Chapter 36 Transport in Plants

1. Chapter 36Transport in Plants

2. For vascular plants the evolutionary movement onto land involved the differentiation of the plant body into roots and shoots • Vascular tissue transports nutrients throughout a plant; this transport may occur over long distances

3. Transport Scale/Distance • 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. 1 2 3 4 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. Overview of Transport in Plants CO2 O2 Light H2O Sugar Sugars are transported as phloem sap to roots and other parts of the plant. O2 H2O CO2 Minerals

5. Selective Permeability of Membranes • The selective permeability of a the plasma membrane controls the movement of solutes into and out of the cell • Specific transport proteins are involved in movement of solutes

6. EXTRACELLULAR FLUID CYTOPLASM – + H+ + – ATP H+ – + H+ Proton pump generates membrane potential and H+ gradient. H+ H+ H+ – H+ + H+ – + Proton Pumps Proton pumps create a hydrogen ion gradient that is a form of potential energy that can be harnessed to do work Contribute to a voltage known as a membrane potential

7. + – CYTOPLASM EXTRACELLULAR FLUID + – Cations ( , for example) are driven into the cell by themembrane potential. K+ K+ + – K+ K+ K+ K+ K+ – + K+ Transport protein – + (a) Membrane potential and cation uptake • Plant cells use energy stored in the proton gradient and membrane potential to drive the transport of many different solutes

8. + – H+ H+ NO3 – + – NO3– + – Cell accumulates anions (, for example) by coupling their transport to theinward diffusion H+ H+ H+ NO3– H+ H+ H+ H+ of through a cotransporter. NO3– – NO3 – + NO3 – – + H+ NO3– – H+ + H+ H+ (b) Cotransport of anions Cotransport • In cotransport a transport protein couples the passage of one solute to the passage of another

9. + – 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) Cotransport of a neutral solute Sucrose uptake • The cotransport is also responsible for the uptake of the sugar sucrose by plant cells

10. Happy 200th Birthday Mr. Darwin Feb 12th 1809

11. Water Potential • To survive plants must balance water uptake and loss • Remember: Osmosis is the passive transport of water across a membrane • Water potential is a measurement that combines the effects of solute concentration and physical pressure (due the presence of the plant cell wall) and determines the direction of movement of water • Water flows from regions of high water potential to regions of low water potential

12. Initial flaccid cell: 0.4 M sucrose solution: Plasmolyzed cell at osmotic equilibrium with its surroundings Plasmolysis • If a flaccid cell (not firm) is placed in an environment with a higher solute concentration (Hypertonic) • The cell will lose water and become plasmolyzed

13. Initial flaccid cell: Distilled water: Turgid cell at osmotic equilibrium with its surroundings Turgidity • If the same flaccid cell is placed in a solution with a lower solute concentration (Hypotonic) • The cell will gain water and become turgid • Healthy plant cells are turgid most of the time Turgidity helps support nonwoody plant parts

14. Wilting • Turgor loss in plants causes wilting which can be reversed when the plant is watered

15. Aquaporins • Water molecules are small enough to move across the lipid bilayer of the plasma membrane, but they also move through specific channels for the passive diffusion of water across the membrane…Aquaporins • Aquaporins don’t effect the direction of water flow, but rather the rate of diffusion.

16. Three Major Compartments of Vacuolated Plant Cells • Transport is also regulated by the compartmental structure of plant cells (3 compartments) • Cell Wall • The plasma membrane controls the traffic of molecules into and out of the protoplast and is the barrier between the cell wall and the cytosol • Cytosol • Vacuole

17. Cell wall Transport proteins in the plasma membrane regulate traffic of molecules between the cytosol and the cell wall. Transport proteins in the vacuolar membrane regulate traffic of molecules between the cytosol and the vacuole. Cytosol 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 Vacuole • The vacuole is a large organelle that can occupy as much as 90% of more of the protoplast’s volume • The vacuolar membrane • Regulates transport between the cytosol and the vacuole

18. Cell-to-cell continuity • The cell walls and cytosol are continuous from cell to cell in most plant tissues • The cytoplasmic continuum is called the symplast • The cell wall continuum is called the apoplast

19. 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 Transport routes between cells. At the tissue level, there are three passages: the transmembrane, symplastic, and apoplastic routes. Short Distance Transport of Water/Solutes in Tissues and Organs • Water and minerals can travel through a plant by one of three routes • Out of one cell, across a cell wall, and into another cell (transmembrane route) • Via the symplast (symplastic route) • Along the apoplast (apoplastic route)

20. 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 • Transpiration (evaporation of water from a leaf) reduces pressure in the leaf xylem. This creates a tension (negative pressure) that pulls material up the xylem • In phloem, hydrostatic pressure (pressure exerted upon the tube from the surrounding tissue) is generated at one end of a sieve tube, which forces sap to the other end of the tube

21. 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 • Soil solutionRoot Hair EpidermisRoot Cortex Root Xylem • Root Hairs • Much of the absorption of water and minerals occurs near root tips, where the epidermis is permeable to water and where root hairs are located • Root hairs account for much of the surface area of roots

22. Root growth Movie

23. 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. Lateral transport of minerals and water in roots

24. Mycorrhizae • Most plants form mutually beneficial relationships with fungi, which facilitate the absorption of water and minerals from the soil • Roots and fungi form mycorrhizae, symbiotic structures consisting of plant roots united with fungal hyphae White mycelium of the fungus around this pine root provides a vast surface area for absorption of water and minerals from the soil.

25. The Endodermis • Is the 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 • Water can cross the cortex via the symplast or apoplast • The waxy Casparian strip of the endodermal wall blocks apoplastic transfer (but not symplastic) of water and minerals from the cortex to the vascular cylinder

26. Ascent of Xylem Sap • Plants lose an enormous amount of water through transpiration and the transpired water must be replaced by water transported up from the roots • Xylem sap rises to heights of more than 100 m in the tallest plants

27. 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, which lowers water potential • Water flows in from the root cortex generating a positive pressure that forces fluid up the xylem. This is upward push is called root pressure

28. Root Pressure-Gluttation • Root pressure sometimes results in guttation, (the exudation of water droplets on tips of grass blades or the leaf margins of some small, herbaceous dicots in the morning). More water enters the leaves than is transpired, and the excess is forced out of the leaf.

29. Pulling Xylem Sap The Transpiration-Cohesion-Tension Mechanism • 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

30. Xylem sap Mesophyll cells Stoma Water molecule Transpiration Atmosphere Xylem cells Adhesion Cell wall Water potential gradient Cohesion, by hydrogen bonding Cohesion and adhesion in the xylem Water molecule Root hair Soil particle Water Water uptake from soil Cohesion and Adhesion in the Ascent of Xylem Sap • The transpirational pull on xylem sap • Is transmitted all the way from the leaves to the root tips and even into the soil solution • It is facilitated by the cohesion and adhesion properties of water

31. 20 µm Transpiration Control • Stomata help regulate the rate of transpiration • Leaves generally have broad surface areas and high surface-to-volume ratios • These characteristics (1) increase photosynthesis (2) Increase water loss through stomata

32. 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 • Transpiration also results in evaporative cooling…which can lower the temperature of a leaf and prevent the denaturation of various enzymes involved in photosynthesis and other metabolic processes

33. Cells turgid/Stoma open Cells flaccid/Stoma closed Radially oriented cellulose microfibrils Cell wall Vacuole Guard cell • About 90% of the water a plant loses escapes through stomata • Each stoma is flanked by guard cells which control the diameter of the stoma by changing shape Guard Cells

34. H2O H2O H2O H2O 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 • Changes in turgor pressure that open and close stomata result primarily from the reversible uptake and loss of potassium ions by the guard cells

35. Cuticle Upper epidermal tissue Lower epidermal tissue Trichomes (“hairs”) 100 m Stomata Xerophyte Adaptations That Reduce Transpiration • Xerophytes are plants adapted to arid climates • They have various leaf modifications that reduce the rate of transpiration • The stomata of xerophytes • Are concentrated on the lower leaf surface • Are often located in depressions that shelter the pores from the dry wind Stomata in recessed crypts of Oleander plant

36. Translocation of Phloem Sap • Organic nutrients are translocated through the phloem (translocation is the transport of organic nutrients in the plant) • Phloem sap • Is an aqueous solution that is mostly sucrose • Travels from a sugar source to a sugar sink • A sugar source is a plant 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

37. Seasonal Changes in Translocation • A storage organ such as a tuber or bulb may be a sugar sink in summer as it stockpiles carbohydrates. • After breaking dormancy in the spring the storage organ may become a source as its stored starch is broken down to sugar and carried away in phloem to the growing buds of the shoot system

38. High H+ concentration Cotransporter H+ Proton pump S ATP . Sucrose H+ H+ S Low H+ concentration Phloem loading • Sugar from mesophyll leaf cells must be loaded into sieve-tube members before being exported to sinks • Depending upon the species, sugar moves by symplastic and apoplastic pathways • In many plants phloem loading requires active transport. • Proton pumping and cotransport of sucrose and H+ enable the cells to accumulate sucrose.