Water Potential PDF

Water 2 Ψ = ΨS + ΨP + ΨM Pressure Potential (ΨP) ΨP is the effect of pressure on water potential (Ψ). ΨP is not affected by [solute], and ΨS is not affected by pressure. Increasing pressure results in an increase in th...

Water 2 Ψ = ΨS + ΨP + ΨM Pressure Potential (ΨP) ΨP is the effect of pressure on water potential (Ψ). ΨP is not affected by [solute], and ΨS is not affected by pressure. Increasing pressure results in an increase in the free energy of the system (i.e. an increase in the chemical potential) So what is the effect of P on osmosis? Water will move from higher P (ΨP) to lower P (ΨP). selectively permeable membrane Giant piston from the sky, pushing down on one side. Can have have net water movement in response to a pressure gradient. H2O Greater pressure (= greater ΨP) on left side of the membrane Same [solute] (= same ΨS) on both sides of the membrane  Water 2 Ψ = ΨS + ΨP + ΨM By definition, standard atmospheric pressure results in ΨP = 0 MPa -standard atmospheric pressure (SAP) = 1 atm = 0.1013 MPa -for a beaker of water exposed to the atmosphere, at std atm P, ΨP = 0 MPa Think about a beaker of water open to the atmosphere: -if atmospheric pressure (“AP”) is above standard atmospheric pressure (“SAP”), then ΨP > 0 MPa -if pressure is below SAP, then ΨP < 0 MPa In general: ΨP = AP – SAP = AP – 0.1013 MPa Beaker of water open to the atmosphere; -high pressure day e.g. AP = 0.1025 MPa ΨP = 0.1025 MPa - 0.1013 MPa = 0.0012 MPa And, we already have units of pressure → don't need to convert to anything else.  Water 2 Ψ = ΨS + ΨP + ΨM Having gone through that exercise, changes in AP are usually ignored when doing plant water relations calculations. Plant tissues may display ΨP values of up to 3 MPa; -allows roots to expand with enough force to split rock or concrete In that context, slight deviations from SAP are trivial; → when we do a few sample calculations, we will always assume AP = SAP = 0.1013 MPa. → ΨP = 0 MPa if AP = SAP → surface water exposed to the atmosphere has ΨP = 0 MPa ΨS = 0 MPa ΨP = 0 MPa at ΨS < 0 MPa if this is pure the top of the water. water by cjp24, Creative Commons Attribution-Share Alike 3.0 Unported  Water 2 Ψ = ΨS + ΨP + ΨM ΨP is responsible for turgor (pressurized rigidity) of the plant cell. When we say that plant cells exhibit turgor, we mean that ΨPcell > 0 MPa; -meaning that the pressure inside a cell is > than atmospheric P Turgor is the driving force for plant cell extension/expansion/elongation. In a plant cell, it is the cell wall that allows the development of ΨP; -without the cell wall, the cell would be blown apart -animal cells cannot develop significant turgor ΨP > 0 MPa is the standard state for plant cells. Modified from Plants in Action 2nd ed.  Water 2 Consequences of very high root cell What happens if plant cell ΨP = 0 MPa? turgor (plant cell ΨP >> atm pressure): (cell internal pressure = atm pressure = Plant roots can split rocks and damage 0.1013 MPa) concrete. Roots lifting slab of concrete. A Wilted squash plants demonstrating loss of demonstration of the power of turgor. ↓ cell turgor. when not happens enough internal Plants in Action 2nd ed. pressure  Water 2 Ψ = ΨS + ΨP + ΨM Matric (Matrix) Potential (ΨM) ΨM = the effect of large (insoluble) polar or charged molecules on Ψ. Water is attracted to large polar charged molecules because of the partial charges on water; - the same partial charges that lead to H-bonding Water is attracted to soil particles (soil particles have a negative charge). Soil particles with a film of water. Modified from https://www.nature.com/scitable/knowledge/library/soil- water-from-molecular-structure-to-behavior-122155909/  Water 2 Ψ = ΨS + ΨP + ΨM Water is attracted to large biological molecules such as starch and cellulose → composed of glucose subunits (glucose is polar); → leads to binding of the water to the surface of the substance Starch attracts water, but is not soluble. By Picasa author kalaya - https://commons.wikimedia.org/w/index.php?curid=9939132 Similar to the situation with solutes, attraction of water to charged or polar large molecules (too large to be dissolved) this leads to a reduction in the water potential. If there is no binding of water to a matrix, then ΨM = 0 Any binding to a matrix will lead to a reduction in potential, hence: ΨMmax = 0 MPa (same reasoning as for ΨS)  Water 2 ΨM not usually very important in well-hydrated tissues (or moist soils). Often important in the initial uptake of water by seeds, also important in dry soil (especially clays). We are going to ignore ΨM in our calculations. Will always assume well-hydrated tissues and reasonable levels of available soil water. That means that our equation, for simple calculations, will be: Ψ = ΨS + ΨP &  Water 2 Water Relations of an Idealized Cell cell wall PM Consider only ΨP and ΨS. membrane The idealized cell: Plasma 1) extremely rigid cell wall (no elasticity) → no volume changes are possible 2) a PM that is perfectly selectively permeable → water can freely pass through, but solutes cannot higher [solute] Place this cell it in a solution that has a lower [solute] than the cytosol. Water will attempt to move into the cell osmotically in response to ΔΨS. lower ΨScell is relatively constant (very little dilution H2O possible, because volume changes are not [solute] possible and liquid water is not very pressurizedb compressible). o Giant idealized plant cell, in a beaker. ΨSsoln > ΨScell (cell/cytosol has higher solute [solute] than the solution). As a consequence of the gradient in ΨS, ΨPcell increases.  Water 2 Eventually equilibrium will be attained cell wall PM → no net water movement (gross water movement still happening) ΔΨ = 0 Ψsoln = Ψcell higher ΨSsoln + ΨPsoln = ΨScell + ΨPcell [solute] But ΨSsoln > ΨScell (keep in mind that “>” means “less negative”) And ΨPsoln < Ψpcell lower [solute] ΨPsoln = 0 MPa (because it’s open to the atm) Ψsoln = Ψcell = ΨScell + ΨPcell  Water 2 Take home message: cell wall PM The turgor of a plant cell is due to: 1) the fact that the cytosol has a higher [solute] than the outside 2) the presence of a selectively permeable PM 3) the presence of a cell wall prevents the cell from rupturing higher → the combination of high cellular [solute] and [solute] a cell wall leads to cellular turgor (ΨPcell > 0) lower [solute]  Water 2 Water Relations of a Less Idealized Cell cell wall PM Consider only ΨP and ΨS. The less idealized cell: 1) cell wall that has some extensibility (= elasticity or stretchiness) → volume changes are possible 2) a PM that is perfectly selectively permeable → water can freely pass through, but solutes higher cannot [solute] Place this cell it in a solution that has a lower [solute] than the cytosol. Water will move into the cell osmotically in lower response to ΔΨS; H2O → cellular solute content is diluted. [solute] As a consequence of the gradient ΨS, ΨPcell The combination of high cellular [solute] increases, but not as much as in the more and a cell wall leads to cellular turgor (ΨPcell > 0) even with a somewhat idealized cell (because of the extensibility). stretchy cell wall.  Water 2 Water Relations of a Less Idealized Cell cell wall PM Consider only ΨP and ΨS. The less idealized cell: 1) cell wall that has some extensibility (= elasticity or stretchiness) → volume changes are possible 2) a PM that is perfectly selectively permeable → water can freely pass through, but solutes lower cannot [solute] Place this cell it in a solution that has a higher [solute] than the protoplasm. H2O higher [solute] Water will move out of the cell osmotically in response to ΔΨS; → cellular solute content is concentrated. As a consequence of the gradient ΨS, ΨPcell decreases.  Water 2 What happens if ΨScell > ΨS outside the cell? [Solute] in cell < [solute] in the environment of the cell. Plasmolysis happens. plasmolyzed cells During plasmolysis, the PM pulls away from the cell wall, and the protoplasm shrinks. In a plasmolyzed cell: ΨPcell = 0 MPa (= atm P) ΨScell = ΨSenvironment (d) is a nice diagram, but perhaps not 100% accurate (there are usually protoplasmic Noggle & Fritz (1983) Introductory strands attached to the cell Plant Physiology, 2nd. ed. Prentice-Hall. wall)  Plants in Action 1st ed. Cells in onion bulb scale leaf epidermis before and after plasmolysis, viewed by confocal microscopy and stained with fluorescent dye DIOC(6). (A) Cytoplasm is seen as pale strands at the cell surface, traversing the large vacuole. Cell boundaries are bright because the surface cytoplasm is intensely fluorescent. (B) Precisely the same field of view after plasmolysis in 0.6 M sucrose. The cell walls are now visible as dark lines between the shrunken protoplasts, which still show brightly fluorescent surfaces. (C) A reconstruction of many planes of focus at higher magnification to show some of the hundreds of stretched strands of plasma membrane that remain tethered to the wall during plasmolysis. (Micrographs courtesy B.E.S. Gunning)  Water 2 Red Onion Epidermal Peels A common system for visualizing plasmolysis. The protoplasm (living part of the cell) is composed largely of vacuole (but there is a nucleus and other organelles, and cytosol as well). The vacuoles contain a type of anthocyanin (a pink/red type). Anthocyanins are common water-soluble pigments (often found in petals of flowers). Epidermal peel incubated in a dilute solution. Cells exhibit turgor (ΨP > 0 MPa). , Red Epidermal peel incubated in a concentrated solution (high [solute]). Protoplasm shrinks and pulls away from the cell wall. Loss of turgor (ΨP = 0 MPa). https://www.microbehunter.com/observing-plasmolysis/ How plants want to be Water 2 – Plasmolysis ↑ Turgid leaf cell (turgor about 0.5 MPa) and flaccid cell (zero turgor) that has lost some water. Plasmolysis occurs when a cell is placed in a solution of osmotic strength greater than that of the cell. Water is withdrawn from the cell until its concentration of solutes equals that of the bathing Treedspresse right solution. Modified from Plants in Action 2nd ed.  Water 2 ΨPcell > 0 MPa ΨPcell = 0 MPa water loss The cells in Figures “b” and “d” have the same ΨP (0 MPa). The cell in “d” has a ΨPcell = 0 MPa lower (more negative) Ψ S. Modified from Plants in Action 2nd ed.  Water 2 Thought Experiment: Beaker of pure water – What are Ψ, ΨS and ΨP? Modified from https://www.freepik.com/free-vector/two- glass-beakers-filled-with-ice- water_7103952.htm Then add a LOT of mannitol to the beaker (mannitol is a highly water soluble sugar alcohol). Mannitol What are Ψ, ΨS and ΨP after the addition of mannitol? You do not need to know the structure of mannitol. By Edgar181 - Own work, Public Domain, https://commons.wikimedia.org/w/inde x.php?curid=2993116