2.7 Tonicity and Osmoregulation

Tonicity

the ability of an extracellular solution to cause a cell to gain or lose water

depends on the concentration of solutes that cannot pass through the cell membrane

cells can be in 3 types of solutions:

isotonic, hypertonic, and hypotonic

Osmoregulation

cells must be able to regulate their solute concentrations and maintain water balance

animal cells will react differently than cells with cell walls, like plants, fungi, and some protists

Isotonic solutions

the concentration of solutes inside the cell is equal to that outside the cell

no net movement of water

water diffuses into the cell at the same rate water moves out of the cell

makes plant cells flaccid(turgor pressure decreases)

ideal for animal cells

Hypertonic solutions

the concentration of solutes is higher outside of the cell

water will move to the extracellular fluid/outside

animal cells will shrivel and die

plant cells experience plasmolysis(vacuole shrinks and the plasma membrane pulls away from the cell wall)

ex. 0.15 M inside cell 0.3 M outside cell, water moves out

Hypotonic solutions

the concentration of solutes is lower outside of the cell

the cell will gain water/moves inside

animal cells will swell and lyse(break),cytolysis

plant cells work optimally(maintains turgor pressure,turgid)

ex. 0.3 M inside cell, 0.15 M outside cell, water moves in

Water potential

a physical property that predicts the direction water will flow

measured in megapascals(MPas) or bars

includes the effects of solute concentration and physical pressure

water will flow from areas of:

high water potential→low water potential

low solute concentration→high solute concentration

high pressure→low pressure

Water potential formula

water potential= solute potential + pressure potential

Pressure potential

the physical pressure on a solution

can be positive or negative relative to atmospheric pressure

open air means the pressure potential is 0 (usually is 0 in problems)

Solute potential

an increase in solutes causes binding to more free water

this reduces water potential

expressed as a negative number

pure/distilled water is 0 MPa(no solute potential)

Solute potential formula

solute potential = -iCRT

i= ionization constant, number of particles formed, if no ions are formed the ionization constant is 1

usually only case where it is not 1 is sodium chloride (NaCl)

C= molar concentration (given in problem)

R= pressure constant, always 0.0831

T= temperature in Kelvin's, degrees Celsius + 273

all formulas are given on tests so no need to memorize