II Water and nutrient acquisition by roots Initial pathways soil to endodermis o Apoplastic non living space cell walls o Symplastic living space membrane Casparian strip waxy layer in apoplast of endodermis that forces water to cross cell membrane before entering xylem Once water and few nutrients in xylem in root now ready for long distance transport Clicker the casparian strip allows the plant to A regulate solute transport into xylem B regulate amount of H2O transport into xylem C prevent solute transport into xylem D prevent H2O transport from soil into xylem E prevent H2O transport from xylem to soil III Transpiration and bulk flow through xylem Transpiration loss of water vapor by diffusion from leaves to the surrounding environment What is actually driving water vapor loss Concentration gradient high low Stomata also necessary for gas exchange for photosynthesis also loses water as consequence Transport Process Osmosis liquid water Diffusion solutes dissolved in water Diffusion water vapor Driving force Water potential gradient Concentration gradient Where important in plants Small scale cell membrane Small scale within cell Concentration gradient Pressure flow bulk flow Pressure gradient p Inside outside of leaf transpiration Xylem less negative more negative pressure Phloem more positive less positive pressure Evaporation from water film in air space between cells How is water lost by transpiration replaced o Surface tension generated by capillarity of water in mesophyll cell walls apoplast p and thus s p but s is negligible o This pulls water from living cells to cell wall which then pulls water from xylem into living cells osmosis due to gradient across each membrane Trees lost all leaves no evaporation photosynthesis tree cells would be in equilibrium with soil Water leaving xylem in leaf creates tensions negative p gradient in the pipe of dead xylem Cohesion tension theory of xylem transport water is pulled up xylem by process of bulk flow driving force is pressure gradient less to more negative p Ring spiral wall thickening of lignin protects against vessel collapse sclerenchyma Xylem vessels and tracheids Clicker why is the transport process in the xylem bulk flow and not osmosis A living cell membranes being crossed B dead cell membranes being crossed C no cell membranes being crossed D no cell walls being crossed E no gradient in xylem What type of cells form the functional xylem pipe A parenchyma with lignified primary cell walls B collengchyma with thickened piramary cell walls C sclerencyhma with lignified secondary cell walls D endodermal cells with suberized casparian strip E epidermal cells with waxy cuticle and stomata Phloem is not sclerenchyma Leaf evaporative water loss transpiration sets up gradient that pulls water up through plant Different transport processes dominate in different parts of pathway from soil to atmosphere Tiny amounts of dissolved nutrients and hormones are carried along by the bulk flow in the xylem sap How is transpirational water loss regulated Leaves have broad SA and high SA V ratio o Increases potential for photosynthesis o Also increases potential water loss But most leaf epidermal cells covered with cuticle that prevent water loss Stomata special epidermal cells help regulated the rate of transpiration Stomatal movement Stomatal opening promoted by light and CO2 depletion closure promoted by lack of light dry hair drought K channels aquaporins and radially oriented cellulose fibers play important roles H pumped out K flow in H2O flow in stomata open o K goes up so s goes down so osmosis occurs due to concentration and gradients Open slowly but close quickly How much water is used 90 of water taken up is transpired o But necessary for CO2 uptake o Where does rest go Transpiration o Cools leaf o Transport mineral from roots to shoot o Peak velocity of xylem sap 15 45 m hr fast How do plants deal with water limitations Ecological time scale o Reduce water loss by stomatal regulation o Increase water uptake with more deeper roots Osmotic adjustment cells make compatible solutes to decrease s more and thus maintain turgor in living cells p as soil dries Evolutionary time scale plants adapted to dry habitats o Escape in time desert animals o Have inherently more conservative water use and water access strategy o Have more conservative physiology ex CAM Examples adaptations to more sever water limitations High root shoot biomass ratio and deep roots juniper in Texas Redwood trees apparently near physical limit of pulling water from soil also trap fog as a leaf water source Ocotillo plant SW US deserts leaves are drought deciduous plant uses low rates of stem photosynthesis to persist through leafless phase old man cacti leaf hairs to reflect sun and CAM photosynthesis so stomata open at night when cooler trade off slow growth IV Phloem transport also via bulk flow but in living cells and positive pressure Transport mostly sugars in water in highly specialized living parenchyma cells sieve cells stacked in pipes and connected via sieve plates Accompanied by companion cells has functional organelles Phloem cells only have cytoplasm exchange from one cell to the next Direction of flow is sugar source where it s produced to sugar sink where it s needed Why is it bulk flow not osmosis Big open pores so no crossing cell membranes How do sugars get in and out of phloem Chloroplast stroma cytoplasm apoplast or symplast pathway for loading phloem mesophyll cell But what about within phloem Process for transport o Bulk flow higher to lower pressure potential Driving force o Pressure gradient more positive to less positive