
conditions will increase transpiration?
Control of Transpiration –
Stomatal Regulation

Microfibril Mechanism
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Guard cells attached at tips
Microfibrils elongate & cause cells to arch open
Microfibrils shorten & cause cells to close
Ion Mechanism
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Uptake of K+ by guard cells during the day
 H2O potential becomes more negative
 H2O enters the guard cells by osmosis
 Guard cells become turgid & buckle open
Loss of K+ by guard cells
 H2O potential becomes more positive
 H2O leaves the guard cells by osmosis
 Guard cells become flaccid & close the stomata
Control of TranspirationStomatal Regulation
Control of Transpiration –
Stomatal Regulation

Three cues that open stomata at
sunrise:

Light Trigger
Blue-light receptor in plasma membrane
 Turns on proton pumps & takes up K+
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Depletion of CO2 in air spaces
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CO2 used up at night by the Calvin Cycle
Internal Clock (Circadian Rhythm)
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Automatic 24-hour cycle
Control of TranspirationAdaptations that Reduce Transpiration
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Small, thick leaves
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Thick cuticle
Stomata on lower leaf
side with depressions
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Reduces surface areato-volume ratio
Depressions shelter the
stomata from wind
May shed leaves during
dry months
Fleshy stems for water
storage
CAM metabolism

Takes in CO2 at night &
can close stomata
during the day
Translocation of Phloem Sap

Phloem Sap


Water & sugar (mostly
sucrose)
Moved through sieve tube
members

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Porous cross walls that allow
sap to move through
Travels in many directions

From source to sink (where
sugar is consumed/stored)
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Source: leaf
Sink: roots, shoots,
stems,& fruits
Translocation of Phloem SapLoading of Sugars

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Flow through the symplast or apoplast in
mesophyll cells into sieve-tube members
Active co-transport of sucrose with H+

Proton pump
Translocation of Phloem SapPressure Flow
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Bulk Flow Movement
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Sugar loaded at the source

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Reduces water potential
Causes H2O to move into sieve-tube
members
Creates a hydrostatic pressure that
pushes sap through the tube
Sucrose is unloaded at the sink
Water moves into xylem & is carried
back up the plant
Phloem Transport
Pressure Flow and
Translocation of Sugars