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Plants are phenomenal hydraulic engineers. Using only the basic laws of physics and the simple manipulation of potential energy, plants can move water to the top of a tree approaching 116 m (~381 ft). Plants can also use hydraulics to generate enough force to split rocks and buckle sidewalks. Plants achieve this because of water potential. Water potential is a measure of the potential energy in water. Plant physiologists are not interested in the energy in any one particular aqueous system, but they are very interested in water movement between two systems. In practical terms, therefore, water potential is the difference in potential energy between a given water sample and pure water (at atmospheric pressure and ambient temperature). Water potential is denoted by the Greek letter ψ (psi) and is expressed in units of pressure (pressure is a form of energy) called megapascals (MPa). The potential of pure water (Ψwpure H2O) is, by convenience of definition, designated a value of zero (although pure water contains plenty of potential energy, that energy is ignored). Water potential values for the water in a plant root, stem, or leaf are therefore expressed relative to Ψw pure H2O. The water potential in plant solutions is influenced by solute concentration, pressure, gravity, and factors called matrix effects. Water only moves in response to ΔΨ, not in response to the individual components. However, because the individual components influence the total Ψsystem, by manipulating the individual components (especially Ψs), a plant can control water movement. Solutes, pressure, gravity, and matric potential are all important for the transport of water in plants. Water moves from an area of higher total water potential to an area of lower total water potential. Solute Potential Solute potential (Ψs), also called osmotic potential, is negative in a plant cell and zero in distilled water. Typical values for cell cytoplasm are –0.5 to –1.0 MPa. Solutes reduce water potential (resulting in a negative Ψw) by consuming some of the potential energy available in the water. Solute molecules can dissolve in water because water molecules can bind to them via hydrogen bonds; a hydrophobic molecule like oil, which cannot bind to water, cannot go into solution. The energy in the hydrogen bonds between solute molecules and water is no longer available to do work in the system because it is tied up in the bond. In other words, the amount of available potential energy is reduced when solutes are added to an aqueous system. Thus, Ψs decreases with increasing solute concentration. Because Ψs is one of the four components of Ψsystem or Ψtotal, a decrease in Ψs will cause a decrease in Ψtotal. Water moves towards areas of lower Ψs (and thus lower Ψtotal). The semipermeable membrane that separates the two sides of the tube allows water but not solutes to pass. In the first tube, solute has been added to the right side. Adding solute to the right side lowers Ψs, causing water to move to the right side of the tube. As a result, the water level is higher on the right side. #NikolaysGeneticsLessons #Osmosis #waterPotential #whatIsOsmosis #howOsmosisWorks #movementOfWater #hypertonic #hypotonic #vocabulary #solutes #solvent #tutorial #Lesson #explanation #cartoon #animation #funny #simplified #TEKS #plants #examples #saltwater #freshwater #examplesOfOsmosis #diffusion #waterPotentialCalculation #waterPotentialFormula #biology #science #educational #waterMolecules #turgorPressure #cellWall #osmose #APBiologyOsmosisLab #highSchoolBiology #quiz
