Chapter3Membranes

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GLEN OAKS COMMUNITY COLLEGE NSB 121 GENERAL BIOLOGY I

CHAPTER 3: CELL MEMBRANE STRUCTURE & FUNCTION

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Outline
 Cell membrane function  Historical discovery of cell membrane function  Cell membrane structure • Fluid mosaic model

 How the cell membrane plays a gate keeper role
• Selective permeability • Membrane proteins

 Diffusion

 Osmosis

• Dye in water
• Bag of sugar in water

 How Diffusion And Osmosis Affects Transport Across The Plasma Membrane  How Do Molecules Move Against A Concentration Gradient? • Active transport • Endocytosis • Exocytosis
• Passive transport • Simple diffusion, facilitated diffusion, osmosis

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PLASMA MEMBRANE FUNCTION  The cell plasma membrane separates the cell contents from the external environment regulating the passage of molecules into and out of the cell.

Copyright © 2009 Pearson Education Inc.

3.1 What Does The Plasma Membrane Do?
 Function/Importance
• A cell membrane separates an internal environment from an external one • A cell membrane functions as a selectively permeable barrier • Permeable • The membrane acts as a gatekeeper, regulating the passage of molecules into and out of the cell • Selective • Allows only certain molecules in, excludes others

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Historical discovery of cell membrane structure
 First correctly intuited by Quincke
• noted that a cell generally forms a spherical shape in water and, when broken in half, forms two smaller spheres • The only other known material to exhibit this behavior was oil • He also noted that a thin film of oil behaves as a semipermeable membrane, precisely as predicted

 Based on these observations, Quincke asserted that the cell membrane comprised a fluid layer of fat less than 100 nm thick  Meyer, Overton (1895) suggests membrane is lipid

• Chemicals which act as general anesthetics are also those soluble in both water and oil. • Early criticisms of this theory was that it included no mechanism for energy-dependent selective transport

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Historical discovery, con’t

 Langmuir (1917) made artificial membranes by adding phospholipids dissolved in benzene to water • Benzene evaporated, phospholipids remained as a film covering the water. Hydrophilic heads in water

 Gorter & Grendel (1925) suggest membrane is phospholipid bilayer • Must be 2 molecules thick • A bilayer could exist as a stable boundary between 2 aqueous compartments

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 Summary- drawings  But still, need to account for associated membrane proteins

Historical discovery, con’t
 Davson & Danielli (1935) propose a “membrane sandwich” model • Accomodates proteins associated with membranes • Early TEM support • Problems • Membranes appear different in different parts of the cell • Amphipathic nature of membrane proteins- hydrophobic regions in aqueous environment

 The fluid mosaic model is published in 1972 by Singer and Nicolson (Science 1972 Feb. 18 175:23 720-31) • the only accepted proper description of a biological membrane • Proteins are inserted into membrane (amphipathic problem solved) • Supported by freeze-fracture studies

 Summary- drawings
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Historical discovery, con’t

 Similar images taken in the 1950s and 1960s confirmed the bilayer nature of the cell membrane

A micrograph from a Transmission Electron Micrograph showing a lipid vesicle. The two dark bands are the two leaflets comprising the bilayer

Copyright © 2009 Pearson Education Inc.

Fig. 7-4

Freeze Fracture and Freeze-etch
TECHNIQUE Extracellular layer RESULTS

Knife

Proteins

Inside of extracellular layer

Plasma membrane

Cytoplasmic layer Inside of cytoplasmic layer

    

Freeze specimen at temp of lq N2 Fracture with cold knife Plane of...
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