DEFIN IT ION:T H E D I F F U S I O N O F WAT E R AC R O S S A S E L E C T I V E LY P E R M E A B L E M E M B RA N E

F R O M A N A R E A O F HIGHER WATER CONCENTRATION T O A N A R E A O F

LO W E R WAT E R C O N C E N T R AT I O N .

OSMOSIS AN IMPORTANT TYPE OF DIFFUSION

WATER…WOW!

• Water is vital to life• Plants and animals use water to carry out

essential life processes• Water particles are small enough to travel

across the cell membrane in both directions• If the water concentration is equal on both

sides, the cell maintains its size.

WHAT HAPPENS WHEN THERE IS AN IMBALANCE?

• If there is more water either inside or outside the cell, the direction in which water moves across the cell membrane adjusts to this imbalance•More water will move in one direction than the other

• Water moves by osmosis from an area with higher water (or lower solute) concentration to an area with lower water (or higher solute) concentration.• Osmosis continues until the concentration of

water is equal on both sides of the membrane.• When concentrations are equal, osmosis ends

and water continues to pass through the membrane in both directions at an equal rate.

Definition: SOLUTEA substance that is dissolved in another substance – ie. Sugar and salt

OSMOSIS

CELLS IN SOLUTION

• Sugars, salts and proteins are common solutes in cells• Cells need to maintain solute

concentrations at certain levels to stay alive and healthy• The movement of water into and out of the cell determines solute concentration

Environment A• Concentration of water

molecules outside of the cell is EQUAL to concentration of water molecules inside the cell.

• Movement of water is continuous

• Shape and size of cell do not change

Environment B• Concentration of water

molecules GREATER OUTSIDE than inside the cell.

• More water molecules move INTO the cell than out

• Cell INCREASES in size• If too much water enters the

cell it may burst and die

Environment C• Concentration of water

molecules GREATER INSIDE the cell than outside

• More water molecules move OUT of the cell

• Cell DECREASES in size• If too much water leaves the

cell, it may die.

TURGOR PRESSURE: OSMOSIS IN ACTION

• Plant cells have a large, central vacuole which is filled with water• This vacuole takes up much of the cell’s

interior space• Water absorbed by the plants roots is

stored in vacuoles• When a cell needs water for cellular

processes, water moves from the vacuole to the parts of the cell where it is needed

• This movement causes a decrease in the water in the cells cytoplasm, and an increase in the concentration of solutes• When solute concentration inside the plant

cell becomes high, water moves into the cell by osmosis• When the vacuole and cytoplasm fill with

water, this exerts pressure against the cell wall, and causes the cell to swell.• This outward pressure on the cell wall is

called TURGOR PRESSURE.

• When cells in the plant’s stem and leaves take in water, they become “turgid” – they stiffen and stay upright (a)

• When cells in a plant’s stem and leaves lose water, the cells become less turgid, and the plant wilts (b)

• Problems occur when plants lose too much water.• If the concentration of solutes increases in

the soil – this lowers the concentration of water in the soil compared to the concentration of water in the plant roots• This causes water to move by osmosis

from the plants roots to the soil• Why is this important to understand in the farming industry when dealing with fertilizers?

EXIT SLIP

• Salting roads in the winter often results in a build up of salt, which causes the plant life beside the roads to die. Why are salty soil conditions not good for plants? Explain.