BIO 107 Lab # 4 Cell membranes, Osmosis & Diffusion

1. BIO 107Lab # 4Cell membranes, Osmosis & Diffusion S. Badran

2. The plasma membrane • Is the boundary that separates the living cell from its nonliving surroundings • Is the minimal requirement for the formation of a cell • Was the first cell part to form during cellular evolution

3. Figure 7.1 Selective permeability • The plasma membrane allows some substances to cross it but not others

4. The fluid mosaic model • States that the cell membrane is a fluid structure with a “mosaic” of various proteins embedded in it • Fluidity results from the movement of phospholipids & proteins.

5. There are four primary types of membrane proteins, each of which performs a different function.

6. The Permeability of the Lipid Bilayer • Small, hydrophobic molecules can pass through the membrane rapidly • Example: CO2, O2 • Polar molecules do not cross the membrane rapidly • Example: H2O, sugars • Charged ions DO NOT cross the lipid bi-layer on their own

7. Facilitated Diffusion by Transport Proteins • Allows passage of hydrophilic substances and ions across the membrane and consist of: • Channels with ionic amino acids for passage of ions such as Na, K and Ca • Carriers that are specific for sugar, water and other hydrophilic molecules • Example: glucose carrier and aquaporins

9. Passive transport • Cell does not expend any extra energy for diffusion • Driving force for diffusion: • kinetic energy of particles diffusing • Difference in their concentration (concentration gradient)

10. (a) Diffusion of one solute. The membrane has pores large enough for molecules of dye to pass through. Random movement of dye molecules will cause some to pass through the pores; this will happen more often on the side with more molecules. The dye diffuses from where it is more concentrated to where it is less concentrated (called diffusing down a concentration gradient). This leads to a dynamic equilibrium: The solute molecules continue to cross the membrane, but at equal rates in both directions. Molecules of dye Membrane (cross section) Equilibrium Net diffusion Net diffusion Figure 7.11 A Diffusion • tendency for molecules of any substance to spread out evenly into the available space • Activity: Diffusion

11. (b) Diffusion of two solutes. Solutions of two different dyes are separated by a membrane that is permeable to both. Each dye diffuses down its own concen- tration gradient. There will be a net diffusion of the purple dye toward the left, even though the total solute concentration was initially greater on the left side. Equilibrium Net diffusion Net diffusion Net diffusion Equilibrium Net diffusion Figure 7.11 B Diffusion • Substances diffuse down their concentration gradient, from high to low concentration

12. Effects of Osmosis on Water Balance • Osmosis is the diffusion of water across a semi-permeable membrane down its concentration gradient • Based on osmosis, solutions are classified as: • Hypotonic (hypo = less) • Isotonic (iso = same) • Hypertonic (hyper = more) • The prefix refers to the SOLUTE concentration NOT the water concentration

13. Tonicity of a solution • The solution with higher concentration of solutes is hypertonic. • The solution with lower concentration of solutes is hypotonic. • These are comparative terms. • Tap water is hypertonic compared to distilled water but hypotonic when compared to sea water. • Solutions with equal solute concentrations are isotonic.

14. Lower concentration of solute (sugar) Higher concentration of sugar Same concentration of sugar Selectively permeable mem- brane: sugar mole- cules cannot pass through pores, but water molecules can Water molecules cluster around sugar molecules More free water molecules (higher concentration) Fewer free water molecules (lower concentration) Osmosis  Water moves from an area of higher free water concentration to an area of lower free water concentration Figure 7.12 Osmosis • Is affected by the concentration gradient of dissolved substances • Activity: osmosis and water balance

15. Pressure due to osmosis

16. Cells in isotonic solutions • The concentration of solutes is the same as it is inside the cell • No net movement of water • Dynamic equilibrium: water moves but at the same rate in opposite directions

17. Cells in hypertonic solutions • The concentration of solutes is greater than it is inside the cell • The cell will lose water • Animal cells shrivel • Plasmolysis occurs in plant cells, when the cytoplasm shrivels and detaches from cell wall

18. Cells in hypotonic solutions • The concentration of solutes is less than it is inside the cell • The cell will gain water • Animal cells burst (or adapt by using contractile vacuoles) • Plant cells become turgid (preferred state)

19. Water Balance of Cells with Walls • Cell walls help maintain water balance

20. 3.9 Osmosis is the diffus the passive diffusion of water

21. Animal adaptations • Paramecium, a protist, is hypertonic when compared to pond water • Adaptations: • membrane is less permeable to water than usual • specialized organelle (contractile vacuole) pumps excess water out

22. Osmosis & Dialysis in today’s lab • Dialysis tubing: an artificial semi-permeable membrane • Molecules can pass if they are small enough to fit through the microscopic pores • Dialysis: diffusion of solute (not osmosis of water) • You will observe: • Osmosis and dialysis using dialysis tubing • Osmosis leading to plasmolysis of Elodea cells treated with high salt concentration

23. Elodea cells in isotonic solution • Osmosis leading to plasmolysis of • Elodea cells treated with high salt concentration

24. A solution of 1 M glucose is separated by a selectively permeable membrane from a solution of 0.2 M fructose and 0.7 M sucrose. The membrane is not permeable to the sugar molecules. Which of the following statements is correct? Side A is hypotonic relative to side B. The net movement of water will be from side B to side A. The net movement of water will be from side A to side B. Side B is hypertonic relative to side A. There will be no net movementof water. Question 1 0

25. For Question 2 0 An artificial cell consisting of an aqueous solution enclosed in a selectively permeable membrane has just been immersed in a beaker containing a different solution. The membrane is permeable to water and to the simple sugars glucose and fructose but completely impermeable to the disaccharide sucrose.

26. Which solute(s) will exhibit a net diffusion into the cell? sucrose glucose fructose Question 3 0

27. Which solute(s) will exhibit a net diffusion out of the cell? sucrose glucose fructose Question 4 0

28. Which solution is hypertonic to the other? the cell contents the environment Question 5 0