Chapter 36

1. Chapter 36 Transport in Vascular Plants

3. Concept 36.1: Physical forces drive the transport of materials in plants over a range of distances • Transport in vascular plants occurs on three scales: • Transport of water and solutes by individual cells, such as root hairs • Short-distance transport of substances from cell to cell at the levels of tissues and organs • Long-distance transport within xylem and phloem at the level of the whole plant

4. LE 36-2_4 CO2 O2 Light H2O Sugar O2 H2O CO2 Minerals

5. Selective Permeability of Membranes: A Review • The selective permeability of the plasma membrane controls movement of solutes into and out of the cell • Specific transport proteins enable plant cells to maintain an internal environment different from their surroundings

6. The Central Role of Proton Pumps • Proton pumps in plant cells create a hydrogen ion gradient that is a form of potential energy that can be harnessed to do work • They contribute to a voltage known as a membrane potential EXTRACELLULAR FLUID CYTOPLASM ATP Proton pump generates mem- brane potential and gradient.

7. LE 36-4a EXTRACELLULAR FLUID CYTOPLASM Cations ( , for example) are driven into the cell by the membrane potential. Transport protein Membrane potential and cation uptake

8. In the mechanism called cotransport, a transport protein couples the passage of one solute to the passage of another Cell accumulates anions ( , for example) by coupling their transport to; the inward diffusion of through a cotransporter. Cotransport of anions

9. LE 36-4c Plant cells can also accumulate a neutral solute, such as sucrose ( ), by cotransporting down the steep proton gradient. Cotransport of a neutral solute

10. Effects of Differences in Water Potential • Water potential is a measurement that combines the effects of solute concentration and pressure • Water flows from regions of higher water potential to regions of lower water potential

11. Quantitative Analysis of Water Potential Addition of solutes 0.1 M solution • The addition of solutes reduces water potential Pure water H2O P = 0 S = –0.23  = 0 MPa P = –0.23 MPa

12. Applying physical pressure Applying physical pressure • Physical pressure increases water potential H2O H2O P = 0.30 P = 0 S = –0.23 S = –0.23  = 0 MPa  = 0 MPa P = 0.07 MPa P = 0 MPa

13. Video: Plasmolysis • 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 become plasmolyzed Initial flaccid cell:  P = 0 S = –0.7  0.4 M sucrose solution: Distilled water: P = –0.7 MPa   P = 0 P = 0  S = –0.9  S = 0  Plasmolyzed cell at osmotic equilibrium  Turgid cell at osmotic equilibrium  udings P = 0 MPa   P = –0.9 MPa P = 0.7  P = 0  S = –0.7 S = –0.9    P = 0 MPa  P = –0.9 MPa   conditions: cellular > environmental 

14. If the same flaccid cell is placed in a solution with a lower solute concentration, the cell will gain water and become turgid Video: Turgid Elodea

15. Aquaporin Proteins and Water Transport • Aquaporins are transport proteins in the cell membrane that allow the passage of water • Aquaporins do not affect water potential

16. The cytoplasmic continuum is called the symplast • The apoplast is the continuum of cell walls and extracellular spaces Key Symplast Apoplast Transmembrane route Apoplast Symplast Symplastic route Apoplastic route Transport routes between cells

17. Concept 36.2: Roots absorb water and minerals from the soil Casparian strip Animation: Transport in Roots Endodermal cell Pathway along apoplast • Water and mineral salts from the soil enter the plant through the epidermis of roots and ultimately flow to the shoot system Pathway through symplast Casparian strip Plasma membrane Apoplastic route Vessels (xylem) Symplastic route Root hair Epidermis Endodermis Vascular cylinder Cortex

18. Casparian strip Endodermal cell Pathway along apoplast • 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 Pathway through symplast Casparian strip Plasma membrane Apoplastic route Vessels (xylem) Symplastic route Root hair Epidermis Endodermis Vascular cylinder Cortex

19. Concept 36.3: Water and minerals ascend from roots to shoots through the xylem • Plants lose an enormous amount of water through transpiration, the loss of water vapor from leaves and other aerial parts of the plant • The transpired water must be replaced by water transported up from the roots

20. Transpiration • Transpiration Cohesion Tension Theory • Transpiration – water loss through leaves • Cohesion – attraction of like molecules • Adhesion – attraction of unlike molecules • Tension – negative pressure

21. Factors Affecting the Ascent of Xylem Sap • Xylem sap rises to heights of more than 100 m in the tallest plants • Is the sap pushed upward from the roots, or is it pulled upward by the leaves?

22. Root pressure sometimes results in guttation, the exudation of water droplets on tips of grass blades or the leaf margins of some small, herbaceous eudicots

23. Cohesion and Adhesion in the Ascent of Xylem Sap Xylem sap  Outside air  = –100.0 MPa Mesophyll cells • The transpirational pull on xylem sap is transmitted all the way from the leaves to the root tips and even into the soil solution • Transpirational pull is facilitated by cohesion and adhesion Stoma  Leaf (air spaces) = –7.0 MPa Water molecule Transpiration  Atmosphere Leaf (cell walls) = –1.0 MPa Xylem cells Adhesion Cell wall Water potential gradient  Trunk xylem = –0.8 Mpa Cohesion, by hydrogen bonding Cohesion and adhesion in the xylem Water molecule Root hair  Root xylem = –0.6 MPa Soil particle  Soil = –0.3 MPa Water Water uptake from soil Animation: Transpiration

24. LE 36-14 20 µm

25. 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 absorption through the roots, the plant will lose water and wilt

26. 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

27. Stomata: Major Pathways for Water Loss Cells turgid/Stoma open Cells flaccid/Stoma closed • About 90% of the water a plant loses escapes through stomata Radially oriented cellulose microfibrils Cell wall Vacuole Guard cell Changes in guard cell shape and stomatal opening and closing (surface view)

28. LE 36-15b Cells turgid/Stoma open Cells flaccid/Stoma closed H2O H2O H2O H2O H2O K+ H2O H2O H2O H2O H2O Role of potassium in stomatal opening and closing

29. Movement from Sugar Sources to Sugar Sinks • Phloem sap is an aqueous solution that is mostly 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

30. Pressure Flow: The Mechanism of Translocation in Angiosperms Sieve tube (phloem) Vessel (xylem) Source cell (leaf) H2O Sucrose • In studying angiosperms, researchers have concluded that sap moves through a sieve tube by bulk flow driven by positive pressure H2O flow stream Pressure Transpiration Sink cell (storage root) Animation: Translocation of Phloem Sap in Summer Sucrose H2O Animation: Translocation of Phloem Sap in Spring

31. LE 36-19 25 µm Sieve- tube member Sap droplet Sap droplet Stylet Stylet in sieve-tube member (LM) Severed stylet exuding sap Aphid feeding