Water Potential

SOLVATION WITH WATER AS THE SOLVENT

  • Solvation – the interaction of a solvent with dissolved molecules or ions.

  • Water is an ideal solvent because of its polarity: the hydrogen ends are relatively positive and the oxygen end is relatively negative.

  • Water molecules can interact with the positive and negative charges of solutes and form hydrogen bonds.

  • Polar solvent molecules can solvate polar solutes and ions because they can orient the appropriate partially charged portion of the molecule towards the solvent through hydrogen bonding or ion-dipole forces.

  • Solvation of sodium chloride in water (conceptual):

    • Na and Cl− ions become surrounded by hydration shells.

    • Na+ is surrounded by water molecules with the oxygen ends facing the ion (O atoms coordinate to the cation).

    • Cl− is surrounded by water molecules with the hydrogen ends facing the anion (H–O dipoles orient toward Cl−).

    • This hydration stabilizes the ions in solution and facilitates dissociation of NaCl into Na and Cl− in water.

WATER MOVEMENT BY OSMOSIS

  • Osmosis is the net (overall) movement of water molecules from a less concentrated (more dilute) solution to a more concentrated solution, across a selectively permeable membrane.

  • Three solution concentrations:

    • HYPOTONIC – external solution is less concentrated than the cell cytoplasm; net inflow of water into the cell by osmosis.

    • HYPERTONIC – external solution is more concentrated than the cell cytoplasm; net outflow of water from the cell by osmosis.

    • ISOTONIC – external solution has the same solute concentration as the cell cytoplasm; no net entry or exit of water by osmosis.

WATER MOVEMENT BY OSMOSIS INTO OR OUT OF CELLS

  • The direction of net water movement can be predicted from the external solution relative to the cell cytoplasm.

  • In an isotonic solution, there is no net movement of water because solute concentration is the same on both sides of the membrane.

  • The isotonic condition is a dynamic equilibrium: water molecules still cross the membrane (dynamic movement), but there is no overall change in water concentration on either side.

CHANGES IN PLANT TISSUE BATHED IN HYPOTONIC AND HYPERTONIC SOLUTIONS

  • Plant tissue bathed in an isotonic solution – tissue retains the same dimensions and mass.

  • Plant tissue bathed in a hypertonic solution – tissue decreases in dimensions and mass.

  • Plant tissue bathed in a hypotonic solution – tissue increases in dimensions and mass.

  • Osmotic concentration – the measure of solute concentration (defined as the number of osmoles (Osm) of solute per litre (L)).

  • Deducing the concentration of the bathing solution isotonic with potato cells (method example used in labs).

EXPERIMENTAL TREND: CHANGE IN MASS VS SOLUTE CONCENTRATION

  • Graphically, as external sucrose concentration increases, the percentage change in mass tends to move from positive (water uptake) toward negative (water loss).

  • The relationship demonstrates osmotically driven water movement relative to external solute concentration.

EFFECTS OF WATER MOVEMENT ON CELLS THAT LACK A CELL WALL (RED BLOOD CELLS)

  • In pure water or hypotonic solutions, cells swell and can lyse due to the influx of water and osmotic swelling.

  • In hypertonic solutions, cells shrink as water leaves cytoplasm; cells are crenated.

  • Isotonic tissue fluid is important to prevent harmful changes in multicellular organisms (osmoregulation).

  • Unicellular organisms in freshwater use contractile vacuoles to remove excess water.

(a) ANIMAL CELL vs (b) PLANT CELL: RESPONSES TO OSMOSIS

  • Hypotonic

    • Animal cell: water enters; Lysed (bursts).

    • Plant cell: water enters; the cell becomes TURGID due to the cell wall

  • Isotonic

    • Animal cell: Normal / equilibrium.

    • Plant cell: Flaccid; cell wall remains intact but lacks turgor.

  • Hypertonic

    • Animal cell: water exits; cells become shriveled.

    • Plant cell: water exits; plasmolyzed.

  • Key terms seen in diagrams:

    • Lysed (animal cells only in hypotonic)

    • Normal (animal cell in isotonic)

    • Shriveled (animal cell in hypertonic)

    • Turgid (plant cell in a hypotonic solution)

    • Flaccid (plant cell in isotonic)

    • Plasmolyzed (plant cell in hypertonic)

CONTRACTILE VACUOLE PROCESS (illustrated cycle in animal cell vs plant cell context)

  • In hypotonic conditions, animal cells may lyse; in freshwater unicellular organisms, contractile vacuoles expel excess water to maintain internal water balance.

  • Steps (as depicted):
    1) Water enters due to osmosis.
    2) Excess water accumulates in the contractile vacuole.
    3) The vacuole moves to the edge of the cell.
    4) Contractile vacuole swells.
    5) Vacuole bursts and expels water.
    6) Cycle repeats.

EFFECTS OF WATER MOVEMENT ON PLANT CELLS WITH A CELL WALL

  • Osmosis in plant cells with a cellulose cell wall yields different outcomes:

    • Hypotonic solution -> TURGOR PRESSURE develops; cell is TURGID; the cell wall prevents bursting and provides mechanical support.

    • Hypertonic solution -> cell becomes FLACCID and plasmolyzed.

MEDICAL APPLICATIONS OF ISOTONIC SOLUTIONS

  • Isotonic solutions are used in intravenous fluids (drips) and in the bathing of transplant organs.

  • If a hypotonic solution were introduced to the body, red blood cells would swell and burst, reducing oxygen delivery to cells.

  • A hypertonic solution would cause red blood cells to shrink and lose water, reducing mobility and increasing clot risk.

  • Transplant organs must be maintained in a saline solution that is isotonic with the cells of the tissues and organs to prevent cellular damage during transit.

WATER POTENTIAL (HL)

  • Water potential – the potential energy of water per unit volume.

  • It quantifies the tendency of water to move from dilute to concentrated solutions due to osmosis.

  • Measured in kilopascals (kPa) and represented by the Greek letter Ψ (psi).

  • Absolute measurement is not possible; values are standardized relative to pure water at standard atmospheric pressure and 20°C.

MOVEMENT OF WATER FROM HIGHER TO LOWER WATER POTENTIAL (HL)

  • A solution with a low solute concentration has a high water potential (Ψw) and thus high potential energy for movement.

  • A solution with a high solute concentration has a low water potential (Ψw).

  • In solutions containing solute, water molecules form hydrogen bonds with solute and have restricted freedom of movement, lowering Ψw.

  • Water moves from regions of higher Ψw (lower solute concentration, higher potential energy) to regions of lower Ψw (higher solute concentration, lower potential energy).

  • Summary: Water potential governs osmosis in and between cells and tissues.

CONTRIBUTIONS OF SOLUTE POTENTIAL AND PRESSURE POTENTIAL TO THE WATER POTENTIAL OF CELLS WITH WALLS (HL)

  • The solute concentration determines the solute potential: ψs\psi_s

  • The pressure within a cell contributes to the pressure potential: ψp\psi_p

  • The total water potential is the sum: ψw\psi_w == ψs\psi_s + ψp\psi_p

  • Solute potentials can range from zero downwards (i.e., lower than or equal to zero): ψs≤0\psi_s \le 0

  • Pressure potentials are generally positive inside cells, though negative pressure potentials occur in xylem vessels where sap is under tension: \psip > 0 \text{in most cells}, \quad \psip < 0 \text{in xylem}

  • Pure water has the highest water potential, which is zero: ψw=0 \psi_{w}=0\,

    For pure water at standard conditions.

WATER POTENTIAL AND WATER MOVEMENTS IN PLANT TISSUE (HL)

  • Hypotonic solution effects:

    • Water moves into the cell, increasing the number of water molecules in the cell.

    • This raises pressure potential (ψp) because there is more water exerting pressure on the plasma membrane.

    • The solute concentration within the cell decreases (relative to before) as water dilutes solutes.

    • Overall, the cell’s water potential (ψw) increases (becomes less negative or more positive).

  • Hypertonic solution effects:

    • Water moves out of the cell, decreasing ψp as there are fewer water molecules.

    • Solute concentration within the cell increases as water leaves.

    • Overall, ψw decreases (becomes more negative).

  • These changes illustrate how ψs and ψp together determine water movement in plant tissues with walls.