Chapter 36

1. Chapter 36 Transport in Vascular Plants

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

3. LE 36-4b Cell accumulates anions ( , for example) by coupling their transport to; the inward diffusion of through a cotransporter. Cotransport of anions

4. 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

5. Effects of Differences in Water Potential • To survive, plants must balance water uptake and loss • Osmosis determines the net uptake or water loss by a cell is affected by solute concentration and pressure

6. LE 36-8b Key Symplast Apoplast Transmembrane route Apoplast Symplast Symplastic route Apoplastic route Transport routes between cells

7. Bulk Flow in Long-Distance Transport • In bulk flow, movement of fluid in the xylem and phloem is driven by pressure differences at opposite ends of the xylem vessels and sieve tubes

8. Concept 36.2: Roots absorb water and minerals from the soil • Water and mineral salts from the soil enter the plant through the epidermis of roots and ultimately flow to the shoot system Animation: Transport in Roots

9. LE 36-9 Casparian strip Endodermal cell Pathway along apoplast Pathway through symplast Casparian strip Plasma membrane Apoplastic route Vessels (xylem) Symplastic route Root hair Epidermis Endodermis Vascular cylinder Cortex

10. 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

11. 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

12. 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

14. Transpirational Pull • Water vapor in the airspaces of a leaf diffuses down its water potential gradient and exits the leaf via stomata • Transpiration produces negative pressure (tension) in the leaf, which exerts a pulling force on water in the xylem, pulling water into the leaf

15. LE 36-13 Xylem sap  Outside air  = –100.0 MPa Mesophyll cells 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

16. Concept 36.4: Stomata help regulate the rate of transpiration • Leaves generally have broad surface areas and high surface-to-volume ratios • These characteristics increase photosynthesis and increase water loss through stomata

17. LE 36-14 20 µm

18. 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

19. LE 36-15a Cells turgid/Stoma open Cells flaccid/Stoma closed Radially oriented cellulose microfibrils Cell wall Vacuole Guard cell Changes in guard cell shape and stomatal opening and closing (surface view)

20. 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

21. 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

22. Pressure Flow: The Mechanism of Translocation in Angiosperms • In studying angiosperms, researchers have concluded that sap moves through a sieve tube by bulk flow driven by positive pressure Animation: Translocation of Phloem Sap in Summer Animation: Translocation of Phloem Sap in Spring

23. LE 36-18 Sieve tube (phloem) Vessel (xylem) Source cell (leaf) H2O Sucrose H2O flow stream Pressure Transpiration Sink cell (storage root) Sucrose H2O

24. What mechanism explains the movement of sucrose from source to sink? • evaporation of water and active transport of sucrose from the sink • osmotic movement of water into the sucrose-loaded sieve-tube members creating a higher hydrostatic pressure in the source than in the sink • tension created by the differences in hydrostatic pressure in the source and sink • active transport of sucrose through the sieve-tube cells driven by proton pumps • the hydrolysis of starch to sucrose in the mesophyll cells that raises their water potential and drives the bulk flow of sap to the sink

25. The main mechanism(s) determining the direction of short-distance transport within a potato tuber is (are) • diffusion due to concentration differences and bulk flow due to pressure differences. • pressure flow through the phloem. • active transport due to the hydrolysis of ATP and ion transport into the tuber cells. • determined by the structure and function of the tonoplast of the tuber cells. • not affected by temperature and pressure.

26. A water molecule could move all the way through a plant from soil to root to leaf to air and pass through a living cell only once. This living cell would be a part of which structure? • the Casparian strip • a guard cell • the root epidermis • the endodermis • the root cortex

27. Photosynthesis begins to decline when leaves wilt because • flaccid cells are incapable of photosynthesis. • CO2 accumulates in the leaves and inhibits photosynthesis. • there is insufficient water for photolysis during light reactions. • stomata close, preventing CO2 entry into the leaf. • the chlorophyll of flaccid cells cannot absorb light.

28. Which of the following experimental procedures would most likely reduce transpiration while allowing the normal growth of a plant? * • subjecting the leaves of the plant to a partial vacuum • increasing the level of carbon dioxide around the plant • putting the plant in drier soil • decreasing the relative humidity around the plant • injecting potassium ions into the guard cells of the plant

29. Water flows into the source end of a sieve tube because • sucrose has diffused into the sieve tube, making it hypertonic. • sucrose has been actively transported into the sieve tube, making it hypertonic. • water pressure outside the sieve tube forces in water. • the companion cell of a sieve tube actively pumps in water. • sucrose has been dumped from the sieve tube by active transport.

30. In the pressure-flow hypothesis of translocation, what causes the pressure? • root pressure • the osmotic uptake of water by sieve tubes at the source • the accumulation of minerals and water by the stele in the root • the osmotic uptake of water by the sieve tubes of the sink • hydrostatic pressure in xylem vessels