BSCI207 – Intercellular TransportSo how do trees get water all the way to the top?- Water Potential = o of pure water = 0o Measure in pascal (Pa) or megapascal (MPa) Force per unit areao Other solutions with solute in it have a lower water potential (expressed as a (-) value) The greater the osmolarity, the lower (more negative) the o Water always moves from high to low water potential!Pascal and psi- Psi is 1 pound of force per square inch- Pascal equals to a force of 1 Newton per square meter. - 1 Psi = 6,894.75729 Pascals (pa)- Gas pressure is +pa- Solute concentration pressure is -pa has two components- Solute potential (s) due to dissolvedsoluteso Water moves due to osmosiso [Solute] in a solution - Pressure potential (p)o Water moves due to pressureo Loss of tugor pressure in cells …leads to wilting!Water enters roots from soil- of soil ~ - 0.3 mPa- of cytoplasm in roots ~ - 0.6 mPao Due to solutes in plant cell cytoplasm Water has to pass through several layers to reach vascular tissue!Importance of the Casparian Strip (suberin)- KEY – To block entry of Na+ & other undesired solutes!So how does water now get to the top of the tree?- Properties of Watero Surface Tensiono Cohesiono Adhesiono Capillary Action MeniscusCohesion-Tension Theory Water Movement in Trees- Describes how water moves from the roots to the leaf- Osmosis causes water to enter the xylem of roots fromthe soil- Due to hydrogen bonding between water molecules,water forms a string of molecules as it moves to thexylem- Constant transpiration (evaporation of water fromplant leaves) at the top of the leaf pulls the watermolecules out of the plant- The difference in water potentials and pressures causethis fairly constant movement of water through theplantEvidence for Cohesion-Tension Theory?- Q: Do daily changes in the diameter of tree trunks support the cohesion-tension theory?- Hypothesis – When transpiration is occurring, xylem u sunder enough tension to make tree trunks shrink slightly.- Prediction – Tree trunks shrink during the day when transpiration is occurring and should expand at night whentranspiration stops- Conclusion – On hot summer days, xylem is under enough tension to shrink tree trunks slightly. This observation supportsthe cohesion-tension theoryPlant Signals Experiment- Experimental setupo Divide roots of many plants into two sideso In experiment group, water on one side. In control group, water on both sideso In both groups, measure water potential of leaves and observe stomata- Prediction – stomata in experimental group will close; stomata in control group will stay open- Results – no difference between experimental and control plants in water potential of leaves; stomata in experimentalgroup closed while stomata in control group will stay opened- Conclusion – Roots can communicate with shoots. Dry roots signal the shoot and cause stomata to close, even thoughleaves are receiving sufficient water (from roots on the wet side of the plant)Stomata- Stomata Opening in response to blue lighto Guard cells change shape in response to ion and water flowo Blue light strikes photoreceptor and H+ is pumped out K+ and Cl-/H+ (co-transporter) enter Water enters viaosmosis cell swells. Stomata open!- Stomata closing in response to ABA and dry conditionso ABA binds to receptors on guard cells and pumps out Cl- change in membrane potential allows K+ to exit Water leaves via osmosis Cell shrinks. Stomata close!Halophytes – plants that are able to thrive in salty environments (low water potential)- Have enzymes and transport proteins that increase the concentrations of ions in their vacuoles and increase theconcentration of sugars and other organic molecules in the cytoplasm o Keep the water potential of their tissues even lower than the water potential of salty soil o Avoid water loss!- Have specialized structures on their leaves to excrete salt via active transportMoving Fluid- Fluid flows due to P- Organisms have evolved muscular organs and tissues to work as a pump- Remember: a pump has two parts to each strokeo Intake due to negative pressureo Output due to compression and + pressureOpen Circulatory Systems (ex. clams, grasshoppers)- Low pressure system = low flow rates- Low energy requirements to move blood- Blood or hemolymph flows into spaces directly around tissueso Minimum diffusion distance!- Blood cannot be directed to tissues with high O2 demand- Only some of the blood gets aerated! Why can insects utilize an “open” system, but you cannot?- Exoskeleton- Size (vs. gravity)- Metabolic rate- PRESSURE!Closed Circulatory Systems- High pressure differences in a closed system = high flow rates- Two types of capillary beds for gas exchangeo Respiratory surface and tissue capillarieso Density of capillaries is proportional to activity of the tissueo Minimize diffusion distance!- Blood can be directed to tissues with high O2 demandPOISEUILLE’S LAW- Q = ml/min- P = pressure difference- r = radius of vessel(s) in a parallel system of pipes- = viscosity of fluid- L = length of system -Evolutionary Perspectives- Closed Systems have evolved in both invertebrates andvertebrateso Annelids (earthworms) Intense muscular activities Use moist skin as respiratory surface Requires high flow rates and respiratorypigment Closed System Design- Pump (two cycles per stroke!)- Muscular arteries to maintain high pressureo Control radii of vessels for distribution of blood- Distensible veins to hold quantities of bloodBlood flow to the brain of a giraffe?- Upright: Blood flow to the brain is via high pressurewith thick arteries and veinso MAP = 193 mm Hgo MAP at head = 131 mm Hg- What about when the giraffe lowers its head to drink? ~0So how does your heart work?- Auto-rhythmic- Cells functions as a unito Linked via gap junctions- Systole and Ejection- Diastole and Filling The BIG Picture- Your heart attempts to maximize the P between your aorta and right atrium- Maximum P results in maximum blood flow!Questions: - Where is the pressure the highest?- Where is the pressure the lowest?- Where is the velocity of blood the highest? and lowest?o Why? Capillary Fluid Movement: Starling’s Hypothesis- At the end of the capillary nearest to an arteriole, the
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