Membrane Structure and Function

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Membrane Structure and Function
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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, theyre 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?
Phospholipids drift 2µm/sec
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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 cell.
• 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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Source of Membrane Carbs
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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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Selective Permeability

• Order of decreasing permeability
• Hydrophobic (non polar) molecules
• Small, uncharged polar molecules
• Large, uncharged polar molecules
• Ions

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Selective Permeability

• Not all membranes are the same
• More double bonds in fatty acids greater
  permeability
• More cholesterol decreases permeability
• Increasing temperature increases permeability

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If a molecule can move freely through the
phospholipid bilayer 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 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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• Extensive irrigation in arid regions causes salts
  to accumulate in the soil. (The water contains
  low conc. of salts, but when the water evaporates
  from the fields, the salts are left behind to
  concentrate in the soil.)
• Based on what you have learned about water
  balance in plant cells, explain why increasing
  salinity has an adverse effect on agriculture.
• Suggest some ways to minimize this damage.
• What costs do they have?

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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.
• Sometimes they are gated

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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 or too large to pass through
  the lipid bilayer.

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Aquaporins

• 1990s Peter Agre (Johns Hopkins) discovers
  water pores in membranes that facilitate
  diffusion of water
• 2003 Nobel Prize in Chemistry along with Roderick
  MacKinnon
• Water is small and can diffuse through bilayer to
  some extent despite polarity
• Aquaporins speed up this diffusion
• May be gated to regulate water diffusion

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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 ions inside the cell
  to ions outside the cell

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What 2 forces drives 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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Review of passive and active transport
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Review of passive and active transport
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What is cotransport ?
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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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Water Potential

• Water moves from higher water potential (?) to
  lower water potential
• ?0 in pure water at atmospheric pressure
• ? and solute concentration have an inverse
  relationship
• ? and pressure have a direct relationship

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Plant Cells and Water Potential

• Plasmolysis protoplast pulls away from the cell
  wall due to loss of water
• The pressure exerted on the cell wall is 0
• Turgor pressure Pressure exerted on cell wall
  by protoplast due to influx of water
• The pressure exerted on the cell wall is positive