2.8 Tonicity and Osmoregulation

Cells have membranes that allow them to establish and maintain internal environments that are different from their external environments. Explain how concentration gradients affect the movement of molecules across membranes. External environments can be hypotonic, hypertonic, or isotonic to internal environments of cells-- Water moves by osmosis from areas of high water potential/low osmolarity/low solute concentration to areas of low water potential/high osmolarity/high solute concentration. Explain how osmoregulatory mechanisms contribute to the health and survival of organisms. Growth and homeostasis are maintained by the constant movement of molecules across membranes. Osmoregulation maintains water balance and allows organisms to control their internal solute composition.water potential.

osmosis

diffusion of water across membrane from high water potential/low osmolarity/low solute concentration to low water potential/high osmolarity/high solute concentration

water potential

measures eagerness of water to flow from high to low potential

• adding solute lowers water potential because water is more likely to enter than exit

water potential equation

water potential = pressure potential + solute potential

solute potential/osmotic potential

tendency of water to move by osmosis in response to differences in solute concentrations, effect of solute on water flow

solute potential of a solution equation

solute potential = -ionization constant * molar concentration * pressure constant * temperature in Kelvin

tonicity

ability of solution/environment (based on solute concentration) to cause cell to lose/gain water, organisms must use osmoregulation to maintain homeostasis (ex. contractile vacuoles)

isotonic solutions

• same concentration of solute

• no net movement of water

• in plants, water doesn’t enter, cell becomes flaccid, plants wilt

hypertonic solutions

• higher concentration of solute

• water moves out of submerged cell

• cell shrivels

• in plants, membrane pulls away (plasymolysis), plant wilts and dies

hypotonic solutions

• lower solute concentration

• water moves into submerged cell

• cell swells and bursts

• in plants, cell wall opposes water uptake, cell is turgid and healthy