Tonicity and Osmoregulation

Tonicity

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

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

Cells can be three types of solutions…

• Isotonic

• Hypertonic

• 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

Have no net movement of water

• the concentration of non-penetrating solutes inside the cell is equal to that outside the cell

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

Hypertonic Solutions

Lose water to their extracellular surroundings

• the concentration of non-penetrating solutes is higher outside of the cell

• Water will move into the extracellular fluid

— Cells will shrivel and die

Plasmolysis

Similar to the reaction of cells in a hypertonic solution

• Vacuole shrinks and the plasma membrane pulls away from cell wallHy

Hypotonic solution

Cells gain water

• the concentration of non-penetrating solutes is lower outside of the cell

• Animal cells swell and lyse (burst)

• Plant cells work optimally as the cell wall (its tension) and central vacuole maintains turgor pressure

Water potential

a physical property that predicts the direction water will flow

• includes the effects of solute concentration and physical pressure

Water will flow from areas of: (water potential)

• High water potential to low water potential

Water will flow from areas of: (solute concentration)

Low solute concentration to areas of high solute concentration

Water will flow from areas of: (pressure)

High pressure to areas of low pressure

Low water potential equals…

high solute concentration

High water potential equals…

low solute concentration

Water potential formula

Water potential equals solute (osmotic) potential plus pressure potential

• measured in megapascals (MPa) or bars

1 MPa

10 bars

Solute potential

an increase in solutes causes binding to more free water

• this reduces water potential

• Expressed as a negative number

Pure water (MPa)

0 MPa

Pressure potential

The physical pressure on a solution

• Can be + or - relative to atmospheric pressure

• “Open air” means 𝚿_p= 0 MPa