Membrane Structure and Function

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Membrane Structure and Function

• Chapter 7

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What is the Function of The Plasma Membrane?

• boundary
• must be selectively permeable
• What, then is the structure that allows the
  membrane to perform this function successfully?

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Phospholipids

• Recall that phospholipids are amphipathic (both
  hydrophilic and hydrophobic)
• Artificial membranes showed phospholipids will
  form a layer in water

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The Bilayer

• Why are membranes organized into a bilayer?
• There are two watery areas that interact with
  the membrane, outside of the cell and inside the
  cell

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Science as a Process

• 1935 Davson-Danielli model The Fat Sandwich.
  The evidence
• Both protein and phospholipids were isolated from
  membranes
• Thickness measured equals the bilayer
• Phospholipids alone are not as attracted to water
  as the real membrane surfaces. Therefore coat
  with proteins!

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Problems with the model

• In what way are membrane proteins a problem for
  this model?
• Hint Membrane proteins are amphipathic
• Hydrophobic parts of the proteins are in
  hydrophilic zones, resulting in an unstable
  structure
• What other problem was there with this model?
• The model suggests that all membranes are
  identical with regard to thickness. They are not!

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Fluid Mosaic Model - 1972

• In what way does this model solve the problems?
• Hydrophobic parts of proteins are embedded within
  the membrane
• Thickness between different membranes is a
  function of the proteins

What evidence is there to support this model?
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Fluid Mosaic Model - 1972

• In what way does this model solve the problems?
• Hydrophobic parts of proteins are embedded within
  the membrane.
• Thickness between different membranes is a
  function of the proteins

Freeze-fracture technique with electron microscope
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What evidence is there for membrane fluidity?
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Membrane Fluidity

• Why is it that membrane phospholipids drift
  laterally, and rarely flip?

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How is this fluidity maintained?

• Kinks in unsaturated fatty acid tails of
  phospholipids
• Cholesterol

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How are proteins arranged to contribute to
membrane function?

• Membrane proteins contribute to the mosaic
  quality of the structure
• Different proteins convey different properties to
  each membrane
• Integral proteins are inserted within the
  membrane
• Peripheral proteins are attached to membrane
  surface
• Proteins attach to cytoskeleton or to
  extracellular fibers to help give animal cells a
  stronger framework

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Membrane Carbohydrates

• Found only on the outside of the membrane
• What is their function?
• Cell to cell recognition
• Sorting cells into tissues
• Immune defense
• Usually oligosaccharides (15 or less sugar units)
• glycolipids or glycoproteins

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How do ions and other polar molecules pass into
and out of cells?

• Transport proteins
• Provide hydrophilic tunnel for ions
• They are specific for the substances they
  transport

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What determines the direction of traffic across a
membrane?

• Diffusion
• What causes diffusion? Why is it spontaneous?
• Concentration gradient represents potential
  energy
• Since the direction of movement decreases the
  free energy of the system, it is spontaneous

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Does the diffusion of more than one kind of
particle work together or separately?
Do the particles stop moving once equilibrium is
reached?
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Does the diffusion of more than one kind of
particle work together or separately?
NO!
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If a molecule can move freely through the
phospholipid bilayer, then what always controls
the direction of its movement?

• Concentration gradient
• Remember that the concentration gradient
  represents potential energy

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Osmosis

• What is osmosis?
• The diffusion of water across a semi-permeable
  membrane

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Since water passes freely across the membrane,
how can the cell control the direction of osmosis?
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• The cell can concentrate solutes that are not
  permeable to the phospholipid bilayer on one side
  of the membrane
• Which way will water move?
• Water will follow the solutes
• What do the terms hypotonic, hypertonic and
  isotonic mean?
• Hypotonic lower solute concentration
• Hypertonic greater solute concentration
• Isotonic equal solute concentration

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?
?
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Which way will the water move?
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WHY?
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Do Water Molecules Stop Moving in Isotonic
Conditions?

• No
• They continue to diffuse, however there is no net
  movement
• In general, which way does water move?
• From hypotonic to hypertonic

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Water Balance in Cells
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Facilitated Diffusion

• What is facilitated diffusion?
• Diffusion of solutes with the help of transport
  proteins
• Is this a passive or an active process?
• passive
• Why do these solutes need a protein to facilitate
  their diffusion?
• They are too polar to pass through the lipid
  bilayer

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Active Transport

• What is active transport?
• Pumps molecules across the membrane against their
  concentration gradients
• Requires energy in the form of ATP
• Used to help maintain ionic gradients across
  membranes
• What do these ionic gradients represent?
• Potential energy

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Membrane Potential

• Membrane potential is the voltage across a
  membrane
• Usually around -70 mV
• How is it maintained?
• An unequal distribution of anions inside the cell
  to cations outside the cell

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What two forces drive the diffusion of ions?

• Concentration gradient of the ion
• Effect of membrane potential (charge) on the ion
• This is called the electrochemical gradient
• Ions diffuse down their electrochemical gradient
• A transport protein that generates voltage across
  a membrane is called an electrogenic pump
• One example is the sodium-potassium pump

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Proton pumps are the main electrogenic pumps of
bacteria, fungi and plants.
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Review of passive and active transport
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Review of passive and active transport
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Review of passive and active transport
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What is co-transport ?
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Transport of large molecules

• Endocytosis
• Phagocytosis endocytosis of large particulate
  substances
• Pinocytosis endocytosis of fluid and dissolved
  solutes
• Exocytosis -

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Phagocytosis
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Pinocytosis