BIOSCI 152 1st Edition Lecture 17Learning Objectives: After suitable revision, you should be able to• Define and use in context the vocabulary words below • Understand differences between passive and active transport. • Recognize which types of molecules diffuse across biological membranes,which require a transmembrane carrier or transporter • Understand what happens to plant cells exposed to higher (hypertonic) orlower (hypotonic) solute concentration • Recognize role of solute concentration and water potential (w) in driving watermovement • Explain how water moves from soil into roots, via xylem to the rest of plant,through leaves and out to the air VocabularyDiffusion Vascular stele TranspirationConcentration gradient Cortex Root pressurePassive transport Epidermis Cohesion-tensionMembrane channel Root hairXerophytic adaptationsMembrane carrier Endodermis Stomatafacilitated diffusion Casparian stripPhloemActive transport TracheidsgirdlingATP Vessel elementsProton pumpWater potential wPhloemTurgor pressureCotransporterPlasmolysisCationOsmolyteAnionOverview• Cellular transport – movement across membranes • Movement within a plant • Water potential • Transport from soil into root • Transport to leaves • Preserving water – xerophytic adaptations Solute diffusion– Diffusion is the movement of solutes (dissolved ions, sugars, amino acids etc) from an area of high concentration to an area of low concentration (concentrationgradient)Diffusion works within cellsCrossing membranes requires transportMembranes are impermeable to charged ions and large organic moleculesSolute transport across membranes• Passive Transport – diffusion across a membrane - occurs without energy expenditure• Active Transport – pumping of solutes across membranes against a concentration gradient – low concentration → high concentration requires energy (ATP)Passive transport across membranes Fig. 5.6• Passive (direct) diffusion (gases - O2, CO2, H2O, small uncharged molecules) • Channels – transmembrane proteins that selectively admit certain ions • Carrier (facilitated diffusion) – transmembrane proteins Active Transport across membranes• Pumping of solutes through a membrane against the concentration gradient i.e.low to high concentration• Cell must expend energy (ATP) • e.g. proton pumps (Fig. 25.10) Proton pump – coupled to other transporters Fig. 25.10Solute diffusion within cells, water movement in and out of cellsOsmosis – water moves to equilibrate concentrations across a membraneIons cannot move Fig. 5.3Wilting in herbaceous plants relates to cell water contentIn class exercise :Isotonic, Hypertonic, Hypotonic solutions – which way does the water move? What happens to cellshape?Water Potentialw depends on solute concentrationHigh solute (ions, sugars, osmolytes) concentration = very low solute potentialWater moves from low to high solute concentrationWater moves from higher to lower wFrom cellular to whole plant transportIon uptake across root-hair membranes Fig. 25.11Nutrient uptake by root hairsRootsFunctions of regions of rootEndodermis1. Casparian strip – layer of waxy suberin - impermeable to water 2. Casparian strip prevents water and minerals from crossing endodermisbetween cell walls 3. Must cross plasma membrane into cytoplasm 4. Endodermal cells can monitor minerals entering vascular tissue 5. Prevents backflow from vascular tissue to cortexCasparian strips forces transport into cell cytoplasm (inside cells)Fig. 25.11. Soil → epidermis (root hair) → root cortex → endodermis→ vascular tissue → rest of plant via root then stemHow does water travel way up tree stems?Xylem sap movement• Plants need lots of water to prevent desiccation • Mature maple tree - 200L water hour-1 in summer • Actively transpiring leaf - replaces all water every hour • Xylem sap - can ascend 100 m in height against gravity! • Transport rates of 15 m hour -1 or faster • Two mechanisms to move xylem sap • root pressure mechanism – a push • transpiration cohesion-tension mechanism – a pullRoot pressure = push• Roots accumulate minerals (solutes) at night • Ions are pumped into xylem but can’t flow out because of endodermis(casparian strip) • w of stele becomes lower • Water moves into xylem into the lower w , generating positive pressure • Forces water up the xylem Guttation – exudation of water in morning on leaf tipsRoot pressure = pushForces water up the xylemMaple syrup harvested from xylem tissue• Water cohesion (hydrogen-bonding) - a column of water in xylem ‘tubes’ • Adhesion of water to xylem cell walls • In small vessel diameter (tracheid) water adhesion overcomes gravity • Water can be ‘pulled’ up the stem • Secondary walls of xylem prevent walls collapsingTranspiration cohesion-tension- pulling water up the stemFig. 25.12• Higher water vapor concentration inside leaf than in air • Water vapor diffuses out of leaf via stomata • Generates negative water pressure in the leaf air spaces – water flows from thesurrounding cells • Draws water out of xylem, through mesophyll cells to stomata Transpiration – balancing water loss from leaves with water uptake via rootsXerophytic adaptations to conserve water water limitation drives adaptationMorphological, physiological, anatomical adaptations• Biochemical – CAM, C4 • Minimal leaf surface area, reduced leaves • Thick epidermis and cuticle • Stomata on lower surface, sunken in depressions • Strong stomatal control • Shed leaves in dry season• Succulent stems or leaves Barrel cacti – low surface area/vol ratio, water storing tissueMarram grass – curled leaves Stomata sunken in cavitiesEucalyptus globulus - leaves hang down, thick cuticle; Casuarina - Highly reducedleavesPhloemTransport of photosynthate (sugars) from leavesPhloem Transport – from leaves to other parts of plantContinuous chain of phloem-conducting cellsGirdling a tree removes bark including phloem - starves tree rootsSummary• Root and stem anatomy • Long-distance transport of water and minerals • How water and minerals move through xylem • Water potential – drives cellular and whole plant transport • Transpiration is driven by push from roots and pull from leaves • Xerophytic adaptations • Phloem