D2.3 WATER POTENTIAL

what is solvation

it is a process by which solvent molecules surround and interact with solute molecules, leading to the dissolution of the solute

outline water as a universal solvent

• water has a high capacity to dissolve ions or polar molecules due to its polarity and ability to form hydrogen bonds with other molecules

• hydrogen bonds form between polar solute and polar water molecules

  • slightly positively charged H atom of water are attracted to negative ions

  • slightly negative charged O atom of water are attracted to positive ions.

explain the formation of hydrogen shells

as the polar regions within water molecules associate with molecules of opposing charges, forming hydrogen shells

explain how water moves from a less concentrated to more concentrated area

• through osmosis which is the simple diffusion involving the movement of free water molecules from a low solute to high solute concentration

explain tonicity

it is the ability of extracellular solutions to make water move in or out of a cell

what is a hypertonic solution

• higher solute concentration outside of cell than inside the cell

  • water is moving out of the cell, cell is shriveled

    • HYPER = HIGH

what is a hypotonic solution

• lower solute concentration outside of the cell than inside the cell

  • water moves into the cell, cell is gaining size

    • HYPO = LOW

isotonic solution

• equal solute concentration outside and inside the cell

  • water moves in and out equally, no net change, cell looks normal

    • ISO = EQUAL

what is osmolarity

• the concentration of solution expressed as a number of solute particles per liter

  • tissues will lose water in hypertonic solution while gaining water in hypotonic solution

  • water loss or gain can be determined by weighting the sample before and after bathing it in the solution

explain the effects of water movement on cells that lack a cell wall

• animal cells lack a cell wall therefore they would go through uncontrolled osmosis

  • in hypertonic solutions: water will leave, cell will shrivel (can shrink and crenate)

  • in hypotonic solutions: water will enter, cell will undergo lysis (can swell or burst)

• unicellular organisms or freshwater organisms possess contractile vacuole to regulate osmotic conditions

  • In hypertonic solutions: water is expelled by the vacuole, causing the cell to shrink, systole

  • In hypotonic solutions: water is absorbed by the vacuole, causing the cell to swell, diastole

explain the effects of water movement on cells with a cell wall

• fungi/plant cells have a cell wall therefore they undergo moderated osmosis due to the their inflexible cell wall

  • in hypertonic solutions: water moves out of the cell, causes decrease in internal pressure (turgor pressure) making cell membranes shrink away from cell walls known as plasmolysis

  • in hypotonic solutions: water moves into the cell, causing an increase in internal pressure against the rigid cell wall (TURGOR PRESSURE), cells prevent bursting and allow plant cells to maintain a “turgid shape”

explain examples of medical applications of isotonic solutions

• organ transplant

  • organ needs to be bathed in a fluid that is isotonic to the cytoplasm of the organs cells

    • prevents loss or gain of water reducing the risk of damage for a successful transplant

• intravenous fluids

  • replace lost fluids, administer drugs, blood transfusion

  • more rapid and direct absorption into the circulatory system if the fluid is isotonic to blood/tissue fluid

what is water potential and why is it used

• it is the potential energy (energy stored in a system), per unit volume of water, relative to pure water (kPa, kilopascals)

  • pure water at standard atmospheric pressure and 20ºC = 0kPa

  • water moves from high water potential to low water potential

why does water move from a high water potential to a low water potential

• osmosis

  • water movement facilitated by osmosis, allows water to flow from areas of lower solute conc (high water potential), to areas of higher solute conc (lower water potential)

• to reach equilibrium

describe the movement of water

water moves from a high to low water concentration, high to low potential energy, or low solute to high solute concentration

explain the contributions of solute potential and pressure potential to the water potential of cells with walls

• water potential is dependent the solute potential + pressure potential

  • Ψ = ΨS + ΨP

  • water potential is the ability of water to move

• solute potential (ΨS) - more solute = less movement

  • solutes bind to water by hydrogen bonds

  • the potential energy of water is transferred into the hydrogen bonds, reducing the potential energy in the water

• pressure potential (ΨP) - more pressure = more movement

  • pressure exerted by water molecules against the cell wall of a cell

explain plant tissues in hypotonic solutions

• solute potential of tissue is more negative than the solution

• water moves from less negative to more negative potential (surrounding cell to plant cell)

• influx of water increases the pressure potential

  • this is due to the cytoplasm pressing against the cell wall

• when the water potential on the outside and insane are equal, the inward movement of water stops and cell returns to turgid state

explain plant tissues in hypertonic solutions

• solute potential of solution is more negative than tissue

• water moves form less negative to more negative potential (from plant cell to surrounding environment)

• efflux of water decreases pressure potential

  • due to reduced volume of cytoplasm which decreases the water pressure potential inside the cell causing the plant to wilt into a flaccid state