1 Lecture #5 – Plant Transport Image of waterfall.
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Transcript of 1 Lecture #5 – Plant Transport Image of waterfall.
2
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
3
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
4
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
5
Diagram – water moves from high water potential to low water potential, sometimes toward a negative value; same next 3 slides
10
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
11
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
13
Micrograph – photosynthetic cells: turgid on left, plasmolyzed on right; same on next 3 slides
What are the little green things???
16
Critical Thinking
• How can you tell this tissue was artificially plasmolyzed?
• Observe the cell on the far right – it is still turgid
17
Image – turgid plant on left, plasmolyzed on right
Crispy means plasmolyzed beyond the permanent
wilting point
18
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
19
Diagram – effect on water potential of adding salts to solutions separated by semi-permeable membrane
Remember, Ψ = P – s
20
Ψ = P – s
Pressure can be +, -, or 0
Solutes always have a negative effect
Simplest way to calculate Ψ is by this equation
26
Flaccid cell in pure water – what happens???…..what do you know???
Ψ = ?
….what do you need to know???
30
Flaccid cell in pure water – what happens???
Ψ = 0 MPa
Ψ = -0.7 MPa?
Will water move into the cell or out of the cell???
31
Flaccid cell in pure water – what happens???
Ψ = 0 MPa
Water moves from high Ψ to low Ψ
Ψ = -0.7 MPa
35
Hands On
• Prepare a section of plump celery and stain with T-blue
• Examine and describe• Introduce a drop of salt water• Any change???• Examine the stalk of celery that was in salt
water vs. one that was in fresh water• Explain your observations in your lab notes.
36
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
37
Diagram – osmosis across a semi-permeable membrane; next slide also
Osmosis
Critical Thinking: Where does water move by osmosis in plants???
38
Osmosis
Critical Thinking: Where does water move by osmosis in plants???
Cell membrane is semi-permeable
39
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
40
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 – no membrane, no
solute gradient!
42
Critical Thinking
• Where does water move by bulk flow in plants???
• Primarily in the xylem, also in phloem and in the cell walls
43
Diagram – apoplast, symplast and transmembrane pathways; same on next slide
Cell WallCell Membrane
Cytoplasm
Routes of water transportsoil root stem leaf atmosphere
46
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
48
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???
49
Critical Thinking
• Apoplast water is forced into the symplast at the Casparian Strip
• What does this mean for the water???• It has to cross a cell membrane (easy for
water!)• What is the function of the Casparian
Strip???
50
Critical Thinking
• Apoplast water is forced into the symplast at the Casparian Strip
• What does this mean for the water???• It has to cross a cell membrane (easy for
water!)• What is the function of the Casparian
Strip???• Solute uptake is regulated at the
membrane!!!
53
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
54
Micrograph – stomata
Stomates are the Valves:as long as the stomata are open, water
will move through the plant
55
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
56
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
58
Critical Thinking
• Why do we have life on this planet and not the others in our solar system???
• Liquid water!• Why do we have liquid water???
59
Critical Thinking
• Why do we have life on this planet and not the others in our solar system???
• 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
61
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
63
Critical Thinking
• Under what conditions does atmospheric water potential approach zero???
Atmospheric water
potential (MPa)
Relative Humidity (%)
0 10080
- 200
- 30
0
asymptotic
64
Critical Thinking
• Under what conditions does atmospheric water potential approach zero???
• Only in the pouring rain
Atmospheric water
potential (MPa)
Relative Humidity (%)
0 10080
- 200
- 30
0
asymptotic
65
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
66
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
• 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
69
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
71
Critical Thinking
• Tension is a strong force!• Why doesn’t the water stream break???• Adhesion and cohesion
• Why doesn’t the xylem collapse???• Lignin!
72
Critical Thinking
• Tension is a strong force!• Why doesn’t the water stream break???• Adhesion and cohesion
• Why doesn’t the xylem collapse???
73
Critical Thinking
• Tension is a strong force!• Why doesn’t the water stream break???• Adhesion and cohesion
• Why doesn’t the xylem collapse???• Lignin!!!
75
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…
76
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!!!
77
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
81
Hands On• Examine variegated plant
Water with dye solutionWhat do you expect???
• Set up experiments with white carnationsVary conditions of light, temperature and air flowRe-cut stems and place in dye solution – why?
• Be sure to develop hypotheses• Discuss findings with team and be prepared
to share conclusions with the class
82
Hands On• Work with team to develop hypotheses
about how different species might vary in water transport – rely on locally available plant species, and vary species only (not environmental conditions)
• As a class, develop several hypotheses• Collect plant samples • Set up potometers, record data• Summarize results and discussions in lab
notes
83
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
85
Critical Thinking
• What happens when soil moisture becomes limited???
• Water stress causes stomata to close• What then???
86
Critical Thinking
• What happens when soil moisture becomes limited???
• Water stress causes stomata to close• What then???• Gas exchange ceases – no CO2 = no
photosynthesis
87
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
88
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
89
• High [K+] lowers water potential in guard cells
• What does water do???
• K+ accumulation is triggered by increased light, low carbon dioxide, circadian rhythms
Normally, stomata open during the day and close at night in response to changes in K+
concentration in stomata guard cells
90
• High [K+] lowers water potential in guard cells
• Water enters, cells swell and buckle
• K+ accumulation is triggered by increased light, low carbon dioxide, circadian rhythms
Normally, stomata open during the day and close at night in response to changes in K+
concentration in stomata guard cells
91
• High [K+] lowers water potential in guard cells
• Water enters, cells swell and buckle
• Pore opens
• K+ accumulation is triggered by increased light, low carbon dioxide, circadian rhythms
Normally, stomata open during the day and close at night in response to changes in K+
concentration in stomata guard cells
92
• High [K+] lowers water potential in guard cells
• Water enters, cells swell and buckle
• Pore opens• Reverse at night
closes the pores
• K+ accumulation is triggered by increased light, low carbon dioxide, circadian rhythms
Normally, stomata open during the day and close at night in response to changes in K+
concentration in stomata guard cells
94
Diagram – hormone mediated stomatal opening and closing
Abscissic acid is the hormone that mediates this response
95
Diagram – spoke-like orientation of cellulose microfibrils
Cellulose orientation determines shape of turgid cells
96
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
99
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
100
Hands On• Work with your team to make hypotheses
about stomata number and placement on various types of leaves
• Use nail polish to make impressions of stomataPut a tab of paper under the polishMake a dry mount of the impression
• Count stomata in the field of view and estimate the number of stomata per mm2
• Be prepared to discuss your findings
101
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
102
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
103
The Pressure Flow Model For
Phloem Transport
• Sources can be leaves, stems or roots
• Sinks can be leaves, stems, roots or reproductive parts (especially seeds and fruits)
104
The Pressure Flow Model For
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
105
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
107
Critical Thinking
• Remember the water potential equation
Ψ = P - s• What happens to Ψ as s increases???
108
Critical Thinking
• Remember the water potential equation
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
109
The Pressure Flow Model For
Phloem Transport
• High [solute] at source end decreases Ψ
• What does water do???
110
Critical Thinking
• Remember the water potential equation
Ψ = P - s• What does water do when Ψ decreases???
111
Critical Thinking
• Remember the water potential equation
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???
112
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!
113
The Pressure Flow Model For
Phloem Transport
• High [solute] at source end decreases Ψ
• Water moves into the source end of the phloem
• What does this do to P at the source end?
115
Critical Thinking
• What will happen to water pressure in any plant cell as water moves in???
• It increases• Why???
116
Critical Thinking
• What will happen to water pressure in any plant cell as water moves in???
• It increases• Why???• The cell wall limits expansion – it “pushes
back”
117
The Pressure Flow Model For
Phloem Transport
• High [solute] at source end decreases Ψ
• Water moves into the source end of the phloemThis increases
the pressure
118
The Pressure Flow Model For
Phloem Transport
• Increased pressure at source end causes phloem sap to move to any area of lower Ψ = sinks
119
The Pressure Flow Model For
Phloem Transport
• At sink end, the sugars are removed by metabolism, by conversion to starch, or by active transport
120
The Pressure Flow Model For
Phloem Transport
• What then happens to the Ψ at the sink end of the phloem???
121
Critical Thinking
• Remember the water potential equation
Ψ = P - s• What happens to Ψ as s decreases???
122
Critical Thinking
• Remember the water potential equation
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
123
The Pressure Flow Model For
Phloem Transport
• Ψ goes up at the sink end of the phloem
• What does water do???
124
Critical Thinking
• Remember the water potential equation
Ψ = P - s• What does water do when Ψ increases???
125
Critical Thinking
• Remember the water potential equation
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???
126
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!
127
The Pressure Flow Model For
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
128
The Pressure Flow Model For
Phloem Transport
• Thus the phloem sap moves – from source to sinkSome xylem
water is cycled into and out of the phloem in the process
129
The Pressure Flow Model For
Phloem Transport
• Active transport is always involved at the source end, but only sometimes at the sink end
130
Micrograph – sieve cells; same next slide
Critical Thinking
• What about the structure of the sieve cells facilitates the movement of phloem sap???
131
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
133
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