Notes .pptx

Good for undergraduates

1. WATER TRANSPORT/TRANSLOCATION Here we shall Learn about: Water absorption from surrounding soils to the xylem vessels Water ascent up in plants Dr. Omony JB

2. Water transport in plants: The same way we drink soda from a straw! Water’s great cohesive forces (molecules sticking to each other) and adhesive forces (attaching to walls of xylem cells)

3. Transpiration-cohesion Theory for water transport in the xylem Evaporation of water in the leaves (through stomates) generates the ‘sucking force’ that pulls adjacent water molecules up the leaf surface

4. Water transport (cont’d…) Like a long chain, water molecules pull each other up the column. The column goes from roots to leaves. What’s amazing is that the water moves up by using the sun’s evaporative energy… Plants control transpiration by opening/closing stomata .

5. Water Moves through the Soil by Bulk Flow Water moves through soils predominantly by bulk flow driven by a pressure gradient , although diffusion also accounts for some water movement. As a plant absorbs water from the soil, it depletes the soil of water near the surface of the roots. Water absorption by the root

6. Root tip—the water absorption zone

7. 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

8. The overall scheme of water movement through the plant 1- From soil to root epidermis – Diffusion to the intercellular space Capillary movement of soil water to plant roots. Plant root removes water. Tension in the soil right around the root increases gradient flow of water from low tension to high. This keeps a source of capillary water flowing to the plant root. – Osmosis to the epidermis cells

9. Apoplast pathway : water moves exclusively through the cell wall without crossing any membranes. (The apoplast is the continuous system of cell walls and intercellular air spaces in plant tissues.) Symplast pathway: water moves through the symplast, traveling from one cell to the next via the plasmodesmata (The symplast consists of the entire network of cell cytoplasm interconnected by plasmodesmata.) Transmembrane pathway: water sequentially enters a cell on one side, exits the cell on the other side. In this pathway, water crosses at least two membranes for each cell in its path . Symplast pathway and transmembrane pathway are two components of cellular pathway , 2- From epidermis to and through cortex

10. Lateral transport of minerals and water in roots 1 2 3 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. Minerals and water that cross the plasma membranes of root hairs enter the symplast. 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. 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. P a t h w a y a l on g apoplast Pathway through s y m p l a s t P l a s ma membrane Apoplastic route S y m p l a s t i c route R oo t hair Epidermis Cortex Endodermis Vascular cylinder 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. Vessels (xylem) C a s p a r i an s t ri p Casparian strip Endodermal cell 4 5 2 1

11. Transversing endodermis Casparian strip? Casparian strip is a band of cell wall material deposited on the radial and transverse walls of the endodermis, which is chemically different from the rest of the cell wall. It is used to block the passive flow of materials, such as water and solutes into the stele of a plant. To transverse casparian strip, apoplast pathway does not work (blocked), only cellular pathway works Stele is the central part of the root or stem containing the tissues derived from the procambium. These include vascular tissue, in some cases ground tissue (pith) and a pericycle, which, if present, defines the outermost boundary of the stele. Outside the stele lies the endodermis.

12. From endodermis to root vessel apoplast pathway and cellular pathway (diffusion or osmosis) . From root vessel to stem vessel to leaf vessel apoplast pathway (mass flow) . From leaf vessel → leaf mesophylls and intercellular space →stomatal cavity → stomata →air (diffusion or osmosis) .

13. ASCENT OF WATER UP IN A TALL PLANT

14. H 2 O Minerals CO 2 O 2 CO 2 O 2 H 2 O S u g ar Lig h t The ascent of xylem sap : Rises to heights of more than 100m in the tallest plants A variety of physical processes are involved in the different types of transport Sugars are produced by photosynthesis in the leaves. 5 Sugars are transported as phloem sap to roots and other parts of the plant. 6 T h r o ugh s to m a t a , l e a v e s take in CO 2 and expel O 2 . The CO 2 provides carbon for p h o t o s yn t h e s i s . S o m e O 2 produced by photosynthesis is used in cellular respiration. 4 3 Transpiration, the loss of water from leaves (mostly through stomata), creates a force within leaves that pulls xylem sap upward . Water and minerals are transported upward from roots to shoots as xylem sap. 2 Roots absorb water and dissolved minerals from the soil. 1 R oo t s e x c h a n g e g a s es w i t h the air spaces of soil, taking in O 2 and discharging CO 2 . In cellular respiration, O 2 supports the breakdown of sugars. 7

15. Factors Affecting the Ascent of Xylem Sap Xylem sap Rises to heights of more than 100 m in the tallest plants 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 .

16. 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

17. 2. Pulling Xylem Sap : The Transpiration- Cohesion-Tension Mechanism Water is pulled upward by negative pressure in the xylem called capillary force

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

19. Transpiration produces negative pressure (tension) in the leaf – Which exerts a pulling force on water in the xylem, pulling water into the leaf 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 p r e s s u r e b e c o m es m o r e n e g a ti v e . T h i s n e g a ti v e p r e s s u r e, or tension, pulls water from the xylem, where the pressure is greater. C uti c l e Upper epidermis Me s o p h yl l Lower e pi d er m i s Cuticle W a t er v a p o r CO 2 O 2 Xylem CO 2 O 2 W a t er v a p o r S t o ma Evaporation At first, the water vapor lost by transpiration is replaced by evaporation from the water film that coats mesophyll cells. In transpiration, water vapor (shown as blue dots) diffuses from the moist air spaces of the leaf to the drier air outside via stomata. A i r s p a c e Cytoplasm C e l l w a l l Evaporation Vacuole W a t er f il m Low rate of t r a n s p i r a t i o n C e l l w a l l A i r s p a c e  = –0.15 MPa  = –10.00 MPa A ir - w a t er interface High rate of t r a n s p i r a t i o n 3 1 2 Air- s p a c e

20. 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 Is facilitated by cohesion and adhesion

21. Ascent of xylem sap Outside air  = –100.0 MPa Leaf  (air spaces) = –7.0 MPa Leaf  (cell walls) = – 1.0 MPa Trunk xylem  = – 0.8 MPa Water potential gradient Root xylem  = – 0.6 MPa Soil  = – 0.3 MPa Xylem sap Mesophyll cells Stoma Water molecule Atmosphere Transpiration X yl em cells Adhesion Cell wall Cohesion, by hydrogen bonding Water molecule R oo t h a i r So i l p a rti c l e Water Cohesion and adhesion in the xylem W a t e r up t a k e from soil

22. S u m m e r y of t h e D r i ving F o r c es of W a t er absorption and movement in the xylem sap Root Pressure Transpiration pull

23. 1- Root Pressure Solute Accumulation in the Xylem Generates “Root Pressure” The root absorbs ions from the dilute soil solution and transports them into the xylem. The buildup of solutes in the xylem sap leads to a decrease in the xylem osmotic potential ( Ψ s) and thus a decrease in the xylem water potential ( Ψ w). This lowering of the xylem Ψ w provides a driving force for water absorption.

24. Guttation Appearance of xylem sap drops on the tips or edges of leaves e.g. grasses Sugars, mineral nutrients and potassium D e w? Transpiration stops at night time due to stomata closing High soil moisture level Lower root water potential Accumulation of water in plants Plants will start bleeding through leaf tips and edges

25. 2-Transpiration Pull T r a n s pira t io n - cohesion theory Transpiration is the loss of water through the stomata in leaves. This loss of water causes an area of low pressure within the plant and water moves from where it is at high pressure to low pressure. The cohesion part is what allows water to do this against gravity.

26. Stomata help regulate the rate of transpiration Leaves generally have: Broad surface areas High surface-to-volume ratios Both of these characteristics Increase photosynthesis Increase water loss through stomata 20 µm

27. 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 Stomata: Major Pathways for Water Loss

28. 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 About 90% of the water a plant loses Escapes through stomata

29. Each stoma is flanked by guard cells – Which control the diameter of the stoma by changing shape Cells flaccid/Stoma closed Cells turgid/Stoma open Radially oriented cellulose microfibrils Cell w a l l V acu o l e G u a r d c e l l (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.

30. Changes in turgor pressure that open and close stomata – Result primarily from the reversible uptake and loss of potassium ions by the guard cells H 2 O H 2 O H 2 O H 2 O 2 H O K + 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 . ( b) H 2 O H 2 O H 2 O H 2 O H 2 O

31. Xerophyte Adaptations That Reduce Transpiration Xerophytes Are plants adapted to arid climates Have various leaf modifications that reduce the rate of transpiration

32. The stomata of xerophytes Are concentrated on the lower leaf surface Are often located in depressions that shelter the pores from the dry wind . L o w er e p i d e r mal tissue C u t i c l e Upper epidermal tissue T ri c ho mes S t o ma t a (“hairs”) 10  m

33. Summary of the plant water relationships Dissolving sucrose in the water to a concentration of 0.1 M: Lowers the osmotic potential to –0.244 Mpa (fig. B) Decreases water potential to –0.244 Mpa If Cell is flaccid, the internal pressure is the same as ambient pressure, so the hydrostatic pressure is MPa If this cell is placed in the beaker containing 0.1 M sucros the water potential of the sucrose solution is greater than the water potential of the cell water will move from the sucrose solution to the cell (from high to low water potential). A slight increase in cell volume causes a large increase in the hydrostatic pressure within the cell .

34. Turgor loss in plants causes wilting – Which can be reversed when the plant is watered