Cell Membranes
Osmosis and Diffusion
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
Phospholipid Bilayer
Fluid Mosaic Model
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.
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.
Methods of Transport Across
Membranes
1. Diffusion
2. Osmosis
3. Facilitated Diffusion
4. Active Transport
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
gradient – energy is in the form of
ATP
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
Diffusion
Osmosis
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.
Plant and Animal Cells put into
various solutions
Types of Transport
Today’s 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?
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.
Test for Starch
Elmhurst College Website
Test for Chloride ions
• Indicator for presence of chloride ions is
silver nitrate, AgNO3
– A white precipitate, AgCl, forms if chloride
is present.
Test for Sulfate ions
• Indicator for the presence of sulfate ions
is Barium chloride, BaCl
– Barium sulfate, BaSO4 - forms a white
precipitate
– BaCl + NaSO4 ----> BaSO4 + NaCl
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
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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