Chapter 4. Membrane Structure and Function. Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

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1 Chapter 4 Membrane Structure and Function Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 1

2 4.1 Plasma Membrane Structure and Function Regulates the entrance and exit of molecules into and out of the cell Phospholipid bilayer with embedded proteins Hydrophilic (water-loving) polar heads Hydrophobic (water-fearing) nonpolar tails Cholesterol (animal cells)

3 4.1 Plasma Membrane Structure and Function Membrane proteins may be Peripheral proteins associated with only 1 side of membrane Integral proteins span the membrane Can protrude from 1 or both sides Can move laterally

4 Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display. plasma membrane carbohydrate chain Outside glycoprotein glycolipid hydrophobic hydrophilic tails heads phospholipid bilayer filaments of cytoskeleton Inside peripheral protein integral protein cholesterol

5 4.1 Plasma Membrane Structure and Function 5 Membrane Protein Functions Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display. a. Channel Protein Allows a particular molecule or ion to cross the plasma membrane freely. Cystic fibrosis, an inherited disorder, is caused by a faulty chloride (Cl ) channel; a thick mucus collects in airways and in pancreatic and liver ducts. b. Carrier Protein Selectively interacts with a specific molecule or ion so that it can cross the plasma membrane. The family of GLUT carriers transfers glucose in and out of the various cell types of the body. Different carriers respond differently to blood levels of glucose.

6 Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display. c. Cell Recognition Protein The MHC (major histocompatibility complex) glycoproteins are different for each person, so organ transplants are difficult to achieve. Cells with foreign MHC glycoproteins are attacked by white blood cells responsible for immunity. Receptor Protein Shaped in such a way that a specific molecule can bind to it. Some types of dwarfism result not because the body does not produce enough growth hormone, but because the plasma membrane growth hormone receptors are faulty and cannot interact with growth hormone. d. e. Enzymatic Protein Catalyzes a specific reaction. The membrane protein, adenylate cyclase, is involved in ATP metabolism. Cholera bacteria release a toxin that interferes with the proper functioning of adenylate cyclase, which eventually leads to severe diarrhea.

7 Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Channel Protein Allows a particular molecule or ion to cross the plasma membrane freely. Cystic fibrosis, an inherited disorder, is caused by a faulty chloride (Cl ) channel; a thick mucus collects in airways and in pancreatic and liver ducts. Carrier Protein Selectively interacts with a specific molecule or ion so that it can cross the plasma membrane. The family of GLUT carriers transfers glucose in and out of the various cell types of the body. Different carriers respond differently to blood levels of glucose. Cell Recognition Protein The MHC (major histocompatibility complex) glycoproteins are different for each person, so organ transplants are difficult to achieve. Cells with foreign MHC glycoproteins are attacked by white blood cells responsible for immunity. a. b. c. Receptor Protein Shaped in such a way that a specific molecule can bind to it. Some types of dwarfism result not because the body does not produce enough growth hormone, but because the plasma membrane growth hormone receptors are faulty and cannot interact with growth hormone. d. e. Enzymatic Protein Catalyzes a specific reaction. The membrane protein, adenylate cyclase, is involved in ATP metabolism. Cholera bacteria release a toxin that interferes with the proper functioning of adenylate cyclase, which eventually leads to severe diarrhea.

8 4.2 Permeability of the Plasma Membrane Differentially permeable Factors that determine how a substance may be transported across a plasma membrane: Size Nature of molecule polarity, charge

9 4.2 Permeability of the Plasma Membrane Concentration gradient More of a substance on one side of the membrane Going down a concentration gradient From an area of higher to lower concentration Going up a concentration gradient From an area of lower to higher concentration Requires input of energy

10 4.2 Permeability of the Plasma Membrane Some molecules freely cross membrane Water, small, noncharged molecules Water may also use aquaporins to cross membrane Other molecules cannot use Channel proteins Carrier proteins Vesicles Endocytosis or exocytosis

11 Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display. charged molecules and ions H 2 O + - noncharged molecules macromolecule + - phospholipid molecule protein

12 Passage of Molecules Into and Out of the Cell

13 4.2 Permeability of the Plasma Diffusion Membrane Movement of molecules from an area of higher to lower concentration Down a concentration gradient Solution contains a solute (solid) and a solvent (liquid)

14 Once the solute and solvent are evenly distributed, their molecules continue to move about, but there is no net movement of either one in any direction Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display. water molecules (solvent) dye molecules (solute) a. Crystal of dye is placed in water b. Diffusion of water and dye molecules c. Equal distribution of molecules results

15 Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Gases can diffuse through a membrane Oxygen and carbon dioxide enter and exit this way O 2 O 2 O 2 O 2 O 2 oxygen O 2 O 2 O 2 O 2 O 2 O 2 O 2 alveolus capillary bronchiole

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17 4.2 Permeability of the Plasma Membrane Several factors influence the rate of diffusion Temperature As temperature increases, the rate of diffusion increases Pressure Electrical currents Molecular size

18 4.2 Permeability of the Plasma Osmosis Membrane Diffusion of water across a differentially permeable membrane Diffusion always occurs from higher to lower concentration Osmotic pressure is the pressure that develops in a system due to osmosis The greater the possible osmotic pressure, the more likely it is that water will diffuse in that direction

19 Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display. less water (higher percentage of solute) 10% water solute < 10% more water (lower percentage of solute) more water (lower percentage of solute) a. 5% thistle tube c. > 5% less water (higher percentage of solute) differentially permeable membrane beaker b. Membrane is not permeable to solute

20 4.2 Permeability of the Plasma Membrane Osmosis Isotonic: the solute concentration is equal inside and outside of a cell Hypotonic: a solution has a lower solute concentration than the inside of a cell Hypertonic: a solution has a higher solute concentration than the inside of a cell

21 Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Animal cells nucleus In an isotonic solution, there is no net movement of water. Isotonic No net gain or loss of water 0.9% NaCl Hypotonic Cell gains water Cytolysis hemolysis Hypertonic Cell loses water Crenation plasma 6.6 µm 6.6 µm 6.6 µm membrane In a hypotonic solution, water enters the cell, which may burst (lysis). David M. Phillips/Photo Researchers, Inc. In a hypertonic solution, water leaves the cell, which shrivels (crenation).

22 Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Plant cells nucleus central vacuole chloroplast cell wall plasma membrane In an isotonic solution, there is no net movement of water. 25 µm 25 µm 40 µm In a hypotonic solution, the central vacuole fills with water, turgor pressure develops, and chloroplasts are seen next to the cell wall. In a hypertonic solution, the central vacuole loses water, the cytoplasm shrinks (plasmolysis), and chloroplasts are seen in the center of the cell. Isotonic No net gain or loss of water Hypotonic Cell gains water Turgor pressure keeps plant erect cell wall Hypertonic Cell loses water Plasmolysis (bottom left, center): Dwight Kuhn; (bottom right): Ed Reschke/Peter Arnold

23 Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Animal cells nucleus In an isotonic solution, there is no net movement of water. Plant cells 6.6 µm 6.6 µm 6.6 µm plasma membrane In a hypotonic solution, water enters the cell, which may burst (lysis). In a hypertonic solution, water leaves the cell, which shrivels (crenation). nucleus central vacuole chloroplast cell wall plasma membrane In an isotonic solution, there is no net movement of water. 25 µm 25 µm 40 µm In a hypotonic solution, the central vacuole fills with water, turgor pressure develops, and chloroplasts are seen next to the cell wall. In a hypertonic solution, the central vacuole loses water, the cytoplasm shrinks (plasmolysis), and chloroplasts are seen in the center of the cell. (all top): David M. Phillips/Photo Researchers, Inc.; (bottom left, center): Dwight Kuhn; (bottom right): Ed Reschke/Peter Arnold

24 4.2 Permeability of the Plasma Membrane Transport by Carrier Proteins Carrier proteins are specific Combine with a molecule or ion to be transported across the membrane Carrier proteins are required for: Facilitated Transport Active Transport

25 Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Inside plasma membrane carrier protein solute Outside Facilitated transport Small molecules that are not lipid-soluble Molecules follow the concentration gradient Energy is not required

26 4.2 Permeability of the Plasma Active Transport Membrane Molecules combine with carrier proteins Often called pumps Molecules move against the concentration gradient Entering or leaving cell Energy is required

27 Sodium-Potassium Pump Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display. carrier protein Outside Inside 1. Carrier has a shape that allows it to take up 3 Na +.

28 Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display. P ATP ADP 2. ATP is split, and phosphate group attaches to carrier.

29 Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display. P 3. Change in shape results and causes carrier to release 3 Na + outside the cell.

30 Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display. P 4. Carrier has a shape that allows it to take up 2.

31 Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display. P 5. Phosphate group is released from carrier.

32 Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 6. Change in shape results and causes carrier to release 2 inside the cell.

33 Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display. carrier protein Outside Inside 1. Carrier has a shape that allows it to take up 3 Na +. P ATP ADP 6. Change in shape results and causes carrier to release 2 inside the cell. 2. ATP is split, and phosphate group attaches to carrier. P P 5. Phosphate group is released from carrier. P 3. Change in shape results and causes carrier to release 3 Na + outside the cell. 4. Carrier has a shape that allows it to take up 2.

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35 4.2 Permeability of the Plasma Membrane Vesicle Formation Membrane-assisted transport Transport of macromolecules Requires energy Keeps the macromolecule contained Exocytosis exit out of cell Endocytosis enter into cell

36 4.2 Permeability of the Plasma Exocytosis Membrane Vesicle fuses with plasma membrane as secretion occurs Membrane of vesicle becomes part of plasma membrane Cells of particular organs are specialized to produce and export molecules Pancreatic cells release insulin when blood sugar rises

37 Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display. plasma membrane Outside secretory vesicle Inside

38 Vesicle Formation Endocytosis Cells take in substances by vesicle formation Phagocytosis: Large, particulate matter Pinocytosis: Liquids and small particles dissolved in liquid Receptor Mediated Endocytosis: A type of pinocytosis that involves a coated pit

39 Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display. plasma membrane paramecium pseudopod of amoeba vacuole forming vacuole a. Phagocytosis µm vesicles forming solute vesicle b. Pinocytosis 0.5 µm receptor protein solute coated pit coated vesicle coated pit coated vesicle c. Receptor-mediated endocytosis a(right): Eric Grave/Phototake; b(right): Don W. Fawcett/Photo Researchers, Inc.; c(both): Courtesy Mark Bretscher