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1 Lecture #5 – Plant Transport Image of waterfall.

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1 1 Lecture #5 – Plant Transport Image of waterfall

2 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 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 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 5 Diagram – water moves from high water potential to low water potential, sometimes toward a negative value; same next 3 slides

6 6

7 7 minus 4 is MORE NEGATIVE than minus 1

8 8 High Low

9 9 Diagram – water potential is universal, including with waterfalls

10 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 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 zero  A cell with positive pressure is turgid  A cell with negative pressure is plasmolyzed  A cell with zero pressure is flaccid

12 12 Turgid P > 0 Plasmolyzed P < 0 Flaccid P = 0

13 13 Micrograph – photosynthetic cells: turgid on left, plasmolyzed on right; same on next 3 slides What are the little green things???

14 14 Turgid Plasmolyzed

15 15 Critical Thinking How can you tell this tissue was artificially plasmolyzed?

16 16 Critical Thinking How can you tell this tissue was artificially plasmolyzed? Observe the cell on the far right – it is still turgid

17 17 Image – turgid plant on left, plasmolyzed on right Crispy means plasmolyzed beyond the permanent wilting point 

18 18 s – Solute Potential s = zero for pure water  Pure H 2 O = nothing else, not a solution Adding solutes ALWAYS decreases the potential energy of water  Some water molecules now carry a load – there is less free water s s s

19 19 Diagram – effect on water potential of adding salts to solutions separated by semi-permeable membrane Remember, Ψ = P – s

20 20 Ψ = P – s Pressure can be +, -, or 0 Solutes always have a negative effect Simplest way to calculate Ψ is by this equation

21 21 Flaccid cell in pure water – what happens??? …..what do you know??? ….what do you need to know???

22 22 Flaccid cell in pure water – what happens??? Ψ = ?

23 23 Flaccid cell in pure water – what happens??? Ψ = ? P = ? s = ?

24 24 Flaccid cell in pure water – what happens??? Ψ = ? P = s = about 0.7 MPa

25 25 Flaccid cell in pure water – what happens??? Ψ = -0.7 MPa P = s = about 0.7 MPa

26 26 Flaccid cell in pure water – what happens??? …..what do you know??? Ψ = ? ….what do you need to know???

27 27 Flaccid cell in pure water – what happens??? Ψ = ? P = ? s = ?

28 28 Flaccid cell in pure water – what happens??? P = s = 0 Ψ = ?

29 29 Flaccid cell in pure water – what happens??? Ψ = 0 MPa P = s = 0

30 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 31 Flaccid cell in pure water – what happens??? Ψ = 0 MPa Water moves from high Ψ to low Ψ Ψ = -0.7 MPa

32 32 Then what happens??? Ψ = 0 MPa Ψ = -0.7 MPa

33 33 Then what happens??? Ψ = 0 MPa Ψ = -0.7 MPa P in cell goes up…..

34 34 Then what happens??? Ψ = 0 MPa Dynamic equilibrium! Ψ = 0 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. 35

36 36 Water Movement Osmosis – the diffusion of water one molecule at a time across a semi-permeable membrane  Controlled by both P and s Bulk Flow – the movement of water in bulk – as a liquid  Controlled primarily by P

37 37 Diagram – osmosis across a semi-permeable membrane; next slide also Osmosis Critical Thinking: Where does water move by osmosis in plants???

38 38 Osmosis Critical Thinking: Where does water move by osmosis in plants??? Cell membrane is semi-permeable

39 39 Water Movement Osmosis – the diffusion of water one molecule at a time across a semi-permeable membrane  Controlled by both P and s Bulk Flow – the movement of water in bulk – as a liquid  Controlled primarily by P

40 40 Water Movement Osmosis – the diffusion of water one molecule at a time across a semi-permeable membrane  Controlled by both P and s Bulk Flow – the movement of water in bulk – as a liquid  Controlled primarily by P – no membrane, no solute gradient!

41 41 Critical Thinking Where does water move by bulk flow in plants???

42 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 43 Diagram – apoplast, symplast and transmembrane pathways; same on next slide Cell Wall Cell Membrane Cytoplasm Routes of water transport soil  root  stem  leaf  atmosphere

44 44 Cell Wall Cell Membrane Cytoplasm Routes of water transport soil  root  stem  leaf  atmosphere

45 45 Diagram – Casparian strip; same on next 2 slides

46 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

47 47 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 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 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 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!!!

51 51 Diagram – review of membrane transport proteins Membrane Transport (review in text if necessary)

52 52 Water is on the move

53 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 54 Micrograph – stomata Stomates are the Valves: as long as the stomata are open, water will move through the plant

55 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 56 Solar Heating Drives the Process Air is dry because of solar heating  The 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

57 57 Critical Thinking Why do we have life on this planet and not the others in our solar system???

58 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 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??? 3 rd rock from the sun!  The Goldilocks Zone – not too hot, not too cold  Plus, we have enough gravity to hold our atmosphere in place  It’s our atmosphere that holds the warmth

60 60 Model – our solar system Life is Random

61 61 Solar Heating Drives the Process Air is dry because of solar heating  The 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

62 62 Atmospheric water potential (MPa) Relative Humidity (%) asymptotic

63 63 Critical Thinking Under what conditions does atmospheric water potential approach zero??? Atmospheric water potential (MPa) Relative Humidity (%) asymptotic

64 64 Critical Thinking Under what conditions does atmospheric water potential approach zero??? Only in the pouring rain Atmospheric water potential (MPa) Relative Humidity (%) asymptotic

65 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 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

67 67 Atmospheric water potential (MPa) Relative Humidity (%) asymptotic

68 68 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 69 Diagram – transpiration gradient from soil to atmosphere The tension gradient is extreme, especially during the day Sunday, 1 October am – RH = 86% Noon – RH = 53% 4 pm – RH = 36% 8 pm – RH = 62% 5am, 23 September – 94% in light rain

70 70 Atmospheric water potential (MPa) Relative Humidity (%) asymptotic

71 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 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 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!!!

74 74 Diagram – transpiration gradient plus pathways

75 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 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 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

78 78 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

79 79 Social Justice I’m not angry with you ……

80 80 Social Justice But I do expect you to DO something !!!

81 Hands On Examine variegated plant  Water with dye solution  What do you expect??? Set up experiments with white carnations  Vary conditions of light, temperature and air flow  Re-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 81

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 82

83 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

84 84 Critical Thinking What happens when soil moisture becomes limited???

85 85 Critical Thinking What happens when soil moisture becomes limited??? Water stress causes stomata to close What then???

86 86 Critical Thinking What happens when soil moisture becomes limited??? Water stress causes stomata to close What then??? Gas exchange ceases – no CO 2 = no photosynthesis

87 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 night  Abscissic acid promotes stomata closure daily, and under water stress conditions  Other structural adaptations limit water loss when stomata are open  Other metabolic pathways (C 4, CAM) limit water loss

88 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 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 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 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 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

93 93 Diagram – open and closed stomata

94 94 Diagram – hormone mediated stomatal opening and closing Abscissic acid is the hormone that mediates this response

95 95 Diagram – spoke-like orientation of cellulose microfibrils Cellulose orientation determines shape of turgid cells

96 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 night  Abscissic acid promotes stomata closure daily, and under water stress conditions  Other structural adaptations limit water loss when stomata are open  Other metabolic pathways (C 4, CAM) limit water loss

97 97 Micrograph – location of stomatal gradient This is the gradient that counts

98 98 Images – structural adaptations to dry environments

99 99 Images and diagrams – metabolic adaptations to dry environments Spatial separation helps C 4 plants be more efficient in hot climates Temporal separation does the same for CAM plants Both use an enzyme that can’t fix O 2 to first capture CO 2 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 stomata  Put a tab of paper under the polish  Make a dry mount of the impression Count stomata in the field of view and estimate the number of stomata per mm 2 Be prepared to discuss your findings 100

101 101 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

102 102 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

103 103 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)

104 104 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

105 105 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

106 106 The Pressure Flow Model For Phloem Transport High [solute] at source end does what to Ψ???

107 107 Critical Thinking Remember the water potential equation Ψ = P - s What happens to Ψ as s increases???

108 108 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

109 109 The Pressure Flow Model For Phloem Transport High [solute] at source end decreases Ψ What does water do???

110 110 Critical Thinking Remember the water potential equation Ψ = P - s What does water do when Ψ decreases???

111 111 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???

112 112 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??? The adjacent xylem – remember structure and function are related!

113 113 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?

114 114 Critical Thinking What will happen to water pressure in any plant cell as water moves in???

115 115 Critical Thinking What will happen to water pressure in any plant cell as water moves in??? It increases Why???

116 116 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”

117 117 The Pressure Flow Model For Phloem Transport High [solute] at source end decreases Ψ Water moves into the source end of the phloem  This increases the pressure

118 118 The Pressure Flow Model For Phloem Transport Increased pressure at source end causes phloem sap to move to any area of lower Ψ = sinks

119 119 The Pressure Flow Model For Phloem Transport At sink end, the sugars are removed by metabolism, by conversion to starch, or by active transport

120 120 The Pressure Flow Model For Phloem Transport What then happens to the Ψ at the sink end of the phloem???

121 121 Critical Thinking Remember the water potential equation Ψ = P - s What happens to Ψ as s decreases???

122 122 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

123 123 The Pressure Flow Model For Phloem Transport Ψ goes up at the sink end of the phloem What does water do???

124 124 Critical Thinking Remember the water potential equation Ψ = P - s What does water do when Ψ increases???

125 125 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???

126 126 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??? The adjacent xylem – remember structure and function are related!

127 127 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

128 128 The Pressure Flow Model For Phloem Transport Thus the phloem sap moves – from source to sink  Some xylem water is cycled into and out of the phloem in the process

129 129 The Pressure Flow Model For Phloem Transport Active transport is always involved at the source end, but only sometimes at the sink end

130 130 Micrograph – sieve cells; same next slide Critical Thinking What about the structure of the sieve cells facilitates the movement of phloem sap???

131 131 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

132 132 The Pressure Flow Model For Phloem Transport Questions???

133 133 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

134 Hands On For tomorrow – bring some soil from your yard and/or garden Put it in a clear, water-tight container (glass jar is easiest) 134


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