Chapter 7: Membrane Structure and Function

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Chapter 7MembraneStructureand Function
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Important Point
If you are having trouble understanding lecture
material Try reading your text before
attending lectures. And take the time to read it
well!
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Lipid Bilayer
The ability of the cell to discriminate in its
chemical exchanges with the environment is
fundamental to life, and it is the plasma
membrane that makes this selectivity possible.
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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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Movement of Phospholipids
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Membrane Fluidity
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Cholesterol Temp. Buffer
Cholesterol reduces membrane fluidity at
moderate temperatures by reducing phospholipid
movement, but at low temperatures it hinders
solidification by disrupting regular packing of
phospholipids. Campbell Reece (2005) p. 126
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Lateral Movement of Proteins
Protein diffusion is not as fast as phospholipid
diffusion
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Not all Proteins Move
Proteins attached to cytoskeleton or ECM dont
move
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Transmembrane Protein
Know definition of transmembrane protein
Note distinct orientation
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Membrane Proteins
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Membrane Protein Functions
Membrane asymmetry allows cells to automatically
differ their intracellular environment from that
existing extracellularly
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Membrane Protein Functions
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Membrane Protein Trafficking
Note the equivalent orientation of endomembrane
lumen and extracelullar environment
Note the orientation of carbohydrate (always
lumen or outside of cell)
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Transport Across Membranes
Active transport is pumping against concentration
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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Passive Transport
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Passive Transport
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Simple Diffusion Across Bilayers

• 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

Selective permeability
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Simple Diffusion Across Bilayers
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Osmosis
More water
Less water
Down waters concentration gradient
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Osmosis H20 Down Conc. Gradient
Equilibrium
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Tonicity
More solute in than out
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Facilitated Diffusion
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Facilitated Diffusion
The parallels between the properties of transport
proteins and enzymes are fairly extensive, to the
point where one may consider a transport protein
simply as an enzyme-like protein that catalyzes
the physical process of movement from one side of
a membrane to another
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Group Translocation
Note that ATP-mediated phosphorylation of glucose
inside of cell drives this transport, making it
an example Group Translocation
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Sodium-Potassium Pump
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Sodium-Potassium Pump

• The sodium-potassium pump is a specific (and
  important) example ATP-powered active transport
• 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!
• As we shall see, there are other means of
  powering active transport that dont involve a
  direct hydrolysis of ATP
• First, though, we will take another look at the
  sodium-potassium pump

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Sodium-Potassium Pump
Why is the sodium-potassium pump considered to be
an eletrongenic pump?
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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
• 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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Movement Across Membranes
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Movement Down Conc. Gradient
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Electrogenic Pump
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Electron Transport Proton Pumping
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Cotranport as Active Transport
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Movement Across Membranes

• Endocytosis
• Phagocytosis
• Pinocytosis
• Receptor mediated endocytosis
• Exocytosis
• These are mechanisms that involve movement into
  and out of the lumen of the endomembrane system
• Not movement directly across membrane
• That is, substances enter the Endomembrane System
  but not the Cytoplasm

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Phagocytosis
Allows digestion in confined space
Taking up solids
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Phagocytosis Examples
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Pinocytosis
Taking up fluids
Increases absorptive membrane area
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Receptor-Mediated Endocytosis
A way of taking up specific substances
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Exocytosis
Exocytosis
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Endocytosis Membrane Orientation
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The End