1 Lecture #5 – Plant Transport. 2 Key Concepts: The importance of water Water potential: Ψ = P -...

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Lecture #5 – Plant Transport

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Key Concepts:

• The importance of water

• Water potential: Ψ = P - s

• How water moves – gradients, mechanisms and pathways

• Transpiration – water movement from soil to plant to atmosphere

• The pressure flow model of phloem transport

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Diagram – movement of water through a tree

WHY WATER???

• Required for metabolism and cytoplasm

• Nutrients are taken up and transported in water-based solution

• Metabolic products are transported in water-based solution

• Water movement through the plant affects gas exchange and leaf T

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Water Potential (Ψ):• Controls the movement of water• A measure of potential energy• Water always moves from an area of HIGH

water potential to an area of LOW water potential

• Controlled by physical pressure, solute concentration, adhesion of water to cell structures and to soil particles, temperature, and gravity

Ψ = P - s

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Diagram – water moves from high water potential to low water potential, sometimes toward a negative value; same next 3 slides

7minus 4 is MORE NEGATIVE than minus 1

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High

Low

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Diagram – water potential is universal, including with waterfalls

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Water Potential (Ψ):• Controls the movement of water• A measure of potential energy• Water always moves from an area of HIGH

water potential to an area of LOW water potential

• Controlled by physical pressure, solute concentration, adhesion of water to cell structures and to soil particles, temperature, and gravity

Ψ = P - s

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P – Pressure Potential

• By convention, set to zero in an open container of water (atmospheric pressure only)

• In the plant cell, P can be positive, negative or zeroA cell with positive pressure is turgidA cell with negative pressure is plasmolyzedA cell with zero pressure is flaccid

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Turgid P > 0

Plasmolyzed P < 0

Flaccid P = 0

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Micrograph – photosynthetic cells: turgid on left, plasmolyzed on right; same on next 3 slides

What are the little green things???

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Turgid Plasmolyzed

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Critical Thinking

• How can you tell this tissue was artificially plasmolyzed?

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Critical Thinking

• How can you tell this tissue was artificially plasmolyzed?

• Observe the cell on the far right – it is still turgid

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Image – turgid plant on left, plasmolyzed on right

Crispy means plasmolyzed beyond the permanent

wilting point

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s – Solute Potential• s = zero for pure water

Pure H2O = nothing else, not a solution

• Adding solutes ALWAYS decreases the potential energy of waterSome water molecules now carry a load – there

is less free water

s

s s

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Diagram – effect on water potential of adding salts to solutions separated by semi-permeable membrane

Remember, Ψ = P – s

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Ψ = P – s

Pressure can be +, -, or 0

Solutes always have a negative effect

Simplest way to calculate Ψ is by this equation

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Flaccid cell in pure water – what happens???…..what do you know???

….what do you need to know???

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Flaccid cell in pure water – what happens???

Ψ = ?

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Ψ = ?

P = ?.......s = ?

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Ψ = ?

P = 0.......s = about 0.7 MPa

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Ψ = -0.7 MPa

P = 0.......s = about 0.7 MPa

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Flaccid cell in pure water – what happens???…..what do you know???

Ψ = ?

….what do you need to know???

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Ψ = ?

P = ?.......s = ?

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P = 0.......s = 0

Ψ = ?

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Ψ = 0 MPa

P = 0.......s = 0

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Ψ = 0 MPa

Ψ = -0.7 MPa?

Will water move into the cell or out of the cell???

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Ψ = 0 MPa

Water moves from high Ψ to low Ψ

Ψ = -0.7 MPa

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Then what happens???

Ψ = 0 MPa

Ψ = -0.7 MPa

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Ψ = 0 MPa

Ψ = -0.7 MPa

P in cell goes up…..

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Ψ = 0 MPa

Dynamic equilibrium!

Ψ = 0 MPa

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Water Movement

• Osmosis – the diffusion of water one molecule at a time across a semi-permeable membraneControlled by both P and s

• Bulk Flow – the movement of water in bulk – as a liquidControlled primarily by P

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Diagram – osmosis across a semi-permeable membrane; next slide also

Osmosis

Critical Thinking: Where does water move by osmosis in plants???

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Osmosis

Critical Thinking: Where does water move by osmosis in plants???

Cell membrane is semi-permeable

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Water Movement


• Bulk Flow – the movement of water in bulk – as a liquidControlled primarily by P

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Water Movement


• Bulk Flow – the movement of water in bulk – as a liquidControlled primarily by P – no membrane, no

solute gradient!

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Critical Thinking

• Where does water move by bulk flow in plants???

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Critical Thinking

• Where does water move by bulk flow in plants???

• Primarily in the xylem, also in phloem and in the cell walls

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Diagram – apoplast, symplast and transmembrane pathways; same on next slide

Cell WallCell Membrane

Cytoplasm

Routes of water transportsoil root stem leaf atmosphere

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Cell WallCell Membrane

Cytoplasm

Routes of water transportsoil root stem leaf atmosphere

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Diagram – Casparian strip; same on next 2 slides

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The Casparian Strip is a band of suberin in the transverse and radial (but not the tangential) walls of

the endodermis cells

Water CANNOT PASS THROUGH the Casparian Strip

Water must GO AROUND the Casparian Strip – through the tangential face of the endodermis

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The Casparian Strip is a band of suberin in the transverse and radial (but not the tangential) walls of

the endodermis cells

Water CANNOT PASS THROUGH the Casparian Strip

Water must GO AROUND the Casparian Strip – through the tangential face of the endodermis

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Critical Thinking

• Apoplast water is forced into the symplast at the Casparian Strip

• What does this mean for the water???

• What is the function of the Casparian Strip???

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Critical Thinking



• It has to cross a cell membrane (easy for water!)


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Critical Thinking



• It has to cross a cell membrane (easy for water!)


• Solute uptake is regulated at the membrane!!!

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Diagram – review of membrane transport proteins

Membrane Transport(review in text if necessary)

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Water is on the move

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Diagram – transpiration

Transpiration

• Movement of water from soil plant atmosphere

• Controlled by HUGE water potential gradient

• Gradient controlled by P Very little s contribution

Ψ = P - s

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Micrograph – stomata

Stomates are the Valves:as long as the stomata are open, water

will move through the plant

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Diagram – transpiration

Transpiration

• Movement of water from soil plant atmosphere

• Controlled by HUGE water potential gradient

• Gradient controlled by P Very little s contribution

Ψ = P - s

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Solar Heating Drives the Process

• Air is dry because of solar heatingThe air molecules bounce around more which

causes air masses to expand Warm air has tremendous capacity to hold

water vapor

• Warm, dry air dramatically reduces the Ψ of the atmosphere

• Daytime gradient is commonly 30+ MPa

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Critical Thinking

• Why do we have life on this planet and not the others in our solar system???

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Critical Thinking


• Liquid water!

• Why do we have liquid water???

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Critical Thinking


• Liquid water!

• Why do we have liquid water???

• 3rd rock from the sun!The Goldilocks Zone – not too hot, not too coldPlus, we have enough gravity to hold our

atmosphere in placeIt’s our atmosphere that holds the warmth

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Model – our solar system

Life is Random

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Solar Heating Drives the Process

• Air is dry because of solar heatingThe air molecules bounce around more which

causes air masses to expand Warm air has tremendous capacity to hold

water vapor

• Warm, dry air dramatically reduces the Ψ of the atmosphere

• Daytime gradient is commonly 30+ MPa

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Atmospheric water

potential (MPa)

Relative Humidity (%)

0 10080

- 200

- 30

0

asymptotic

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Critical Thinking

• Under what conditions does atmospheric water potential approach zero???

Atmospheric water

potential (MPa)


0 10080

- 200

- 30

0

asymptotic

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Critical Thinking

• Under what conditions does atmospheric water potential approach zero???

• Only in the pouring rain

Atmospheric water

potential (MPa)


0 10080

- 200

- 30

0

asymptotic

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Gradient is HUGE

• Pressure plumbing ~ 0.25 MPa• Fully inflated car tire ~ 0.2 MPa

• Only in the pouring rain does atmospheric Ψ approach zero

• Soil Ψ is ~ zero under most conditions

• Remember – gradient is NEGATIVE• Water is pulled into plant under TENSION

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Gradient is HUGE





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Atmospheric water

potential (MPa)


0 10080

- 200

- 30

0

asymptotic

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Gradient is HUGE





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Diagram – transpiration gradient from soil to atmosphere

The tension gradient is extreme, especially

during the day

Sunday, 1 October 20068 am – RH = 86%Noon – RH = 53%4 pm – RH = 36%8 pm – RH = 62%

5am, 23 September – 94% in light rain

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Atmospheric water

potential (MPa)


0 10080

- 200

- 30

0

asymptotic

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Critical Thinking

• Tension is a strong force!

• Why doesn’t the water stream break???

• Adhesion and cohesion

• Why doesn’t the xylem collapse???

• Lignin!

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Critical Thinking





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Critical Thinking





• Lignin!!!

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Diagram – transpiration gradient plus pathways

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Table – water use by various crops

One hectare (2 football fields) of corn transpires about 6 million liters of water per growing season – the equivalent of 2’ of water over the entire hectare…

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Transpiration is a powerful force!

• A single broadleaf tree can move 4000 liters of water per day!!! (about 1000 gallons)

• If humans had to drink that much water we would drink about 10 gallons per day!

• Transpiration accounts for 90% of evapotranspiration over most terrestrial surfaces

• Plants are the most important component of the hydrological cycle over land!!!

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Image – deforestation snaps water cycle and also results in erosion

Tropical deforestation is leading to ecological and social disaster

• Poverty, famine and forced migration

• 250 million victims of ecological destruction – that’s about how many people live in the US!….and just a tiny fraction of the world’s

impoverished people

Panama

You can help change this!!!

Guatemala

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Tropical deforestation is leading to ecological and social disaster

• Poverty, famine and forced migration

• 250 million victims of ecological destruction – that’s about how many people live in the US!….and just a tiny fraction of the world’s

impoverished people

Panama

You MUST help change this!!!

Guatemala

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Social JusticeI’m not angry with you……

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Social JusticeBut I do expect

you to DO something

!!!

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Transpiration is a Natural Process

• It is a physical process that occurs as long as the gradient exists and the pathway is open

• Under adequate soil moisture conditions the enormous water loss is not a problem for the plant

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Critical Thinking

• What happens when soil moisture becomes limited???

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Critical Thinking


• Water stress causes stomata to close

• What then???

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Critical Thinking


• Water stress causes stomata to close

• What then???

• Gas exchange ceases – no CO2 = no photosynthesis

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What happens when soil moisture becomes limited???

• Water stress causes stomata to close• Closed stomata halt gas exchange

P/T conflict P/T compromise

• Stomata are generally open during the day, closed at nightAbscissic acid promotes stomata closure daily, and

under water stress conditionsOther structural adaptations limit water loss when

stomata are openOther metabolic pathways (C4, CAM) limit water loss

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Micrograph – turgid guard cells; same next 4 slides

Normally, stomata open during the day and close at night in response to changes in K+

concentration in stomata guard cells

• High [K+] does what to Ψ???

• K+ accumulation is triggered by increased light, low carbon dioxide, circadian rhythms

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• High [K+] lowers water potential in guard cells

• What does water do???




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• Water enters, cells swell and buckle




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• Pore opens




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• Pore opens• Reverse at night

closes the pores




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Diagram – open and closed stomata

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Diagram – hormone mediated stomatal opening and closing

Abscissic acid is the hormone that mediates this response

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Diagram – spoke-like orientation of cellulose microfibrils

Cellulose orientation determines shape of turgid cells

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What happens when soil moisture becomes limited???

• Water stress causes stomata to close• Closed stomata halt gas exchange

P/T conflict P/T compromise

• Stomata are generally open during the day, closed at nightAbscissic acid promotes stomata closure daily, and

under water stress conditionsOther structural adaptations limit water loss when

stomata are openOther metabolic pathways (C4, CAM) limit water loss

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Micrograph – location of stomatal gradient

This is the gradient that

counts

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Images – structural adaptations to dry environments

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Images and diagrams – metabolic adaptations to dry environments

Spatial separation helps C4 plants be

more efficient in hot climates

Temporal separation does the same for

CAM plants

Both use an enzyme that can’t fix O2

to first capture CO2

Both adaptations allow photosynthesis

to proceed with stomata largely

closed during the day

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Phloem Transport

• Most of phloem sap is water (70% +)

• Solutes in phloem sap are mostly carbohydrates, mostly sucrose for most plant species

• Other solutes (ATP, mineral nutrients, amino acids, hormones, secondary metabolites, etc) can also be translocated in the phloem

• Phloem transport driven by water potential gradients, but the gradients develop due to active transport – both P and s are important

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Diagram – pressure flow model of phloem flow; this diagram is repeated throughout this section

The Pressure Flow Model For

Phloem Transport

• Xylem transport is uni-directional, driven by solar heating

• Phloem flow is multi-directional, driven by active transport – source to sink

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Phloem Transport

• Sources can be leaves, stems or roots

• Sinks can be leaves, stems, roots or reproductive parts (especially seeds and fruits)

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Phloem Transport

• Sources and sinks vary depending on metabolic activity, which varies daily and seasonally

• Most sources supply the nearest sinks, but some take priority

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Diagram – the transport proteins that actively transport sucrose into the phloem cells from the leaf cells

Active transport (uses ATP) builds high sugar concentration in sieve

cells adjacent to source

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Phloem Transport

• High [solute] at source end does what to Ψ???

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Critical Thinking

• Remember the water potential equation

Ψ = P - s

• What happens to Ψ as s increases???

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Critical Thinking


Ψ = P - s

• What happens to Ψ as s increases???

• Water potential is reduced

• This is what happens at the source end of the phloem

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Phloem Transport

• High [solute] at source end decreases Ψ


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Critical Thinking


Ψ = P - s

• What does water do when Ψ decreases???

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Critical Thinking


Ψ = P - s

• What does water do when Ψ decreases???

• Water moves toward the area of lower water potential

• This is what happens at the source end of the phloem

• Where does the water come from???

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Critical Thinking

• Remember the water potential equationΨ = P - s

• What does water do when Ψ decreases???• Water moves toward the area of lower water

potential• This is what happens at the source end of

the phloem• Where does the water come from???• The adjacent xylem – remember structure

and function are related!

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Phloem Transport


• Water moves into the source end of the phloem

• What does this do to P at the source end?

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Critical Thinking

• What will happen to water pressure in any plant cell as water moves in???

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Critical Thinking


• It increases

• Why???

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Critical Thinking


• It increases

• Why???

• The cell wall limits expansion – it “pushes back”

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Phloem Transport


• Water moves into the source end of the phloemThis increases

the pressure

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Phloem Transport

• Increased pressure at source end causes phloem sap to move to any area of lower Ψ = sinks

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Phloem Transport

• At sink end, the sugars are removed by metabolism, by conversion to starch, or by active transport

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Phloem Transport

• What then happens to the Ψ at the sink end of the phloem???

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Critical Thinking


Ψ = P - s

• What happens to Ψ as s decreases???

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Critical Thinking


Ψ = P - s

• What happens to Ψ as s decreases???

• Water potential is increased

• This is what happens at the sink end of the phloem

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Phloem Transport

• Ψ goes up at the sink end of the phloem


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Critical Thinking


Ψ = P - s

• What does water do when Ψ increases???

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Critical Thinking


Ψ = P - s

• What does water do when Ψ increases???

• Water moves away from the area of higher water potential

• This is what happens at the sink end of the phloem

• Where does the water go???

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Critical Thinking

• Remember the water potential equationΨ = P - s

• What does water do when Ψ increases???• Water moves away from the area of higher

water potential• This is what happens at the sink end of the

phloem• Where does the water go???• The adjacent xylem – remember structure

and function are related!

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Phloem Transport

• Ψ goes up at the sink end of the phloem

• Water leaves the phloem at the sink end, thus reducing Ψ

• Adjacent xylem provides and accepts the water

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Phloem Transport

• Thus the phloem sap moves – from source to sinkSome xylem

water is cycled into and out of the phloem in the process

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Phloem Transport

• Active transport is always involved at the source end, but only sometimes at the sink end

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Micrograph – sieve cells; same next slide

Critical Thinking

• What about the structure of the sieve cells facilitates the movement of phloem sap???

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Critical Thinking

• What about the structure of the sieve cells facilitates the movement of phloem sap???

• The open sieve plate

• The lack of major organelles

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Phloem Transport

Questions???

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Key Concepts: Questions???

• The importance of water

• Water potential: Ψ = P - s

• How water moves – gradients, mechanisms and pathways

• Transpiration – water movement from soil to plant to atmosphere

• The pressure flow model of phloem transport

1 Lecture #5 – Plant Transport. 2 Key Concepts: The importance of water Water potential: Ψ = P -...

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Transcript of 1 Lecture #5 – Plant Transport. 2 Key Concepts: The importance of water Water potential: Ψ = P -...