Diffusion, Osmosis and Biological Membranes Lab 4

Phospholipid Bilayer Hydrophobic Hydrophilic

Phospholipid Bilayer

Lipids Triglycerides (Neutral Fat) Fats are synthesized from 2 components: 1. Fatty acid: long chain C and H atoms ending in a COOH group 2. Glycerol: a three C molecule; note, glycerol is an alcohol Glycerol forms a backbone to which 3 fatty acids are attached via a dehydration reaction fat molecule Provides long term energy storage, insulation

Mechanisms by which substances move into and out of the cell Diffusion – Mode by which small, uncharged molecules enter of leave a cell Osmosis – Diffusion of water Dialysis – Diffusion of dissolved solute Endocytosis – Ingestion of large molecules into a cell – 2 types: Phagocytosis: Cell eating Pinocytosis: Cell drinking Membrane Carrier Proteins – Transport liquid insoluble material (ions, glucose, amino acids) by 3 mechanisms: Facilitated Diffusion: A carrier protein in the membrane combines with a substance moving across the membrane down its [gradient] Protein Channels: Passage of nonlipid soluble molecules Active Transport: Transfers molecules DOWN OR AGAINST [gradient] with use of ENERGY!!

Why so many mechanisms to gets things into and out of the cell? Because the membrane is selectively permeable. It allows easy entry of some molecules and more difficult entry of others, possibly preventing entry at all!

- Molecules move from area of [highest] to areas of [lower], DOWN the concentration gradient - Molecules eventually reach equilibrium

Factors that affect diffusion Temperature -Temperature is an increase in molecular speed (kinetic energy) -Therefore, will higher temperature increase or decrease the rate of diffusion? Molecular Weight -Larger molecules will have more ‘drag’ across the membrane thus producing a slower rate of diffusion -Test using HCl (36.461) and NH 4 OH (35.050)

Lab A: Molecular Motion - Diffusion Use a coverslip with water on it to observe the diffusion of carmine dye. Lab B: Effects of Temperature on Diffusion Use 2 test tubes with water and a crystal of potassium permanganate. Observe the diffusion when one test tube has cold water and the other has warm water. Lab C: Effects of Molecular Weight on Diffusion Use hydrochloric acid (HCl) and ammonium hydroxide (NH4OH) to observe the rate of molecular weight on diffusion. HCl: Molecular weight = 36 Na4OH: Molecular weight = 35

Lab D: Membrane Permeability Use dialysis tubing to observe permeability of molecules. Dialysis tubing acts as a molecular sieve through which molecules SMALLER then the holes in the tubing membrane pass. We will be testing permeability of starch and glucose solutions and deciphering which solution is permeable (due to its smaller size) using Benedict’s Reagent. Use potato cylinders to test the direction of water movement. Osmosis will be determined by the [solute] of a salt solution as compared to that of the [solute] of the potato Lab E: Osmosis Observe plasmolysis in the plant cell Elodea. Plant cells have a rigid outer cell wall. View the difference between normal and salt-treated Elodea cells. Lab F: Plasmolysis