BIOL 1442 1nd Edition Lecture 5 Outline of Last Lecture I: Adaptation: underground plantsII: Shoot architecture and light captureIII: Root architecture and nutrient acquisitionIV: Short distance transport mechanismsOutline of Current Lecture I: Bulk FlowII: TranspirationIII: Apoplastic/Symplastic RoutesIV: Root pressure and Xylem sapV: Cohesion-Tension HypothesisVI: Stomata: Major pathways of water lossVII: Sugar Sources and Sugar SinksCurrent Lectureo Aquaporins: Facilitating Diffusion of Water:-Aquaporins are transport proteins in the cell membrane that allow the passage of water-Provide a positive hydrophilic environment for water molecules or solutes to diffuse from a higher to a lower concentration-These selective channels affect the rate at which water osmotically moves-Permeability is decreased either by increases in cytosolic Ca2+ or decreases in cytosolic pHo Long-Distance Transport: The Role of Bulk Flow:-Efficient long distance transport of fluid requires bulk flow, the movement of a fluid driven by pressure gradient-Independent from solute concentration-Efficient movement is possible because mature tracheids and vessel elements (perforation plates) have no cytoplasm, and sieve-tube elements (sieve plates) have few organelles in their cytoplasm-This is the ideal set-up-Bulk flow relies on Pressure Potential, not solute (osmotic) potentialThese notes represent a detailed interpretation of the professor’s lecture. GradeBuddy is best used as a supplement to your own notes, not as a substitute.o Transpiration drives the transport of water and minerals from roots to shoots via the xylem:-Plants can move a large volume of water from their roots to shootso Absorption of Water and Minerals by Root Cells:-Most water and mineral absorption occurs near root tips, where root hairs are located and the epidermis is permeable to water-The concentration of essential minerals is greater in the roots than soil because of active transport-Epidermis is usually not permeable due to waxy cuticleo Transport of Water and Minerals into the Xylem (via apoplastic/symplastic routes):-The endodermis is the innermost layer of cells in the root cortex-It surrounds the vascular cylinder and is the last checkpoint for selective passage of minerals from the cortex into the vascular tissue (**acts as a filter)-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 cylinderWater and minerals in the apoplast must cross the plasma membrane of an endodermal cell to enter the vascular cylindero Details to explain Figure 36.10:1. Apoplastic Route: uptake of soil solution provides access to apoplast, water/minerals diffuse into cortex along matrix of walls and extracellular spaces2. Symplastic Route: water/minerals that cross plasma membrane of root hairs enter symplast3. Transmembrane Route: as soil solution moves through apoplast, some water/minerals are transported into cell protoplasts and move inward via symplast4. Endodermis: the Casparian Strip (waxy Suberin) blocks passage for water and minerals, only minerals in the symplast or that entered via plasma membrane of endodermal cell can bypass Casparian Strip5. Transport in Xylem: endodermal cells and living cells of the vascular cylinder discharge water and minerals into their walls (apoplast). Xylem vessels transport via bulk flow into the shoot systemo Bulk Flow Transport via the Xylem:-Xylem sap, water and dissolved minerals, is transported from roots to leaves by bulk flow (pressure)-The transport of xylem sap involves transpiration, the evaporation of water from a plant’s surface-Transpiration: evaporation of water from plant leaves due to transporting xylem sapo Pushing Xylem Sap: Root Pressure:-At night 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 (a push of xylem sap)-Root pressure sometimes results in guttation-Pushing water up and out of leaf tips (**not morning dew)-Due to more water entering leaves than what is being transpired-This positive pressure is only for short distances-Xylem sap is more commonly pulled up rather than pushed up because root pressure is too weako Pulling Xylem Sap: The Cohesion-Tension Hypothesis:-According to the cohesion-tension hypothesis, transpiration and water cohesion pull water from shoots to roots-Transpirational Pull:-Water vapor in the airspaces of a leaf diffuses down its water potential gradient and exits the leaf via stomata-As water evaporates, the air-water interface retreats further into the mesophyll cell walls, increasing surface tension-The surface tension of water creates a negative pressure potentialo Details to explain Figure 36.12:1. Water vapor diffuses from moist air spaces in leaf to drier air outside via stomata2. At first, water vapor lost by transpiration is replaced by evaporation from the water film that coats mesophyll cells3. Evaporation of water film causes the air-water interface to retreat farther in cell wall and become curved, which increases surface tension and transpiration rate4. The increased surface tension from 3. Pulls water from surrounding cells/air spaces5. Water from xylem is pulled to replace water that was losto Adhesion and Cohesion in the Ascent of Xylem Sap:-Adhesion: attractive force between water molecules and other polar substances-Cohesion: attractive force between molecules of the same substance-Water molecules are attracted to cellulose in xylem cell walls through adhesion-Adhesion of water molecules to xylem cell walls helps offset the force of gravity-Cavitation: formation of a water vapor pocket, which disrupts transpirational pull by breaking the water molecular chaino The rate of transpiration is regulated by stomata:-Leaves generally have broad surface areas and high surface-to-volume ratios, which increase photosynthesis and increase water loss through stomatao Stomata: Major Pathways for Water Loss:-About 95% of the water a plant loses escapes through stomata-Waxy cuticle tries to prevent much of water loss-Each stoma is flanked by a pair of guard cells, which control the diameter of the stoma by changing shape-Quantity of lost water depends on:1. number of stomata2. average size of their poreso Mechanisms of Stomatal Opening and Closing:-Changes in turgor pressure open and close stomata