Plasma Membrane : Structure and Function

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Plasma Membrane Structure and Function

• Tuesday, July 15

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Membrane Functions

• Compartmentalization
• Encloses contents of the entire cell
• Encloses internal cellular spaces in which
  specialized activities take place
• Providing a selectively permeable barrier
• Permits only appropriate substances through
• Transporting solutes
• Into, out of, and around the cell
• Responding to external signals
• Signal transduction
• Intercellular interactions
• Energy transduction

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Membrane Structure

• Plasma membranes are selectively permeable,
  isolating internal components from the outside
  while allowing uptake of nutrients and
  elimination of waste products
• Membrane components lipids and proteins
• Fluid Mosaic Model fluid membrane with various
  proteins embedded in or attached to a double
  layer (bilayer) of phospholipids

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Figure 8.1 Artificial membranes
Amphipathic phospholipids have a hydrophillic
head group that interacts with water and a
hydrophobic tail that is excluded from water
A bilayer forms a stable boundary between two
aqueous compartments
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Incorporation of Proteins

• Singer and Nicolson developed the fluid mosaic
  model
• Amphipathic proteins are dispersed in the fluid
  lipid membrane
• Protein and lipid content of membranes vary in
  number and kind which are specific to the
  function of that membrane

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Figure 8.3 Freeze-fracture Method to Study
Membranes
The frozen cell is fractured down the middle of
the membrane
The proteins do not split, but follow either of
the membrane layers
A mist of platinum is sprayed onto the surface at
an angle creating shadows indicating depth.
Carbon is added to strength and the specimen is
digested, leaving the platinum-carbon film which
is then examined by electron microscopy.
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Membrane Fluidity
Membranes are held together by hydrophobic
interactions lipids can move laterally and some
proteins have directed movement
Fluidity is enhanced by unsaturated tails that
prevent tight packing of lipids
Fluidity is enhanced by unsaturated tails that
prevent tight packing of lipids
Fluidity is reduced by cholesterol at high temps
by restraining phospholipid movement, but
inhibiting solidification at lower temps by
disrupting the regular packing of phospholipids
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Figure 8.5 Evidence for the drifting of membrane
proteins
Fuse Cells
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Figure 8.6 Plasma membrane
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Membrane Proteins

• Integral proteins transmembrane protein
• Hydrophllic amino acids found on the parts of the
  protein exposed to solution on either side
• Hydrophobic nonpolar amino acids found in the
  interior of the membrane
• These usually form an coiled a-helix structure
  that spans the membrane
• Peripheral proteins loosely bound to the
  surface of the membrane
• Lipid-anchored proteins
• Membrane proteins held in place by attachment to
  the cytoskeleton (interior) or ECM (exterior)

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Figure 8.7 The structure of a transmembrane
protein
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Figure 8.8 Sidedness of the plasma membrane

• The two lipid layer differ in
• specific lipid composition
• protein directional orientation
• carbohydrates (restricted to exterior surface)
• This asymmetrical distribution is generated
  during synthesis of the membrane components in
  the ER
• Extracellular face inside of Golgi and ER
• Vesicle fusion secretes internal components and
  enlarges membrane

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Figure 8.9 Some functions of membrane proteins

• Hydrophillic channel selective for substance
• Actively pump substances

Proteins of adjacent cells hooked together to
form a junction
Sequential steps in a pathway carried out by
ordered enzymes in membrane
Glycoproteins serve as tags recognized by other
cells
Chemical messenger binds to protein causing a
conformation change that relays message to inside
the cell

• Maintain cell shape and protein location
• Coordinate extra-intracellular changes

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Membrane Carbohydrates

• Branched oligosaccharides bound to proteins
  (glycoproteins) or lipids (glycolipids)
• Diversity and location of the molecules serves as
  a marker to distinguish cells
• Important for cell-cell recognition
• Rejection of foreign cells
• Sorting cells into tissues and organs

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Membrane Traffic

• Ability to regulate transport across cellular
  boundaries semi-permeable
• Permeable hydrophobic molecules dissolve in the
  bilayer (CO2, O2, hydrocarbons)
• Impermeable polar molecules and ions, large
  molecules (sugars)
• Transport proteins allow transport of specific
  hydrophillic molecules
• Channels, carriers, pumps
• Proteins transported at different rates

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Membrane Transport

• Passive Transport (no energy input)
• Diffusion - down a concentration gradient
• Osmosis diffusion of water
• Facilitated Diffusion protein channels
• Active Transport (requires energy)
• Protein pump against concentration gradient
• Cotransport active transport coupled to passive
  transport
• Endocytosis and exocytosis
• Transport of large molecules by plasma vesicles

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Figure 8.10 The diffusion of solutes across
membranes
Solute diffuses down concentration gradient
Dynamic equilibrium molecules move at equal rates
Membrane permeable to solute
Solutes move down their specific gradient not the
total solute gradient
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Membrane Transport

• Passive Transport (no energy input)
• Diffusion - down a concentration gradient
• Osmosis diffusion of water
• Facilitated Diffusion protein channels
• Active Transport (requires energy)
• Protein pump against concentration gradient
• Cotransport active transport coupled to passive
  transport
• Endocytosis and exocytosis
• Transport of large molecules by plasma vesicles

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Figure 8.11 Osmosis
Isotonic solutions
Water moves down the concentration gradient of
total solute
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Figure 8.12 The water balance of living cells
Animals cells not in isotonic solutions have
adaptations for osmoregulation - Membrane less
permeable to H2O - Contractile vacuole
Plant cells The elastic cell wall exerts a back
pressure on the cell that opposes further uptake
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Membrane Transport

• Passive Transport (no energy input)
• Diffusion - down a concentration gradient
• Osmosis diffusion of water
• Facilitated Diffusion protein channels
• Active Transport (requires energy)
• Protein pump against concentration gradient
• Cotransport active transport coupled to passive
  transport
• Endocytosis and exocytosis
• Transport of large molecules by plasma vesicles

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Figure 8.14 Facilitated diffusion
Molecules that cannot freely diffuse through
membranes (polar molecules and ions) must rely on
the help of protein transporters that are
specific for their substance
Protein translocators Substrate binds binding
site on protein which triggers a shape change
that translocates the substance across the
membrane
Protein channels Open aquaporin Gated
requires a chemical or voltage stimulus to open
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Membrane Transport

• Passive Transport (no energy input)
• Diffusion - down a concentration gradient
• Osmosis diffusion of water
• Facilitated Diffusion protein channels
• Active Transport (requires energy)
• Protein pump against concentration gradient
• Cotransport active transport coupled to passive
  transport
• Endocytosis and exocytosis
• Transport of large molecules by plasma vesicles

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Active Transport

• Proteins that pump molecules against their
  gradient use energy (ATP)
• Concentration gradient (separation of molecules)
• Electrochemical gradient (separation of charge)
• Membrane potential voltage across a membrane
• Inside of a cell is negative compared to the
  outside
• Passive transport of cations into and anions out
  of the cell
• A pump that generates membrane potential
  electrogenic pump

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Figure 8.17 An electrogenic pump
Plants use ATP energy to power the proton pump
which creates an voltage gradient used to drive
other processes
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Figure 8.15 The sodium-potassium pump a
specific case of active transport
Inside High K Neg. charge
Outside High Na Pos. charge
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Figure 8.16 Review passive and active transport
compared
Hydrophobic and small uncharged polar molecules
Spontaneous movement of molecules down gradients
Hydrophillic molecules (water)
Energy required to move molecules against
gradients
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Membrane Transport

• Passive Transport (no energy input)
• Diffusion - down a concentration gradient
• Osmosis diffusion of water
• Facilitated Diffusion protein channels
• Active Transport (requires energy)
• Protein pump against concentration gradient
• Cotransport active transport coupled to passive
  transport
• Endocytosis and exocytosis
• Transport of large molecules by plasma vesicles

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Figure 8.18 Cotransport
Single ATP powered pump transporting specific
solutes can indirectly drive the active transport
of other solutes. As protons are pumped out of
the cell they diffuse back down their gradient,
releasing energy that is used by a cotransport
protein to drive the active transport of sucrose
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Membrane Transport

• Passive Transport (no energy input)
• Diffusion - down a concentration gradient
• Osmosis diffusion of water
• Facilitated Diffusion protein channels
• Active Transport (requires energy)
• Protein pump against concentration gradient
• Cotransport active transport coupled to passive
  transport
• Endocytosis and exocytosis
• Transport of large molecules by plasma vesicles

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Figure 8.19 The three types of endocytosis in
animal cells
When two membranes come in contact, the lipids
rearrange so the membranes can fuse together.
The contents of the vesicle spill into
(endocytosis) or out of (exocytosis) the cell.
These processes also allow for membrane component
turnover
Engulfing large particles
Nonspecific transport
Engulfing small particles (water)
Specific transport
Ligand
Allows for the uptake of substances present in
low concentrations