Chapter 5.5
Which Way Will Water Move?
AP Biology
Fall 2010
Bell Ringer
Objectives
• Analyze the 3 conditions possible when
describing the tonicity of a solution
• Understand what osmosis is
• Describe the effects of fluid pressure
Movement of Water
• Bulk Flow: mass movement of one or more
substances in response to pressure, gravity, or
another external force
– Ex. Running faucet, Niagara Falls, beating heart
and blood, sap flowing in trees
Movement of Water
• Osmosis: the passive movement (diffusion) of
water across a differentially permeable
membrane from high to low concentration
– In response to solute concentration gradients,
pressure gradients, or both
Movement of Water
• Ex. Osmosis
– If a bag containing a sugar solution is placed in
pure water, the water will diffuse inward (higher
to lower)
Effects of Tonicity
• Tonicity: refers to the relative solute
concentrations of two fluids
– Extracellular fluid and cytoplasmic fluid
• Most free-living cells counteract shift in
tonicity by selectively transporting solutes
across the cell membrane
Effects of Tonicity
• Hypotonic Solution
– Has a lower concentration of solutes than the fluid
in the cell
– Water moves inward
– Cells immersed in it may swell and even burst
– Ex. 2% sucrose cell, immersed in 1L distilled water
Effects of Tonicity
• Hypertonic Solution
– Has a greater concentration of solutes than the
fluid in the cell
– Water moves out
– Cells immersed in it may shrivel
– Ex. 2% sucrose cell, immersed in 1L of 10%
sucrose solution
Effects of Tonicity
• Isotonic Solution
– Has the same concentration of solutes as the fluid
in the cell
– Water moves in and out in equal proportions
– Immersion in it causes no net movement of water,
cell shape stays the same
– Ex. 2% sucrose cell, immersed in 1L of 2% sucrose
solution
Effects of Fluid Pressure
• Cells are either dependent on relatively
constant (isotonic) environments or are
adapted to hypotonic and hypertonic ones
Effects of Fluid Pressure
• Hydrostatic pressure: force directed against a
membrane by a fluid
– Greater the solute concentration, greater the
hydrostatic pressure it exerts
– Plants = is called turgor pressure
Effects of Fluid Pressure
• Osmotic pressure: of any fluid is one measure
of the tendency of water to follow its water
concentration gradient and move into that
fluid
– Can prevent further increase in the volume of the
solution
• When hydrostatic pressure and osmotic
pressure are equal in magnitude, osmosis
stops completely
Effects of Fluid Pressure
• When plants lose water there is shrinkage of
the cytoplasm
– Plasmolysis
Review
1. T/F Osmosis is the diffusion of water across a
semi permeable membrane from an area of
high to low concentration.
2. T/F A cell that is placed in a hypotonic
solution will most likely shrivel up.
3. T/F Turgor pressure refers to the shrinkage of
the cytoplasm.
Answers
1. True
2. False
3. False