Cell Membranes Osmosis and Diffusion

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Cell MembranesOsmosis and Diffusion
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Functions of Membranes

• 1. Protect cell
• 2. Control incoming and outgoing substances
• 3. Maintain ion concentrations of various
  substances
• 4. Selectively permeable - allows some molecules
  in, others are kept out

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Phospholipid Bilayer
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Fluid Mosaic Model
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Blood-Brain Barrier

• Allows some substances into the brain, but
  screens out toxins and bacteria
• Substances allowed to cross include
• water, CO2, Glucose, O2, Amino Acids, Alcohol,
  and antihistamines. HIV and bacterial meningitis
  can cross the barrier.

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Solutions

• Solutions are made of solute and a solvent
• Solvent - the liquid into which the solute is
  poured and dissolved. We will use water as our
  solvent today.
• Solute - substance that is dissolved or put into
  the solvent. Salt and sucrose are solutes.

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Methods of Transport Across Membranes

• 1. Diffusion
• 2. Osmosis
• 3. Facilitated Diffusion
• 4. Active Transport

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Methods of Transport Across Membranes

• 1. Diffusion -passive transport - no energy
  expended
• 2. Osmosis - Passive transport of water
  across membrane
• 3. Facilitated Diffusion - Use of proteins to
  carry polar molecules or ions across
• 4. Active Transport- requires energy to
  transport molecules against a concentration gra
  dient energy is in the form of ATP

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Diffusion

• Movement of molecules from an area of high
  concentration to an area of low concentration.
• Movement from one side of a membrane to another,
  un-facilitated

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Diffusion
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Osmosis
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Tonicity is a relative term

• Hypotonic Solution - One solution has a lower
  concentration of solute than another.
• Hypertonic Solution - one solution has a higher
  concentration of solute than another.
• Isotonic Solution - both solutions have same
  concentrations of solute.

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Plant and Animal Cells put into various
solutions
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Types of Transport
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Todays Lab

• We are using dialysis tubing as the cell membrane
  - It is selectively permeable
• The solute is either the eosin starch solution or
  the sugar solution
• What is the solvent?

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Membrane Permeability

• Solute is eosin-starch-chloride solution
• Solvent is Water
• Indicator for presence of starch is IKI
• Starch is made of amylose and amylopectin
  amylopectin is insoluble
• Iodine is not very soluble in water, but with KI
  it forms a I3- (triiodine ion) which is soluble.
  
• I3- combines with the amylose and the starch
  molecule turns blue-black.

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Test for Starch
Elmhurst College Website
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Test for Chloride ions

• Indicator for presence of chloride ions is silver
  nitrate, AgNO3
• A white precipitate, AgCl, forms if chloride is
  present.

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Test for Sulfate ions

• Indicator for the presence of sulfate ions is
  Barium chloride, BaCl
• Barium sulfate, BaSO4 - forms a white precipitate
• BaCl NaSO4 ----gt BaSO4 NaCl

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Living Cells

• Beet cubes will be used to see the result of
  boiling and adding alcohol to a live membrane.
• Yeast cells are used to see effects of heat
• Fern gametophytes are used to see result of
  putting live cells in solutions of varying
  tonicity

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Osmosis

• We will make an osmometer to see osmosis
• Sugar solution in a dialysis tube is used to
  simulate a cell membrane.
• Various concentrations of solute may be used
  around the room.

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