OSMOSIS
AN IMPORTANT TYPE OF
DIFFUSION
DEFINITION:
THE DIFFUSION OF WATER ACROSS A
SELECTIVELY PERMEABLE MEMBRANE FROM
AN AREA OF HIGHER WATER
CONCENTRATION TO AN AREA OF LOWER
WATER CONCENTRATION.
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
Definition: SOLUTE
A substance that is dissolved in another substance – ie. Sugar and salt
• 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.
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.