Resource Acquisition and Transport in Vascular Plants

1
Chapter 36
Resource Acquisition and Transport in Vascular
Plants
2
Overview Underground Plants

• Stone plants (Lithops) are adapted to life in the
  desert
• Two succulent leaf tips are exposed above ground
  the rest of the plant lives below ground

3
Figure 36.1
4

• The success of plants depends on their ability to
  gather and conserve resources from their
  environment
• The transport of materials is central to the
  integrated functioning of the whole plant

5
Concept 36.1 Adaptations for acquiring
resources were key steps in the evolution of
vascular plants

• The algal ancestors of land plants absorbed
  water, minerals, and CO2 directly from the
  surrounding water
• Early nonvascular land plants lived in shallow
  water and had aerial shoots
• Natural selection favored taller plants with flat
  appendages, multicellular branching roots, and
  efficient transport

6

• The evolution of xylem and phloem in land plants
  made possible the long-distance transport of
  water, minerals, and products of photosynthesis
• Xylem transports water and minerals from roots to
  shoots
• Phloem transports photosynthetic products from
  sources to sinks

7
Figure 36.2-1
H2O
H2Oand minerals
8
Figure 36.2-2
O2
CO2
H2O
O2
H2Oand minerals
CO2
9
Figure 36.2-3
O2
CO2
Light
Sugar
H2O
O2
H2Oand minerals
CO2
10

• Adaptations in each species represent compromises
  between enhancing photosynthesis and minimizing
  water loss

11
Shoot Architecture and Light Capture

• Stems serve as conduits for water and nutrients
  and as supporting structures for leaves
• There is generally a positive correlation between
  water availability and leaf size

12

• Light absorption is affected by the leaf area
  index, the ratio of total upper leaf surface of a
  plant divided by the surface area of land on
  which it grows
• Self-pruning is the shedding of lower shaded
  leaves when they respire more than photosynthesize

13
Figure 36.4
Ground areacovered by plant
Plant ALeaf area ? 40of ground area(leaf area
index ? 0.4)
Plant BLeaf area ? 80of ground area(leaf area
index ? 0.8)
14
Root Architecture and Acquisition of Water and
Minerals

• Soil is a resource mined by the root system
• Taproot systems anchor plants and are
  characteristic of gymnosperms and eudicots
• Root growth can adjust to local conditions
• For example, roots branch more in a pocket of
  high nitrate than low nitrate
• Roots are less competitive with other roots from
  the same plant than with roots from different
  plants

15

• Roots and the hyphae of soil fungi form
  mutualistic associations called mycorrhizae
• Mutualisms with fungi helped plants colonize land
• Mycorrhizal fungi increase the surface area for
  absorbing water and minerals, especially phosphate

16
Figure 36.5
Roots
Fungus
17
Concept 36.2 Different mechanisms transport
substances over short or long distances

• There are two major pathways through plants
• The apoplast
• The symplast

18
The Apoplast and Symplast Transport Continuums

• The apoplast consists of everything external to
  the plasma membrane
• It includes cell walls, extracellular spaces, and
  the interior of vessel elements and tracheids
• The symplast consists of the cytosol of the
  living cells in a plant, as well as the
  plasmodesmata

19

• Three transport routes for water and solutes are
• The apoplastic route, through cell walls and
  extracellular spaces
• The symplastic route, through the cytosol
• The transmembrane route, across cell walls

20
Figure 36.6
Cell wall
Apoplastic route
Cytosol
Symplastic route
Transmembrane route
Key
Plasmodesma
Apoplast
Plasma membrane
Symplast
21
Short-Distance Transport of Solutes Across Plasma
Membranes

• Plasma membrane permeability controls
  short-distance movement of substances
• Both active and passive transport occur in plants
• In plants, membrane potential is established
  through pumping H? by proton pumps
• In animals, membrane potential is established
  through pumping Na? by sodium-potassium pumps

22
Figure 36.7a
EXTRACELLULAR FLUID
CYTOPLASM
?

H
Hydrogen ion

?
ATP
?

H
H
H
H
H
H

?
H
Proton pump
?

(a) H and membrane potential
23

• Plant cells use the energy of H? gradients to
  cotransport other solutes by active transport

24
Figure 36.7b
?

H
H
S
?
H

H
?
H

H
H
S
S
H
H
H
S
S
S
?

H
?

Sucrose(neutral solute)
H/sucrosecotransporter
?

(b) H and cotransport of neutral solutes
25
Figure 36.7c
?

H
H
NO3?
?

NO3?
H
?

H
H
H
Nitrate
H
H
NO3?
NO3?
NO3?
?

NO3?
?

H
HNO3?cotransporter
H
H
?

(c) H and cotransport of ions
26

• Plant cell membranes have ion channels that allow
  only certain ions to pass

27
Figure 36.7d
?

Potassium ion
K
?

K
?

K
K
K
K
K
?

Ion channel
?

(d) Ion channels
28
Short-Distance Transport of Water Across Plasma
Membranes

• To survive, plants must balance water uptake and
  loss
• Osmosis determines the net uptake or water loss
  by a cell and is affected by solute concentration
  and pressure

29

• Water potential is a measurement that combines
  the effects of solute concentration and pressure
• Water potential determines the direction of
  movement of water
• Water flows from regions of higher water
  potential to regions of lower water potential
• Potential refers to waters capacity to perform
  work

30

• Water potential is abbreviated as ? and measured
  in a unit of pressure called the megapascal (MPa)
• ? 0 MPa for pure water at sea level and at room
  temperature

31
How Solutes and Pressure Affect Water Potential

• Both pressure and solute concentration affect
  water potential
• This is expressed by the water potential
  equation ? ? ?S ? ?P
• The solute potential (?S) of a solution is
  directly proportional to its molarity
• Solute potential is also called osmotic potential

32

• Pressure potential (?P) is the physical pressure
  on a solution
• Turgor pressure is the pressure exerted by the
  plasma membrane against the cell wall, and the
  cell wall against the protoplast
• The protoplast is the living part of the cell,
  which also includes the plasma membrane

33

• Consider a U-shaped tube where the two arms are
  separated by a membrane permeable only to water
• Water moves in the direction from higher water
  potential to lower water potential

34
Figure 36.8a
Solutes have a negative effect on ? by binding
water molecules.
Adding solutes to the right arm makes ? lower
there, resulting in net movement of water to
the right arm
Pure water at equilibrium
Pure water
Solutes
Membrane
H2O
H2O
35
Figure 36.8b
Positive pressure has a positive effect on ? by
pushing water.
Applying positive pressure to the right arm
makes ? higher there, resulting in net movement
of water to the left arm
Positivepressure
Pure water at equilibrium
H2O
H2O
36
Figure 36.8c
Solutes and positive pressure have opposing
effects on watermovement.
In this example, the effect of adding solutes is
offset by positive pressure, resulting in no
net movement of water
Positivepressure
Pure water at equilibrium
Solutes
H2O
H2O
37
Figure 36.8d
Negative pressure (tension) has a negative effect
on ? by pulling water.
Applying negative pressure to the right arm
makes ? lower there, resulting in net movement
of water to the right arm
Negativepressure
Pure water at equilibrium
H2O
H2O
38
Water Movement Across Plant Cell Membranes

• Water potential affects uptake and loss of water
  by plant cells
• If a flaccid cell is placed in an environment
  with a higher solute concentration, the cell will
  lose water and undergo plasmolysis
• Plasmolysis occurs when the protoplast shrinks
  and pulls away from the cell wall

Video Plasmolysis
39
Figure 36.9
Initial flaccid cell
0.4 M sucrose solution
Pure water
?P
0
?
?S
0
?
?
0 MPa
Plasmolyzedcell at osmoticequilibrium withits
surroundings
Turgid cellat osmoticequilibrium withits
surroundings
?
?P
0.7
?
?S
?0.7
?
?
0 MPa
?
(a) Initial conditions cellular ? ?
environmental ?
(b) Initial conditions cellular ? ?
environmental ?
40
Figure 36.9a
Initial flaccid cell
0.4 M sucrose solution
Plasmolyzedcell at osmoticequilibrium withits
surroundings
(a) Initial conditions cellular ? ?
environmental ?
41
Figure 36.9b
Initial flaccid cell
Pure water
Turgid cellat osmoticequilibrium withits
surroundings
(b) Initial conditions cellular ? ?
environmental ?
42

• If a flaccid cell is placed in a solution with a
  lower solute concentration, the cell will gain
  water and become turgid
• Turgor loss in plants causes wilting, which can
  be reversed when the plant is watered

Video Turgid Elodea
43
Aquaporins Facilitating Diffusion of Water

• Aquaporins are transport proteins in the cell
  membrane that allow the passage of water
• These affect the rate of water movement across
  the membrane

44
Long-Distance Transport The Role of Bulk Flow

• Efficient long distance transport of fluid
  requires bulk flow, the movement of a fluid
  driven by pressure
• Water and solutes move together through tracheids
  and vessel elements of xylem, and sieve-tube
  elements of phloem
• Efficient movement is possible because mature
  tracheids and vessel elements have no cytoplasm,
  and sieve-tube elements have few organelles in
  their cytoplasm

45
Concept 36.3 Transpiration drives the transport
of water and minerals from roots to shoots via
the xylem

• Plants can move a large volume of water from
  their roots to shoots

46
Absorption of Water and Minerals by Root Cells

• Most water and mineral absorption occurs near
  root tips, where root hairs are located and the
  epidermis is permeable to water
• Root hairs account for much of the surface area
  of roots
• After soil solution enters the roots, the
  extensive surface area of cortical cell membranes
  enhances uptake of water and selected minerals

Animation Transport in Roots
47

• The concentration of essential minerals is
  greater in the roots than soil because of active
  transport

48
Transport of Water and Minerals into the Xylem

• The endodermis is the innermost layer of cells in
  the root cortex
• It surrounds the vascular cylinder and is the
  last checkpoint for selective passage of minerals
  from the cortex into the vascular tissue

49

• Water can cross the cortex via the symplast or
  apoplast
• The waxy Casparian strip of the endodermal wall
  blocks apoplastic transfer of minerals from the
  cortex to the vascular cylinder
• Water and minerals in the apoplast must cross the
  plasma membrane of an endodermal cell to enter
  the vascular cylinder

50
Figure 36.10
Casparian strip
Endodermalcell
Pathway alongapoplast
Pathwaythroughsymplast
Plasmamembrane
Casparian strip
Apoplasticroute
Vessels(xylem)
Symplasticroute
Roothair
Endodermis
Epidermis
Vascular cylinder(stele)
Cortex
51

• The endodermis regulates and transports needed
  minerals from the soil into the xylem
• Water and minerals move from the protoplasts of
  endodermal cells into their cell walls
• Diffusion and active transport are involved in
  this movement from symplast to apoplast
• Water and minerals now enter the tracheids and
  vessel elements

52
Bulk Flow Transport via the Xylem

• Xylem sap, water and dissolved minerals, is
  transported from roots to leaves by bulk flow
• The transport of xylem sap involves
  transpiration, the evaporation of water from a
  plants surface
• Transpired water is replaced as water travels up
  from the roots
• Is sap pushed up from the roots, or pulled up by
  the leaves?

53
Pushing Xylem Sap Root Pressure

• At night root cells continue pumping mineral ions
  into the xylem of the vascular cylinder, lowering
  the water potential
• Water flows in from the root cortex, generating
  root pressure
• Root pressure sometimes results in guttation, the
  exudation of water droplets on tips or edges of
  leaves

54
Figure 36.11
55
Pulling Xylem Sap The Cohesion-Tension Hypothesis

• According to the cohesion-tension hypothesis,
  transpiration and water cohesion pull water from
  shoots to roots
• Xylem sap is normally under negative pressure, or
  tension

56
Transpirational Pull

• Water vapor in the airspaces of a leaf diffuses
  down its water potential gradient and exits the
  leaf via stomata
• As water evaporates, the air-water interface
  retreats further into the mesophyll cell walls
• The surface tension of water creates a negative
  pressure potential

57

• This negative pressure pulls water in the xylem
  into the leaf
• The transpirational pull on xylem sap is
  transmitted from leaves to roots

58
Figure 36.12
Xylem
Cuticle
Upper epidermis
Microfibrils incell wall ofmesophyll cell
Mesophyll
Airspace
Lower epidermis
Cuticle
Stoma
Microfibril(cross section)
Waterfilm
Air-waterinterface
59
Figure 36.13
Xylem sap
Outside air ?
Mesophyll cells
? ?100.0 MPa
Stoma
Leaf ? (air spaces)
Water molecule
? ?7.0 MPa
Atmosphere
Transpiration
Leaf ? (cell walls)
Adhesion byhydrogen bonding
? ?1.0 MPa
Xylemcells
Cell wall
Water potential gradient
Trunk xylem ?
Cohesion byhydrogen bonding
? ?0.8 MPa
Cohesion andadhesion inthe xylem
Water molecule
Root hair
Trunk xylem ?
Soil particle
? ?0.6 MPa
Water
Soil ?
Water uptakefrom soil
? ?0.3 MPa
60
Adhesion and Cohesion in the Ascent of Xylem Sap

• Water molecules are attracted to cellulose in
  xylem cell walls through adhesion
• Adhesion of water molecules to xylem cell walls
  helps offset the force of gravity

Animation Water Transport
Animation Transpiration
61

• Water molecules are attracted to each other
  through cohesion
• Cohesion makes it possible to pull a column of
  xylem sap
• Thick secondary walls prevent vessel elements and
  tracheids from collapsing under negative pressure
• Drought stress or freezing can cause cavitation,
  the formation of a water vapor pocket by a break
  in the chain of water molecules

62
Xylem Sap Ascent by Bulk Flow A Review

• The movement of xylem sap against gravity is
  maintained by the transpiration-cohesion-tension
  mechanism
• Bulk flow is driven by a water potential
  difference at opposite ends of xylem tissue
• Bulk flow is driven by evaporation and does not
  require energy from the plant like
  photosynthesis it is solar powered

63
Concept 36.4 The rate of transpiration is
regulated by stomata

• Leaves generally have broad surface areas and
  high surface-to-volume ratios
• These characteristics increase photosynthesis and
  increase water loss through stomata
• Guard cells help balance water conservation with
  gas exchange for photosynthesis

64
Figure 36.14
65
Stomata Major Pathways for Water Loss

• About 95 of the water a plant loses escapes
  through stomata
• Each stoma is flanked by a pair of guard cells,
  which control the diameter of the stoma by
  changing shape
• Stomatal density is under genetic and
  environmental control

66
Mechanisms of Stomatal Opening and Closing

• Changes in turgor pressure open and close stomata
  
• When turgid, guard cells bow outward and the pore
  between them opens
• When flaccid, guard cells become less bowed and
  the pore closes

67
Figure 36.15
Guard cells turgid/Stoma open
Guard cells flaccid/Stoma closed
Radially orientedcellulose microfibrils
Cellwall
Vacuole
Guard cell
H2O
H2O
H2O
H2O
H2O
K?
H2O
H2O
H2O
H2O
H2O
(b) Role of potassium in stomatal opening and
closing
68

• This results primarily from the reversible uptake
  and loss of potassium ions (K?) by the guard cells

69
Stimuli for Stomatal Opening and Closing

• Generally, stomata open during the day and close
  at night to minimize water loss
• Stomatal opening at dawn is triggered by
• Light
• CO2 depletion
• An internal clock in guard cells
• All eukaryotic organisms have internal clocks
  circadian rhythms are 24-hour cycles

70

• Drought, high temperature, and wind can cause
  stomata to close during the daytime
• The hormone abscisic acid is produced in response
  to water deficiency and causes the closure of
  stomata

71
Effects of Transpiration on Wilting and Leaf
Temperature

• Plants lose a large amount of water by
  transpiration
• If the lost water is not replaced by sufficient
  transport of water, the plant will lose water and
  wilt
• Transpiration also results in evaporative
  cooling, which can lower the temperature of a
  leaf and prevent denaturation of various enzymes
  involved in photosynthesis and other metabolic
  processes

72
Adaptations That Reduce Evaporative Water Loss

• Xerophytes are plants adapted to arid climates

73
Figure 36.16
Ocotillo(leafless)
Oleander leaf cross section
Upper epidermal tissue
Cuticle
Ocotillo afterheavy rain
Oleanderflowers
100 ?m
Stoma
Lower epidermaltissue
Trichomes(hairs)
Crypt
Ocotillo leaves
Old man cactus
74

• Some desert plants complete their life cycle
  during the rainy season
• Others have leaf modifications that reduce the
  rate of transpiration
• Some plants use a specialized form of
  photosynthesis called crassulacean acid
  metabolism (CAM) where stomatal gas exchange
  occurs at night

75
Concept 36.5 Sugars are transported from sources
to sinks via the phloem

• The products of photosynthesis are transported
  through phloem by the process of translocation

76
Movement from Sugar Sources to Sugar Sinks

• In angiosperms, sieve-tube elements are the
  conduits for translocation
• Phloem sap is an aqueous solution that is high in
  sucrose
• It travels from a sugar source to a sugar sink
• A sugar source is an organ that is a net producer
  of sugar, such as mature leaves
• A sugar sink is an organ that is a net consumer
  or storer of sugar, such as a tuber or bulb

77

• A storage organ can be both a sugar sink in
  summer and sugar source in winter
• Sugar must be loaded into sieve-tube elements
  before being exposed to sinks
• Depending on the species, sugar may move by
  symplastic or both symplastic and apoplastic
  pathways
• Companion cells enhance solute movement between
  the apoplast and symplast

78
Figure 36.17
Key
Apoplast
Symplast
Companion(transfer) cell
High H? concentration
Cotransporter
Mesophyll cell
Protonpump
H?
Cell walls (apoplast)
Sieve-tubeelement
S
Plasma membrane
Plasmodesmata
ATP
Sucrose
H?
H?
S
Bundle-sheath cell
Phloemparenchyma cell
Low H? concentration
Mesophyll cell
(a)
(b)
79

• In many plants, phloem loading requires active
  transport
• Proton pumping and cotransport of sucrose and H
  enable the cells to accumulate sucrose
• At the sink, sugar molecules diffuse from the
  phloem to sink tissues and are followed by water

80
Bulk Flow by Positive Pressure The Mechanism of
Translocation in Angiosperms

• Phloem sap moves through a sieve tube by bulk
  flow driven by positive pressure called pressure
  flow

Animation Translocation of Phloem Sap in Summer
Animation Translocation of Phloem Sap in Spring
81
Figure 36.18
Sieve tube(phloem)
Source cell(leaf)
Vessel(xylem)
Loading of sugar
H2O
Sucrose
H2O
Uptake of water
Bulk flow by positive pressure
Bulk flow by negative pressure
Unloading of sugar
Sink cell(storageroot)
Water recycled
Sucrose
H2O
82

• The pressure flow hypothesis explains why phloem
  sap always flows from source to sink
• Experiments have built a strong case for pressure
  flow as the mechanism of translocation in
  angiosperms
• Self-thinning is the dropping of sugar sinks such
  as flowers, seeds, or fruits

83
Figure 36.19
EXPERIMENT
25 ?m
Sieve-tubeelement
Sapdroplet
Sap droplet
Stylet
Separated styletexuding sap
Stylet in sieve-tubeelement
Aphid feeding
84
Concept 36.6 The symplast is highly dynamic

• The symplast is a living tissue and is
  responsible for dynamic changes in plant
  transport processes

85
Changes in Plasmodesmata

• Plasmodesmata can change in permeability in
  response to turgor pressure, cytoplasmic calcium
  levels, or cytoplasmic pH
• Plant viruses can cause plasmodesmata to dilate
  so viral RNA can pass between cells

86
Figure 36.20
Plasmodesma
Virus particles
Cell wall
100 nm
87
Phloem An Information Superhighway

• Phloem is a superhighway for systemic transport
  of macromolecules and viruses
• Systemic communication helps integrate functions
  of the whole plant

88
Electrical Signaling in the Phloem

• The phloem allows for rapid electrical
  communication between widely separated organs
• For example, rapid leaf movements in the
  sensitive plant (Mimosa pudica)