Osmosis:Osmosis:Striking a Striking a BalanceBalance

Maintaining A BalanceMaintaining A Balance Cells are surrounded by watery solutions Cells are surrounded by watery solutions

and are filled by watery solutions.and are filled by watery solutions. A solution is a mixture in which one or A solution is a mixture in which one or

more substances (Solutes) are dissolved more substances (Solutes) are dissolved in another substance (Solvent)in another substance (Solvent)

The concentration of a solute is important The concentration of a solute is important to organisms. to organisms.

Organisms cannot live unless the Organisms cannot live unless the concentration of dissolved substances concentration of dissolved substances stays within a narrow range.stays within a narrow range.

Balance and Balance and HomeostasisHomeostasis

This balance of conditions is called This balance of conditions is called HomeostasisHomeostasis

The movement of water across the The movement of water across the membrane helps to balance the membrane helps to balance the concentrations of dissolved substances in concentrations of dissolved substances in a cella cell

The diffusion of water across a selectively The diffusion of water across a selectively permeable membrane is called permeable membrane is called osmosisosmosis

OsmosisOsmosis Like other molecules Like other molecules the movement of water the movement of water

is controlled by differences in concentrationis controlled by differences in concentration (a (a concentration gradient)concentration gradient)

Water will move in the direction where there is Water will move in the direction where there is a high concentration of solute (hence a lower a high concentration of solute (hence a lower concentration of water)concentration of water)

A simple rule to remember is SALT SUCKSA simple rule to remember is SALT SUCKS Salt is a solute, when it is concentrated inside Salt is a solute, when it is concentrated inside

or outside a cell, it will draw water in its or outside a cell, it will draw water in its directiondirection

TonicityTonicity The measurement of solute concentration The measurement of solute concentration

on either side of a membrane is known as on either side of a membrane is known as TonicityTonicity

The tonicity of a cell’s environment affects The tonicity of a cell’s environment affects its rate of osmosis.its rate of osmosis.

IsotonicIsotonic In an isotonic solution, the concentration of In an isotonic solution, the concentration of

solute is the same on both sides of the solute is the same on both sides of the membrane (inside the cell and outside).membrane (inside the cell and outside).

A cell placed in an isotonic solution A cell placed in an isotonic solution neither gains or loses water. neither gains or loses water.

Most cells in the body are in an isotonic Most cells in the body are in an isotonic solution. solution.

Why?Why?

Red Blood CellsRed Blood Cells Plant CellsPlant Cells

HypertonicityHypertonicity A A hypertonic solutionhypertonic solution

is one that has a high is one that has a high solute (Less water) solute (Less water) concentration. concentration.

Cells in a hypertonic Cells in a hypertonic solution solution will lose will lose water. water.

HypertonicityHypertonicityAs a result of a difference in structure, As a result of a difference in structure,

plant and animal cells respond plant and animal cells respond differently differently

Animal Cells shrink Animal Cells shrink crenatecrenatePlant cells, maintain shape because Plant cells, maintain shape because

of their cell wall, the cell membrane of their cell wall, the cell membrane breaks away from the cell wall breaks away from the cell wall resulting in resulting in PlasmolyisPlasmolyis

Red Blood CellsRed Blood Cells Plant CellsPlant Cells

HypotonictyHypotonicty A A hypotonic solutionhypotonic solution is is

one that has less one that has less solute (more water). solute (more water).

When placed in a When placed in a hypotonic solution, hypotonic solution, cells tend to cells tend to gain gain waterwater and swell and swell

HypotonictyHypotonicty If an animal cell swells too much it will If an animal cell swells too much it will

burst. This is called burst. This is called LysisLysis..

Plant cells rarely burst because of their Plant cells rarely burst because of their cell wall. As water fills the cell the cell wall. As water fills the cell the membrane is pushed up against the wall membrane is pushed up against the wall (turgor pressure) making the plant cell (turgor pressure) making the plant cell turgidturgid. .

Red Blood CellsRed Blood Cells Plant CellsPlant Cells

HypotonicityHypotonicity Many single cell organisms live in Many single cell organisms live in

hypotonic solution so they may be in hypotonic solution so they may be in danger of constantly bursting.danger of constantly bursting.

To help these organisms they have a To help these organisms they have a contractile vacuolecontractile vacuole to pump water out to pump water out of the cell.of the cell.

QuestionsQuestions How can we determine which solution is How can we determine which solution is

isotonic to a given set of cells?isotonic to a given set of cells? Is the concentration the same for all cells?Is the concentration the same for all cells?