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