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					Cell Membranes Osmosis 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

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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