History of Fluid Mosaic Model

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History of Fluid Mosaic Model
For more on this history, see http//www1.umn.edu
/ships/9-2/membrane.htm
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PhospholipidsLipid Bilayar
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Membrane Fluidity
Lipid bilayers are fluid, at normal body
(organismal) temperatures, but typically only in
two dimensions!
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Membrane Proteins
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Membrane Protein Functions
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Membrane Assymetry
This asymmetry allows the cell to automatically
differ its intracellular environment from that
existing extracellularly
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Membrane Glycoproteins
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Fluid Mosaic Model
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Movement Across Membranes
Passive Transport Simple diffusion (passive
diffusion) Facilitated diffusion Transporters
Uniporters Cotransporters Channel
proteins Active Transport Pumps Group
Translocation Electrochemical Gradients
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Movement Across Membranes
Passive Transport Simple diffusion (passive
diffusion) Facilitated diffusion Transporters
Uniporters Cotransporters Channel
proteins Active Transport Pumps Group
Translocation Electrochemical Gradients
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Primer on Diffusion

• Diffusion is movement from area of high
  concentration to low
• Diffusion is Passive Transport no energy is
  required

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Simple Diffusion Across Bilayers

• Passive diffusion
• - unaided by transport proteins
• - no metabolic energy expended
• - movement is down chemical concentration
  gradient
• - diffusion rate is proportional to
  concentration gradient and hydrophobicity
• - rate limiting step is movement across
  hydrophobic portion of membrane
• - the greater the hydrophobicity of a
  water-soluble molecule, the faster it diffuses
  across phospholipid bilayer.

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Simple Diffusion Across Bilayers
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Diffusion Across Membranes
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Osmosis H20 Down Conc. Gradient
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Osmosis H20 Down Conc. Gradient
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Tonicity H20 Down Conc. Gradient
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Movement Across Membranes
Passive Transport Simple diffusion (passive
diffusion) Facilitated diffusion Transporters
Uniporters Cotransporters Channel
proteins Active Transport Pumps Group
Translocation Electrochemical Gradients
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Protein-Mediated Diffusion
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Proteins Involved in Membrane Transport

• Transport of molecules and ions across the cell
  membrane is mediated by transport proteins
• - integral (intrinsic) membrane proteins
  (transmembrane contain multiple transmembrane a
  helices).
• - Peripheral (extrinsic) membrane proteins
  (examples spectrin and actin in RBC protein
  Kinase C)

It is Integral Membrane Proteins that are
involved in Membrane Transport
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Membrane Transport Proteins

• All are integral membrane proteins
• All exhibit high degree of specificity for
  substance transported
• Three major classes

• ATP powered pumps- ATPases that use energy of
  ATP hydrolysis to move ions or small molecules
  across membrane against a chemical
  concentration gradient or electrical potential
  active transport
• Channel proteins form water filled pores
  across the bilayer through which ions move down
  their concentration or electrical potential
  gradients at rapid rates
• Transporters bind only specific substrate
  molecule at a time (binding of substrate
  induces conformational change)
• -if transported molecule crosses membrane
  passively then downhill process called passive
  transport or facilitated diffusion

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Uniporters and Cotransporters
We can divide Transporters into two distinct
types

• Uniporters
• move one molecule at a time down a
  concentration gradient

• Symporter or Antiporter (cotransporters)
• couple movement of one or more ions down their
  concentration gradient with movement of another
  ion or molecule against its concentration
  gradient
• former is passive transport latter is active
  transport

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Uniporter Catalized Transport

• Uniporters enable amino acids, nucleosides, and
  sugars to enter and leave cells down their
  concentration gradients
• Accelerate a reaction that is already
  thermodynamically favored
• Facilitated transport (or facilitated diffusion)
• - accelerated rate
• - like enzyme-mediated catalysis, transport is
  stereospecific (ie. D-glucose)
• - transport occurs via limited number of
  uniporter molecules

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Uniporter Voltage-Gated Channel
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Movement Across Membranes
Passive Transport Simple diffusion (passive
diffusion) Facilitated diffusion Transporters
Uniporters Cotransporters Channel
proteins Active Transport Pumps Group
Translocation Electrochemical Gradients
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Active Transport
Active transport requires energy to move
substances from areas of lower concentration to
areas of higher concentration
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Sodium-Potassium Pump
In addition to their intrinsic relevance,
studying Membrane-Transport Proteins allows us to
appreciate Mechanisms of Protein-Mediated
Catalysis without getting bogged down in the
details of Chemical Reactions!
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Electrochemical Gradient

• An Electrochemical Gradient is a Concentration
  Gradient with Ions
• - These ions want to move down their
  concentration gradient
• - These ions (particularly) also want to move
  towards the opposite charge found on the other
  side of the membrane
• - This attraction for the other side of
  membranes (membrane potential) can be harnessed
  to do work
• - Electrochemical gradients essentially are
  batteries, i.e., means of physically storing
  electrical energy

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Membrane Potential
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Proton Pump
Proton pumps are used by plants, bacteria, and
fungi to create electrochemical gradients
(sodium-potassium pumps are employed by animals
for the same purpose)
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Cotranport as Active Transport
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Cotransport Catalized Transport
Net equation for two Na / one-glucose
symporter 2Na out glucose out ? 2Na in
glucose in
14 membrane-spanning ? helices
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Uniporter Facilitated Diffusion
Note that phosphorylation of glucose inside of
cell drives this transport, making it an example
Group Translocation
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Movement Across Membranes
Movement Across Membranes
Passive Transport Simple diffusion (passive
diffusion) Facilitated diffusion Transporters
Uniporters Cotransporters Channel
proteins Active Transport Pumps Group
Translocation Electrochemical Gradients
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Movement Across Membranes
Movement Across Membranes
Endocytosis Phagocytosis Pinocytosis Receptor
mediated Exocytosis These are mechanisms that
involve movement into and out of the lumen of the
endomembrane system Not movement directly across
membranes
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Endocytosis
Note that the substance enters the Endomembrane
System but not the Cytoplasm
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Phagocytosis
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Pinocytosis
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Endocytosis Receptor Mediated
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Endocytosis Receptor Mediated
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Exocytosis
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Link to Next Presentation
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Acknowledgements
www.uiowa.edu/c156201/PDFLecs/Schmidt/Schmidt01.p
pt www.avs.uidaho.edu/Rod20Class20Notes/AVS221-B
iol21220-20Topic203-2.ppt bio.winona.edu/wilson
/cell20biology/924.ppt biology.ucf.edu/courses/b
sc2010/08-2010C-02.PPT www.floyd.edu/ddaugherty/10
10/membrane.ppt www.biosci.ohiou.edu/courses/2002
03/ fall/bios/103schutte/chap03n.ppt
www.aw.com/bc/ppt/marieb_ap/chap03c.ppt