Transcript Transpiration

Transpiration
Transport
Overview
 Plants need CO2, Sunlight
and H2O in the leaves
ONLY H2O needs to be
transported to the leaves
 CO2 gets in via stomata
 Water is most of the mass
of a plant
 Carbon accounts for most
of the mass of a dried plant
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Fundamental Forces
 Physical forces drive transport of materials in plants
 Movement by concentration gradient
-- Movement due to random molecular motion
-- Diffusion or facilitated diffusion for things other than water
-- Osmosis is for water
-- Solutes move independently of water concentration
 Movement by pressure gradient
-- Bulk Flow – movement of water and solvents due to
pressure gradient
Slide 3 of 32
3 Types of Transport in Vascular Plants
1. Transport of water & solutes by individual cells
-- ALWAYS accomplished by diffusion
-- Example: from soil to root hair cell
-- Example 2: from one tracheid to another tracheid
2. Short-Distance transport of substances between cells at the
tissue level
-- ALWAYS accomplished by diffusion
3. Long-distance transport within the xylem & phloem among the
entire plant
-- ALWAYS accomplished by bulk flow (pressure gradient)
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Individual Cell Movement
 Passive Transport – movement down a gradient

Does NOT require energy
 Simple diffusion, osmosis or facilitated diffusion
 Active Transport – Movement against a
electrochemical gradient

Requires energy
 Most solutes must use transport proteins

Aquaporin – channel (transport) protein for
water
Slide 5 of 32
Water Potential (Ψ)
 Water moves from High concentration (of water, not solute
concnetration) to Low concentration via osmosis
 Water mover from high pressure to low pressure via bulk flow
 Water potential is the combined effect of

Solute Concentration
 Physical Pressure
 Ψ = Ψs + Ψp
 Conclusion: water moves from high water potential to low water
potential
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Solute Potential (Ψs)
 Solute potential (Ψs) is proportional to the number of
dissolved solute particles
 Also called Osmotic Potential
 Ψs = -iCRT
 Ψs of water = 0
 Addition of solute  Decrease in water potential
 More solute = less water (realtively) = lower water potential
 Ψs ≤ 0
Slide 7 of 32
Pressure Potential (Ψp)
 Pressure Potential (Ψp)
 Physical pressure on a solution
 Created by placing physical pressure (+) or by vacuum/sucking
(-)
 Water is usually under a positive pressure potential
 Turgor pressure – when cell contents press the plasma
membrane against the cell wall
 Drying out = Negative pressure potential
Slide 8 of 32
Water Potential Examples
Slide 9 of 32
Short-Distance Transport
Symplast
Cytoplasmic continuum (called
Symplast) consists of the cytosol
of cells and the plasmodesmata
connecting the cytosols.
 Crosses membrane early in the
process
Apoplast
 Continuum of cell walls +
extracellular spaces
 Only crosses a membrane at
endodermis
Transmembrane
 Self-evident & highly inefficient
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Long Distance Transport
 Accomplished by Bulk Flow
 Water movement from regions of high pressure to regions of
low pressure
 Movement in both xylem and phloem is driven by pressure
differences between opposite ends of vessels or sieve tubes.
 Diffusion is a poor driver over long distances (roots to
leaves)
 In xylem, water & minerals travel by negative pressure
 Transpiration and root push
 In phloem, hydrostatic pressure forces materials down
Slide 11 of 32
Follow a molecule of water or
mineral…
Roots & Water Absorption
 Root hairs = absorption of water

Root hairs increase surface area for absorption
 Hydrophilic cell walls absorbs soil solution (water and minerals)
 Mycorrhizae are important for absorption as well
 Root epidermis  cortex  vascular cylinder (xylem)

Called Lateral Transport (Short Distance Transport)
 To rest of plant via xylem
Slide 13 of 32
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Casparian Strip
 In the endodermis
 Waxy material encircling the cells of the
endodermis
 Ensures that any water or solutes must
pass through a plasma membrane
before entering xylem
 Impedes apoplastic transfer
 Critical control point
 Again, plasma membrane controls what
can enter the xylem
Slide 15 of 32
Xylem moves vertically, how?
 After water or minerals gets past the endodermis, most
will find its way to the xylem
 BULK FLOW, not concentration differences drives this
transport
 2 PRESSURE differences drive this
 Root Pressure or root push
 Transpiration (much more important)
Slide 16 of 32
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Root Pressure
 Water diffusing into the root cortex = positive pressure
 This pressure forces fluid up the xylem
 Weak force – can only propel fluids up a couple of feet
Slide 18 of 32
Transpiration
 Your book calls this: transpiration-cohesion-tension
mechanism
 In leaves, water is lost through stomata
 Why? Lower water pressure in air than in leaves
 Water is drawn up in to this area of negative pressure
 Water molecules pull up other water molecules
 Cohesion – water on water action
 Adhesion – water to cell wall action
 Via Hydrogen bonds
Slide 19 of 32
Transpiration (Page 2)
 Transmitted all the way from Leaves to the soil solution
 Again, due to PRESSURE differential, not concentration
 Small diameter of vessel elements and tracheids
increases adhesion
 Transpiration is ultimately due to stomata
 Necessary water loss for CO2 uptake and O2 removal
 If stomata closed, then less photosynthesis and plant
may overheat
Slide 20 of 32
Transpiration (Page 3)
 1 molecule of H2O evaporates due to transpiration,
another molecule is drawn from the roots to replace it.
 Factors that influence transpiration
 High humidity = DECREASE transpiration
 Wind = INCREASE transpiration
 Increasing light intensity = INCREASE transpiration
 Close stomata = NO transpiration
Slide 21 of 32
 90% of water lost
by plants is
through stomata
 Stomata account
for 1% of leaf
surface area
 Guard cells
control opening
& closing of
stomata
Slide 22 of 32
Slide 23 of 32
Phloem Translocation
 Photosynthetic products (Phloem Sap) are translocated
through the phloem
 Translocation literally means “movement from place to
place”
 30% of phloem sap is sucrose, but it can be any
assimilate form of sugars (G3P)
 Translocation is NOT a one-way transport mechanism
 Sieve tube elements carry sugar from source to sink
 Source – leaves (net producer of sugar)
 Sink – roots (net consumer of sugar)
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Slide 25 of 32
Sucrose is added at the sugar source
(leaves)
Sucrose first moves in by
diffusion
H2O follows
Once sucrose concentration is
too high, an electrochemical
gradient is created to move
sucrose into phloem by
cotransport
Decreases water potential in phloem,
so creates positive pressure
Phloem sap is propelled away from
the source
Where sugar is used, negative
pressure is found
Used in respiration
Converted to starch or cellulose
Slide 26 of 32
 Sugar loading into the sieve-tubes is necessary prior to any bulk flow
Movement through the sugar source cells can be either apoplastic or
symplastic
Symplastic movement occurs via plasmodesmata
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Where sugar is used = sink
Concentration in sink is lower
than in phloem
So sugar concentration
gradient = diffusion of sugar and
then water out of the phloem
So lower pressure at the sink
Sugar may be
-- Used in respiration
-- Converted to starch
Slide 28 of 32
Pressure Flow Hypothesis
 Also called mass flow (bulk flow) hypothesis
 Phloem sap moves from source to sink at 1 m/hr, which
is far faster than diffusion or cytoplasmic streaming
 So it is the PRESSURE differential that moves phloem
sap
 Pressure builds at source
 Pressure falls at sink
Slide 29 of 32
Sucrose Loading
 From cell to cell through
the plasmodesmata
(Symplast)
OR Along cell walls
(apoplast)
 Surface membranes of
companion cells actively
pump sucrose into the
sieve tube’s cytoplasm.
Slide 30 of 32
The accumulation of sucrose
and other solutes, such as
amino acids, in sieve elements
lowers the water potential so
that water diffuses in by
osmosis from adjacent cells
and from the xylem.
This creates pressure in
the sieve elements
causing the liquid
(phloem sap) to flow out
of the leaf.
Slide 31 of 32
Sucrose is unloaded at
sinks.
This is taken up by the cells
and is respired or stored as
starch.
This reduces the
concentration of phloem
sap and lowers the
pressure, so helping to
maintain a pressure
gradient form source to
sink so the sap keeps
flowing in the phloem.
Slide 32 of 32