1. Chapter 36: Resource Acquisition and Transport in Vascular Plants
Mrs. Valdes
AP Biology

2. Overview: Underground Plants
Success of plants depends on their ability to gather & conserve resources from environment
Diffusion, active transport, and bulk flow work together to transfer water, minerals, and sugars
 “Stone Plants” live mostly subterranean with two leaf tips full of clear jelly that acts as lenses to channel light underground

3. H2O H2O and minerals Fig. 36-2-1
Figure 36.2 An overview of resource acquisition and transport in a vascular plant
H2O and minerals

4. CO2 O2 H2O O2 CO2 H2O and minerals Fig. 36-2-2
Figure 36.2 An overview of resource acquisition and transport in a vascular plant O2
H2O and minerals
CO2

5. CO2 O2 Light H2O Sugar O2 CO2 H2O and minerals Fig. 36-2-3
Figure 36.2 An overview of resource acquisition and transport in a vascular plant O2
H2O and minerals
CO2

6. Concept 36.1: Land plants acquire resources both above and below ground
Algal ancestors of land plants absorbed water, minerals, and CO2 directly from surrounding water
Evolution of xylem and phloem in land plants  long-distance transport of water, minerals, and products of photosynthesis
Adaptations in each species represent compromises between enhancing photosynthesis and minimizing water loss
Stems: conduits for water and nutrients; supporting structures for leaves
Phyllotaxy: arrangement of leaves on stem; specific to each species

7. Light absorption affected by leaf area index
total upper leaf surface of plant divided by surface area of land on which it grows
affected by light absorption

8. Root Architecture and Acquisition of Water and Minerals
Soil: resource mined by root system
Taproot systems: anchor plants; characteristic of most trees
Roots + hyphae of soil fungi form symbiotic associations called mycorrhizae
Mutualisms with fungi helped plants colonize land

9. Concept 36.2: Transport occurs by short-distance diffusion or active transport and by long-distance bulk flow
Transport begins with absorption of resources by plant cells
Movement of substances into/out of cells regulated by selective permeability
Diffusion across a membrane = passive
Pumping of solutes across a membrane = active AKA needs energy
Most solutes pass through transport proteins in cell membrane
Proton pump: most important transport protein for active transport
plant cells create hydrogen ion gradient AKA form of potential energy that can be harnessed to DO WORK
contribute to voltage known as membrane potential

10. Plant cells use energy stored in proton gradient and membrane potential to drive transport of many different solutes
Cotransport: transport protein couples diffusion of one solute to active transport of another
“Coattail” effect of cotransport: responsible for uptake of sugar sucrose by plant cells

11. Diffusion of Water (Osmosis)
Osmosis: determines net uptake/water loss by cell; affected by solute concentration and pressure
Water potential: measurement that combines effects of solute concentration and pressure
Determines direction of movement of water (in/out)
Water flows from regions of higher water potential to regions  lower water potential
Symbol:Ψ
Units: measure pressure; megapascals (MPa)
Ψ = 0 MPa for pure water at sea level and room temperature

12. How Solutes and Pressure Affect Water Potential
REMEMBER: Pressure and solute concentration affect water potential
Solute potential (ΨS): proportional to number of dissolved molecules
also called osmotic potential
Pressure potential (ΨP): physical pressure on a solution
Turgor pressure: pressure exerted by plasma membrane against the cell wall, and cell wall against the protoplast

13. Measuring Water Potential
Water moves from higher water potential  lower water potential
Addition of solutes reduces water potential
Physical pressure increases water potential
Negative pressure decreases water potential

14. (a) 0.1 M solution Pure water H2O ψP = 0 ψS = 0 ψP = 0 ψS = −0.23
Fig. 36-8a
(a)
0.1 M solution
Pure water
Figure 36.8a Water potential and water movement: an artificial model
H2O
ψP = 0 ψS = 0
ψP = 0 ψS = −0.23
ψ = 0 MPa
ψ = −0.23 MPa

15. (b) Positive pressure H2O ψP = 0 ψS = 0 ψP = 0.23 ψS = −0.23 ψ = 0 MPa
Fig. 36-8b
(b)
Positive pressure
Figure 36.8b Water potential and water movement: an artificial model
H2O
ψP = 0 ψS = 0
ψP = ψS = −0.23
ψ = 0 MPa
ψ = 0 MPa

16. (c) H2O ψP = 0 ψS = 0 ψP = ψS = −0.23 ψ = 0 MPa ψ = 0.07 MPa
Fig. 36-8c
(c)
Increased positive pressure
Figure 36.8c Water potential and water movement: an artificial model
H2O
ψP = 0 ψS = 0
ψP =   ψS = −0.23
0.30
ψ = 0 MPa
ψ = MPa

17. Negative pressure (tension)
Fig. 36-8d
(d)
Negative pressure (tension)
Figure 36.8d Water potential and water movement: an artificial model
H2O
ψP = −0.30 ψS =
ψP = ψS = −0.23
ψ = −0.30 MPa
ψ = −0.23 MPa

18. Water potential affects uptake and loss of water by plant cells
Flaccid cell: placed in environment with higher solute concentration
cell will lose water and undergo plasmolysis
If same flaccid cell placed in solution with lower solute concentration
cell will gain water and become turgid
Turgor loss in plants causes wilting
can be reversed when the plant is watered

19. Aquaporins: Facilitating Diffusion of Water
Aquaporins: transport proteins in cell membrane that allow passage of water
rate of water movement likely regulated by phosphorylation of aquaporin proteins

20. Three Major Pathways of Transport
Transport also regulated by compartmental structure of plant cells
1: Plasma membrane directly controls traffic of molecules into/out of the protoplast
2: Plasma membrane is barrier between two major compartments, the cell wall and the cytosol
3: Vacuole, large organelle that occupies as much as 90% or more of protoplast’s volume
Vacuolar membrane regulates transport between cytosol and vacuole
In most plant tissues, cell wall and cytosol continuous from cell to cell
Symplast: cytoplasmic continuum
Plasmodesmata: cytoplasm of neighboring cells connected by channels
Apoplast: continuum of cell walls and extracellular spaces

21. Water and minerals can travel through plant by three routes:
Transmembrane route: out of one cell, across a cell wall, and into another cell
Symplastic route: via continuum of cytosol
Apoplastic route: via cell walls and extracellular spaces

22. Bulk Flow in Long-Distance Transport
Bulk flow: movement of a fluid driven by pressure; required for efficient long distance transport of fluid
Water and solutes move together through:
tracheids and vessel elements of xylem
sieve-tube elements of phloem
Efficient movement possible because mature tracheids and vessel elements have no cytoplasm, and sieve-tube elements have few organelles in their cytoplasm

23. Concept 36.3: Water and minerals are transported from roots to shoots
Plants can move large volume of water from their roots to shoots
Most water and mineral absorption occurs near root tips, where epidermis is permeable to water and root hairs are located
Root hairs account for much of surface area of roots
After soil solution enters roots, extensive surface area of cortical cell membranes enhances uptake of water and selected minerals

24. Transport of Water and Minerals into Xylem
Endodermis: innermost layer of cells in root cortex
Surrounds vascular cylinder
Last checkpoint for selective passage of minerals from cortex into vascular tissue
Water can cross the cortex via the symplast or apoplast
Casparian strip: waxy endodermal wall blocks apoplastic transfer of minerals from cortex to vascular cylinder

25. Bulk Flow Driven by Negative Pressure in the Xylem
Transpiration: evaporation of water from a plant’s surface; Plants lose large volume of water
Xylem sap: Water replaced by bulk flow of water and minerals; from steles of roots to stems and leaves
Is sap mainly pushed up from the roots, or pulled up by the leaves?

26. Pushing Xylem Sap: Root Pressure
At night, transpiration very low, root cells continue pumping mineral ions into xylem of vascular cylinder, THUS! Lowering water potential
Positive root pressure relatively weak and is minor mechanism of xylem bulk flow
Water flows in from the root cortex, generating root pressure
sometimes results in guttation
exudation of water droplets on tips or edges of leaves

27. Pulling Xylem Sap: The Transpiration-Cohesion-Tension Mechanism
Water pulled upward by negative pressure in xylem
Transpiration Pull:
Water vapor in airspaces of leaf diffuses down water potential gradient  exits leaf via stomata
Transpiration produces negative pressure (tension) in leaf exerts pulling force on water in xylem RESULT: pulling water into leaf

28. Cohesion and Adhesion in Ascent of Xylem Sap
Transpirational pull on xylem sap transmitted from leaves to root tips and even into soil solution!
facilitated by:
cohesion of water molecules to each other
adhesion of water molecules to cell walls
Drought stress OR freezing can cause cavitation
formation of water vapor pocket by a break in chain of water molecules

29. Water molecule Root hair Water Water uptake from soil Soil particle
Fig a
Water molecule
Root hair
Soil particle
Figure Ascent of xylem sap
Water
Water uptake from soil

30. Adhesion by hydrogen bonding
Fig b
Adhesion by hydrogen bonding
Xylem cells
Cell wall
Figure Ascent of xylem sap
Cohesion by hydrogen bonding
Cohesion and adhesion in the xylem

31. Xylem sap Mesophyll cells Stoma Water molecule Transpiration
Fig c
Xylem sap
Mesophyll cells
Stoma
Water molecule
Figure Ascent of xylem sap
Transpiration
Atmosphere

32. Xylem Sap Ascent by Bulk Flow
Know…
1- Movement of xylem sap against gravity is maintained by transpiration-cohesion- tension mechanism
2- Transpiration lowers water potential leaves which generates negative pressure (tension) that pulls water UP through xylem
3- NO energy cost to bulk flow of xylem sap

33. Concept 36.4: Stomata help regulate rate of transpiration
Leaves generally have broad surface areas and high surface-to-volume ratios
THIS increases photosynthesis and increases water loss through stomata
About 95% of water a plant loses escapes through stomata
Each stoma is flanked by a pair of guard cells, which control diameter of stoma by changing shape

34. Mechanisms of Stomatal Opening and Closing
Changes in turgor pressure open and close stomata
Result from reversible uptake/loss of potassium ions by guard cells

35. Guard cells turgid/Stoma open Guard cells flaccid/Stoma closed
Fig a
Guard cells turgid/Stoma open Guard cells flaccid/Stoma closed
Radially oriented cellulose microfibrils
Cell wall
Figure 36.17a Mechanisms of stomatal opening and closing
Vacuole
Guard cell
(a) Changes in guard cell shape and stomatal opening and closing (surface view)

36. Guard cells turgid/Stoma open Guard cells flaccid/Stoma closed
Fig b
Guard cells turgid/Stoma open Guard cells flaccid/Stoma closed
H2O
H2O
H2O
H2O
H2O K+
H2O
H2O
Figure 36.17b Mechanisms of stomatal opening and closing
H2O
H2O
H2O
(b) Role of potassium in stomatal opening and closing

37. Stimuli for Stomatal Opening and Closing
Generally, stomata open during day and close at night to minimize water loss
Stomatal opening at dawn triggered by:
light,
CO2 depletion
Internal “clock” in guard cells
All eukaryotic organisms have internal clocks; circadian rhythms are 24-hour cycles

38. Effects of Transpiration on Wilting and Leaf Temperature
Plants lose A LOT of water by transpiration
Lose H2O and H2O not replace by H2O transport  plant wilt
Transpiration also results in evaporative cooling:
can lower temperature of leaf  prevent denaturation of various enzymes involved in photosynthesis and other metabolic processes

39. Adaptations That Reduce Evaporative Water Loss
Xerophytes: plants adapted to arid climates
have leaf modifications that reduce rate of transpiration
crassulacean acid metabolism (CAM): form of photosynthesis where stomatal gas exchange occurs at night

40. Figure 36.18 Some xerophytic adaptations
Ocotillo (leafless)
Oleander leaf cross section and flowers
Cuticle
Upper epidermal tissue
100 µm
Trichomes (“hairs”)
Crypt
Stomata
Lower epidermal tissue
Figure Some xerophytic adaptations
Ocotillo leaves
Ocotillo after heavy rain
Old man cactus

41. Concept 36.5: Sugars transported from leaves and other sources to sites of use or storage
Translocation: process of transporting products of photosynthesis through phloem
Phloem sap : aqueous solution high in sucrose
travels from sugar source to sugar sink
sugar source: organ that is a net producer of sugar
Ex: mature leaves
sugar sink: organ that is a net consumer or storer of sugar, such as a tuber or bulb
Storage organ can be both sugar sink in summer and sugar source in winter
Sugar MUST BE loaded into sieve-tube elements before being exposed to sinks
Depending on species, sugar move by:
symplastic
both symplastic and apoplastic pathways
Transfer cells: modified companion cells that enhance solute movement between apoplast and symplast

42. Phloem parenchyma cell Symplast Mesophyll cell
Fig a
Mesophyll cell
Cell walls (apoplast)
Companion (transfer) cell
Sieve-tube element
Plasma membrane
Plasmodesmata
Key
Figure 36.19a Loading of sucrose into phloem
Apoplast
Bundle- sheath cell
Phloem parenchyma cell
Symplast
Mesophyll cell

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

44. Bulk Flow by Positive Pressure: The Mechanism of Translocation in Angiosperms
In studying angiosperms, researchers concluded sap moves through sieve tube by bulk flow driven by positive pressure
Pressure flow hypothesis explains why phloem sap always flows from source to sink

45. Concept 36.6: Symplast is highly dynamic
Symplast: living tissue responsible for dynamic changes in plant transport processes
Plasmodesmata can change in permeability in response to:
turgor pressure
cytoplasmic calcium levels
cytoplasmic pH
Plant viruses can cause plasmodesmata to dilate
Mutations that change communication within symplast can lead to changes in development

46. Electrical Signaling in the Phloem
Phloem allows for rapid electrical communication between VERY separated organs
Phloem is “superhighway” for systemic transport of macromolecules and viruses
Systemic communication helps integrate functions of whole plant

47. You should now be able to:
Describe how proton pumps function in transport of materials across membranes
Define the following terms: osmosis, water potential, flaccid, turgor pressure, turgid
Explain how aquaporins affect the rate of water transport across membranes
Describe three routes available for short-distance transport in plants
Relate structure to function in sieve-tube cells, vessel cells, and tracheid cells
Explain how the endodermis functions as a selective barrier between the root cortex and vascular cylinder
Define and explain guttation
Explain this statement: “The ascent of xylem sap is ultimately solar powered”
Describe the role of stomata and discuss factors that might affect their density and behavior
Trace the path of phloem sap from sugar source to sugar sink; describe sugar loading and unloading