D2.3: Water Potential

Solvation

Combination of solvent and solute

Water's Solvent Properties

Based on water's polarity, dissolves due to hydrogen bonds between partially +/- parts of water and solutes

Forms hydration shells around solutes to prevent precipitation

Osmosis

New movement of H2O across membrane due to attractions between water and solutes.

Occurs from area of low solute [] to an area of high solute [] until dynamic equilibrium is reached

Solutions w/ greater concentrations have more osmotically active solutes, therefore more intermolecular interactions w/ H2O

Why does Osmosis occur?

Solutes in H2O can move, but cannot separate from H2O, therefore they restrict H2O movement.

Some solutes cannot cross the cell membrane, resulting in movement of water, rather than solutes

Concentration

the solute per volume of solution (mol DM^-3) or (mol/L)

Osmolarity

measure of total concentration of solute particles

Hypertonic

high concentration of solute, therefore gains water

Hypotonic

low concentration of solute, therefore loses water

Isotonic

Same solute concentration, therefore no net water movement

Hypertonic Effect on Animal/Plant Cells

Animal: H2O leaves the cell, causing cell to shrivel (crenation)

Plant: H2O leaves cell, causing cytoplasm to detach from cell wall (plasmolysis), kills cell

Hypotonic Effect on Animal/Plant Cells

Animal: H2O enters cell, cell swells and bursts (lysis)

Plant: H2O enters, cytoplasm expands but cell does not lyse (turgid), optimal for plants, lets them resist gravity w/ turgor pressure

Isotonic Effect on Animal/Plant Cells

Animal: Optimal, no net water movement

Plant: Cell is not filled w/ max amount of H2O (flaccid), leaves and stem become limp/wilt

Contractile Vacuoles

Used in freshwater unicellular organisms to remove incoming water, typically needed when in hypotonic solution (ex. paramecium)

Tonicity

Ability of solution to cause a cell to gain/lose water

Tissues/organs must be stored in isotonic solution to prevent cell damage

Usage of Normal Saline (5)

Isotonic NaCl solution (need Na+ b/c essential in nervous system)

Uses:

-put into blood system to help w/ dehydration

-rinse wounds/abrasions

-keep damaged skin hydrated (burns)

-eye drops/contact solution

-slush for cooling donor organs

Water Potential

Potential energy of H2O to do work, measured in kilopascals (kPa)

At Ψw = 0 for pure H2O at standard pressure and 20 Degrees C, max Ψw for cells is 0, therefore can only be zero or negative for cells

Water moves from higher to lower water potential

Water potential formula

Ψw = Ψs + Ψp

w = water

s = solute potential

p = pressure

Solute Potential

Increase solute concentration, decrease water potential (less potential energy)

Can only be negative (b/c H2O cannot hold less solutes then none)

Pressure Potential

As pressure increases, water potential increase

Can be positive or negative (suction, like in evaporation and transpiration)