Transport Pathways in Plants: Survival and Evolutionary Journey

1. Chapter 36 • Transport in Plants

2. Pathways for Survival • For vascular plants • The evolutionary journey onto land involved the differentiation of the plant body into roots and shoots

3. Figure 36.1 • Vascular tissue • Transports nutrients throughout a plant; such transport may occur over long distances

4. Transport occurs on 3 scales • Transport of water and solutes by individual cells, such as root hairs • Short-distance transport of substances from cell to cell • Long-distance transport within xylem and phloem

5. 1 2 4 3 Through stomata, leaves take in CO2 and expel O2. The CO2 provides carbon for photosynthesis. Some O2produced by photosynthesis is used in cellular respiration. Sugars are produced by photosynthesis in the leaves. Transpiration, the loss of water from leaves (mostly through stomata), creates a force within leaves that pulls xylem sap upward. 6 5 7 Water and minerals are transported upward from roots to shoots as xylem sap. Roots absorb water and dissolved minerals from the soil. Roots exchange gases with the air spaces of soil, taking in O2 and discharging CO2. In cellular respiration, O2 supports the breakdown of sugars. • Types of transport CO2 O2 Light H2O Sugar Sugars are transported as phloem sap to roots and other parts of the plant. O2 H2O CO2 Minerals Figure 36.2

6. EXTRACELLULAR FLUID CYTOPLASM – + H+ + – ATP H+ – + H+ Proton pump generates membrane potential and H+ gradient. H+ H+ H+ – H+ + H+ – + Central Role of Proton Pumps • Create a H+ (proton) gradient PE that can be harnessed to do work •  membrane potential Figure 36.3

7. + – H+ H+ NO3 – + – NO3– + – H+ Cell accumulates anions (, for example) by coupling their transport to theinward diffusion H+ H+ NO3– H+ H+ H+ H+ of through a cotransporter. NO3– – NO3 – + NO3 – – + H+ NO3– – H+ + H+ H+ (b) Cotransport of anions Figure 36.4b • Cotransport • Coupled transport

8. + – H+ H+ H+ S + – Plant cells can also accumulate a neutral solute, such as sucrose ( ), by cotransporting down the steep proton gradient. H+ – + H+ H+ S H+ – S H+ H+ H+ S S S – H+ + – + H+ S H+ + – (c) Contransport of a neutral solute Figure 36.4c • “coattail” effect of cotransport • Responsible for the uptake of sucrose by plant cells

9. Effects of Differences in Water Potential • Plants must balance water uptake and loss • Osmosis • Determines uptake or loss • Affected by solute conc. & pressure

10. Water potential (y) • Solute concentration and pressure  Direction of movement of water • Water • Flows f/ high y to low y

11. Solute potential • Proportional to # of dissolved molecules • Pressure potential

12. (a) 0.1 M solution Pure waer H2O P= 0 S= 0.23 = 0.23 MPa = 0 MPa • Addition of solutes • Reducesy Figure 36.5a

13. (b) (c) H2O H2O P= 0.23 S= 0.23 = 0 MPa P= 0.30 S= 0.23 = 0.07 MPa = 0 MPa = 0 MPa • Application of pressure • Increasesy Figure 36.5b, c

14. (d) H2O P= 0.30 S= 0 = 0.30 MPa P= 0 S= 0.23 = 0.23 MPa • Negative pressure • Decreasesy Figure 36.5d

15. Initial flaccid cell: P= 0 S= 0.7 0.4 M sucrose solution: = 0.7 MPa P= 0 S= 0.9 = 0.9 MPa Plasmolyzed cell at osmotic equilibrium with its surroundings P= 0 S= 0.9 Figure 36.6a = 0.9 MPa y • Affects uptake and loss of water by plant cells • Flaccid cells in higher solute concentration • Cells lose water  plasmolyzed

16. Initial flaccid cell: P= 0 S= 0.7 Distilled water: = 0.7 MPa P= 0 S= 0 = 0 MPa Turgid cell at osmotic equilibrium with its surroundings P= 0.7 S= 0.7 = 0 MPa Figure 36.6b • If Flaccid cell solution w/ low conc • Cell gains water  turgid

17. Turgor loss (wilting) • Reversed when the plant is watered Figure 36.7

18. Aquaporin Proteins • Transport proteins in the cell membrane that allow the passage of water • Do not affect y

19. Plasma membrane • Controls traffic of molecules into and out of the protoplast

20. Cell wall Transport proteins in the plasma membrane regulate traffic of molecules between the cytosol and the cell wall. Cytosol Transport proteins in the vacuolar membrane regulate traffic of molecules between the cytosol and the vacuole. Vacuole (a) Cell compartments. The cell wall, cytosol, and vacuole are the three main compartments of most mature plant cells. Vacuolar membrane (tonoplast) Plasmodesma Plasma membrane Figure 36.8a • Vacuolar membrane • Regulates transport between the cytosol and the vacuole

21. Cell walls and cytosol continuous from cell to cell • Cytoplasmic continuum  symplast • Extracellular continuum  apoplast

22. Key Symplast Apoplast Transmembrane route Apoplast The symplast is the continuum of cytosol connected by plasmodesmata. The apoplast is the continuum of cell walls and extracellular spaces. Symplast Symplastic route Apoplastic route (b) Transport routes between cells. At the tissue level, there are three passages: the transmembrane, symplastic, and apoplastic routes. Substances may transfer from one route to another. Figure 36.8b

23. Bulk Flow in Long-Distance Transport • Movement of fluid in the xylem and phloem is driven by pressure differences

24. Water and mineral salts f/ soil epidermis of roots shoot system

25. Casparian strip Endodermal cell Pathway along apoplast Pathway through symplast 1 Uptake of soil solution by the hydrophilic walls of root hairs provides access to the apoplast. Water and minerals can then soak into the cortex along this matrix of walls. Casparian strip 2 Plasma membrane 1 Minerals and water that cross the plasma membranes of root hairs enter the symplast. Apoplastic route 2 Vessels (xylem) 3 As soil solution moves along the apoplast, some water and minerals are transported into the protoplasts of cells of the epidermis and cortex and then move inward via the symplast. Root hair Symplastic route Epidermis Endodermis Vascular cylinder Cortex 5 4 Endodermal cells and also parenchyma cells within the vascular cylinder discharge water and minerals into their walls (apoplast). The xylem vessels transport the water and minerals upward into the shoot system. Within the transverse and radial walls of each endodermal cell is the Casparian strip, a belt of waxy material (purple band) that blocks the passage of water and dissolved minerals. Only minerals already in the symplast or entering that pathway by crossing the plasma membrane of an endodermal cell can detour around the Casparian strip and pass into the vascular cylinder. Figure 36.9 Lateral transport

26. Root hairs account for much of the surface area of roots

27. 2.5 mm Mycorrhizae • Mutually beneficial relationships (symbiosis) with fungal hyphae, facilitate the absorption of water and minerals Figure 36.10

28. The Endodermis: A Selective Sentry • Innermost layer of cells in the root cortex • Surrounds the vascular cylinder and functions as the last checkpoint for the selective passage of minerals from the cortex into the vascular tissue

29. Waxy Casparian strip of the endodermal wall blocks apoplastic transfer

30. Water and minerals ascend from roots to shoots through the xylem • Plants lose an enormous amount of water through transpiration, loss of water f/ leaves

31. Xylem sap • Rises to heights of more than 100 m in the tallest plants

32. Pushing Xylem Sap: Root Pressure • At night, when transpiration is very low • 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

33. Figure 36.11 • Root pressure results in guttation, (exudation) of water f/ tips of leaf

34. Pulling Xylem Sap: The Transpiration-Cohesion-Tension Mechanism • Water is pulled upward by negative pressure in the xylem • Based on cohesion of water molecules which is based on hydrogen bonding

35. Transpirational Pull • Water vapor in the airspaces of a leaf • Diffuses down its water potential gradient and exits the leaf via stomata

36. Evaporation causes the air-water interface to retreat farther into the cell wall and become more curved as the rate of transpiration increases. As the interface becomes more curved, the water film’s pressure becomes more negative. This negative pressure, or tension, pulls water from the xylem, where the pressure is greater. 3 Y = –0.15 MPa Y = –10.00 MPa Cell wall Air-water interface Airspace Low rate of transpiration High rate of transpiration Cuticle Upper epidermis Cytoplasm Evaporation Mesophyll Airspace Air-space Cell wall Lower epidermis Evaporation Vacuole Water film Stoma CO2 O2 Xylem CO2 O2 Cuticle Water vapor Water vapor In transpiration, water vapor (shown as blue dots) diffuses from the moist air spaces of theleaf to the drier air outside via stomata. At first, the water vapor lost by transpiration is replaced by evaporation from the water film that coats mesophyll cells. 1 2 Figure 36.12 • Transpiration produces negative pressure (tension) in the leaf • Pulls on water in the xylem, pulling water into the leaf

37. Cohesion and Adhesion in the Ascent of Xylem Sap • Transpirational pull on xylem sap • Transmitted f/ leaves to roots and even into the soil solution • Facilitated by cohesion and adhesion

38. Xylem sap Outside air Y = –100.0 MPa Mesophyll cells Stoma Water molecule Leaf Y (air spaces) = –7.0 MPa Transpiration Atmosphere Leaf Y (cell walls) = –1.0 MPa Xylem cells Adhesion Cell wall Water potential gradient Trunk xylem Y = – 0.8 MPa Cohesion, by hydrogen bonding Cohesion and adhesion in the xylem Water molecule Root xylem Y = – 0.6 MPa Root hair Soil Y = – 0.3 MPa Soil particle Water Water uptake from soil Figure 36.13 • Ascent of xylem sap

39. Stomata help regulate the rate of transpiration

40. 20 µm Figure 36.14 Stomata regulate transpiration rate, also: • Photosynthetic rate • Water loss rate

41. Stomata • About 90% of the water a plant loses through stomata

42. Cells turgid/Stoma open Cells flaccid/Stoma closed (a) Changes in guard cell shape and stomatal opening and closing (surface view). Guard cells of a typical angiosperm are illustrated in their turgid (stoma open) and flaccid (stoma closed) states. The pair of guard cells buckle outward when turgid. Cellulose microfibrils in the walls resist stretching and compression in the direction parallel to the microfibrils. Thus, the radial orientation of the microfibrils causes the cells to increase in length more than width when turgor increases. The two guard cells are attached at their tips, so the increase in length causes buckling. Radially oriented cellulose microfibrils Cell wall Vacuole Guard cell Figure 36.15a • Stomata flanked by guard cells • Control the diameter of the stoma by changing shape

43. H2O H2O H2O H2O (b) Role of potassium in stomatal opening and closing. The transport of K+ (potassium ions, symbolized here as red dots) across the plasma membrane and vacuolar membrane causes the turgor changes of guard cells. H2O K+ H2O H2O H2O H2O H2O Figure 36.15b • Chgs. in turgor pressure open and close stomata • From uptake and loss of K+ by the guard cells

44. Xerophytes • Plants adapted to arid climates • Leaf modifications •  reduce transpiration

45. Cuticle Upper epidermal tissue 100 m Lower epidermal tissue Trichomes (“hairs”) Stomata Figure 36.16 • Stomata of xerophytes concentrated on the lower leaf surface, often located in depressions

46. Translocation • Transport of organic nutrients (e.g. sugar) in the plant

47. Sugar Movement • Phloem sap • Aqueous sucrose soln. • Travels from a sugar source to a sugar sink

48. Sugar source • Sugar producer, e.g. leaves • Sugar sink • Consumer or storer of sugar, e.g.tuber or bulb

49. Sieve-tube member Companion (transfer) cell Mesophyll cell Cell walls (apoplast) Plasma membrane Plasmodesmata (a) Sucrose manufactured in mesophyll cells can travel via the symplast (blue arrows) to sieve-tube members. In some species, sucrose exits the symplast (red arrow) near sieve tubes and is actively accumulated from the apoplast by sieve-tube members and their companion cells. Phloem parenchyma cell Bundle- sheath cell Figure 36.17a Mesophyll cell • Sugar loaded into sieve-tube members  

50. High H+ concentration Cotransporter H+ Proton pump S (b) A chemiosmotic mechanism is responsible for the active transport of sucrose into companion cells and sieve-tube members. Proton pumps generate an H+ gradient, which drives sucrose accumulation with the help of a cotransport protein that couples sucrose transport to the diffusion of H+ back into the cell. Key ATP Sucrose H+ H+ Apoplast S Low H+ concentration Symplast Figure 36.17b • Phloem loading requires active transport • Proton pumping and cotransport of sucrose and H+