Plasmodesmata Channels connec+ng neighboring cells Cell membrane and cytosol are con+nuous from cell to cell Symplast Cytoplasmic con+nuum Apoplast Compartments and Transport Through plasmodesmata con+nuum of cell walls and extracellular spaces including interior of dead cells Three Major Pathways of Transport Water and minerals can travel through a plant by three routes: 1. Transmembrane route out of one cell, across a cell wall/membrane into another cell 2. Symplas7c route via the con+nuum of cytosol 3. Apoplas7c route Cell wall Cytosol Vacuole Plasmodesma Vacuolar membrane Plasma membrane (a) Cell compartments Transmembrane route Apoplast Symplast Symplastic route Apoplastic route (b) Transport routes between cells Key Apoplast Symplast inside cell walls and extracellular spaces Absorp+on of Water and Minerals by Root Cells Bulk flow Most water and mineral absorp+on Occurs near root +ps Epidermis is permeable to water Loca+on of root hairs Add surface area Soil solu+on enters the roots Via symplas+c or apoplas+c route 1
Transport of Water and Minerals into the Xylem Endodermis Innermost layer of cells in the root cortex surrounds the vascular cylinder last checkpoint for selec+ve passage of minerals from the cortex into the vascular +ssue Transport of Water and Minerals into the Xylem Water can cross the cortex via the symplast or apoplast Casparian strip Suberin layer in the endodermal wall Blocks apoplas+c transfer of minerals from the cortex to the vascular cylinder Forces the symplas+c route Allows filtering of soil solu+on by selec+ve permeability Fig. 36-12 Casparian strip Pathway along apoplast Endodermal cell Pathway through symplast Casparian strip Plasma membrane Apoplastic route Symplastic route Root hair Vessels (xylem) Epidermis Cortex Endodermis Stele (vascular cylinder) 2
Bulk Flow Driven by Nega+ve Pressure in the Xylem Transpira7on evapora+on of water from a plant s surface Plants lose a large volume of water in this way Water replaced by the bulk flow of water and minerals called xylem sap from the steles of roots to the stems and leaves Is sap mainly pushed up from the roots, or pulled up by the leaves? Pushing Xylem Sap: Root Pressure At night transpira+on is very low root cells con+nue pumping mineral ions into the xylem of the vascular cylinder lowering the water poten+al Water flows in from the root cortex genera+ng posi+ve root pressure Gu?a7on Pushing Xylem Sap: Root Pressure Root pressure some+mes results in the exuda+on of water droplets on +ps or edges of leaves 3
Posi+ve root pressure is rela+vely weak Pushing Xylem Sap: Root Pressure minor mechanism of xylem bulk flow Nega+ve pressure Cuticle Upper epidermis Xylem Water is pulled upward in the xylem Mesophyll Air space Microfibrils in cell wall of mesophyll cell Transpira+onal pull Lower epidermis produces nega+ve pressure (tension) in the leaf Cuticle Stoma Microfibril (cross section) Water Air-water film interface Exerts a pulling force on water in the xylem, pulling water into the leaf Cohesion and Adhesion in the Ascent of Xylem Sap Cavita+on Forma+on of a water vapor pocket in xylem By a break in the chain of water molecules Caused by drought stress or freezing Stomata regulate the rate of transpira+on Leaves generally have broad surface areas and high surface-to-volume ra+os increases photosynthesis But also increases water loss through stomata ~95% of plant water loss Each stoma Flanked by a pair of guard cells Control opening of stoma by changing shape 4
Mechanisms of Stomatal Opening and Closing Guard cells turgid/stoma open Guard cells flaccid/stoma closed Changes in turgor pressure in guard cells Cell wall Radially oriented cellulose microfibrils open and close stomata primarily from reversible uptake/ loss of potassium ions Vacuole Guard cell (a) Changes in guard cell shape and stomatal opening and closing (surface view) Guard cells turgid/stoma open Guard cells flaccid/stoma closed H 2 O K+ co-transported K + by proton pumps (b) Role of potassium in stomatal opening and closing Generally S+muli for Stomatal Opening and Closing stomata open during the day close at night to minimize water loss Stomatal opening at dawn is triggered by: Light Blue light receptors s+mulate K+ uptake CO 2 deple+on Internal clock in guard cells Func+ons even in the dark circadian rhythm 24-hour cycles of behavior Adapta+ons That Reduce Evapora+ve Water Loss Xerophytes plants adapted to arid climates Ocotillo (leafless) Oleander leaf cross section and flowers leaf modifica+ons Cuticl e Upper epidermal tissue reduce the rate of transpira+on Crassulacean acid metabolism (CAM) 100 µm Trichomes Crypt Stomata Lower epidermal ( hairs ) tissue Specialized form of photosynthesis Ocotillo leaves where stomatal gas exchange occurs at night Ocotillo after heavy rain Old man cactus 5
Sugar Transport Transloca7on Process of transpor+ng the products of photosynthesis through phloem Movement from Sugar Sources to Sugar Sinks Phloem sap Aqueous solu+on that is high in sucrose Travels from a sugar source to a sugar sink Sugar source An organ that is a net producer of sugar mature leaves Sugar sink An organ that is a net consumer or storage unit of sugar tuber or bulb A storage organ can be both a sugar sink in summer and sugar source in winter Movement from Sugar Sources to Sugar Sinks Sugar Must be loaded into sieve-tube elements before being traveling to sinks Depending on the species May move by symplas+c or both symplas+c and apoplas+c pathways Transfer cells Modified companion cells enhance solute movement between the apoplast and symplast Via proton pump co-transport Mesophyll cell Cell walls (apoplast) Plasma membrane Companion (transfer) cell Sieve-tube element High H + concentration Cotransporter Proton H + pump S Plasmodesmata Key Apoplast Symplast Mesophyll cell Bundle- Phloem sheath cell parenchyma cell ATP H + H+ Low H + concentration S Sucrose 6
Movement from Sugar Sources to Sugar Sinks In many plants Phloem loading requires ac+ve transport Proton pumping and cotransport of sucrose and H + Enable the cells to accumulate sucrose At the sink Sugar molecules diffuse (facilitated) from the phloem to sink +ssues followed by water Mesophyll cell Cell walls (apoplast) Plasma membrane Companion (transfer) cell Sieve-tube element High H + concentration Cotransporter Proton H + pump S Plasmodesmata Key Apoplast Symplast Mesophyll cell Bundle- Phloem sheath cell parenchyma cell ATP H + H+ Low H + concentration S Sucrose Bulk Flow by Posi+ve Pressure Sap moves through a sieve tube by bulk flow driven by posi+ve pressure Vessel (xylem) Sieve tube Source cell (phloem) (leaf) H 2O 1 Sucrose H 2O 2 1 Loading of sugar Called pressure flow Increasing pressure at source end Reduced pressure at sink end Bulk flow by negative pressure 4 H 2O Bulk flow by positive pressure 3 Sink cell (storage root) Sucrose 2 Uptake of water 3 Unloading of sugar 4 Water recycled You should now be able to: 1. Describe how proton pumps func+on in transport of materials across membranes 2. Define the following terms: osmosis, water poten+al, flaccid, turgor pressure, turgid 3. Explain how aquaporins affect the rate of water transport across membranes 4. Describe three routes available for short-distance transport in plants 7
You should now be able to: 5. Relate structure to func+on in sieve-tube cells, vessel cells, and tracheid cells 6. Explain how the endodermis func+ons as a selec+ve barrier between the root cortex and vascular cylinder 7. Define and explain guia+on 8. Explain this statement: The ascent of xylem sap is ul+mately solar powered 9. Describe the role of stomata and discuss factors that might affect their density and behavior 10. Trace the path of phloem sap from sugar source to sugar sink; describe sugar loading and unloading 8