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

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Presentation on theme: "BIO 107 Lab # 4 Cell membranes, Osmosis & Diffusion S. Badran."— Presentation transcript:

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

2 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 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 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Selective permeability The plasma membrane allows some substances to cross it but not others Figure 7.1

4 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 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 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings The Permeability of the Lipid Bilayer Small, hydrophobic molecules can pass through the membrane rapidly Example: CO 2, O 2 Polar molecules do not cross the membrane rapidly Example: H 2 O, sugars Charged ions DO NOT cross the lipid bi-layer on their own

7 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 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

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9 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 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 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Diffusion tendency for molecules of any substance to spread out evenly into the available space Activity: Diffusion Figure 7.11 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) Net diffusion Equilibrium (a)

11 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Diffusion Substances diffuse down their concentration gradient, from high to low concentration Figure 7.11 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. (b) Net diffusion Equilibrium

12 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 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 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 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 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Osmosis Is affected by the concentration gradient of dissolved substances Activity: osmosis and water balance Figure 7.12 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 More free water molecules (higher concentration) Water molecules cluster around sugar molecules Fewer free water molecules (lower concentration) Water moves from an area of higher free water concentration to an area of lower free water concentration  Osmosis

15 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Pressure due to osmosis

16 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 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 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 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 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 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 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 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 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 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 Animal adaptations

22 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 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 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Osmosis leading to plasmolysis of Elodea cells treated with high salt concentration Elodea cells in isotonic solution

24 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Question 1 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 movement of water.

25 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings For Question 2 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 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Question 3 Which solute(s) will exhibit a net diffusion into the cell? – sucrose – glucose – fructose

27 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Question 4 Which solute(s) will exhibit a net diffusion out of the cell? – sucrose – glucose – fructose

28 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Question 5 Which solution is hypertonic to the other? – the cell contents – the environment


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