Membranes

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Membranes

• Chapter 5

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Membrane Structure

• Phospholipids arranged in a bilayer
• Globular proteins inserted in the lipid bilayer
• Fluid mosiac model mosaic of proteins floats in
  or on the fluid lipid bilayer like boats on a pond

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Membrane Components

• Cellular membranes have 4 components
• Phospholipid bilayer
• Flexible matrix, barrier to permeability
• Transmembrane proteins
• Integral membrane proteins
• Interior protein network
• Peripheral membrane proteins
• Cell surface markers
• Glycoproteins and glycolipids

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• Transmission electron microscope (TEM) of the
  cell membrane

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Phospholipids

• Structure consists of
• Glycerol a 3-carbon polyalcohol
• 2 fatty acids attached to the glycerol
• Phosphate group attached to the glycerol
• Spontaneously forms a bilayer
• Fatty acids are on the inside
• Phosphate groups are on both surfaces

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• Bilayers are fluid
• Hydrogen bonding of water holds the 2 layers
  together
• Individual phospholipids and unanchored proteins
  can move through the membrane

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Membrane Proteins

• Various functions
• Transporters
• Enzymes
• Cell-surface receptors
• Cell-surface identity markers
• Cell-to-cell adhesion proteins
• Attachments to the cytoskeleton

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Structure relates to function

• Diverse functions arise from the diverse
  structures of membrane proteins
• Have common structural features related to their
  role as membrane proteins
• Peripheral proteins
• Anchoring molecules attach membrane protein to
  surface

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Integral membrane proteins

• Span the lipid bilayer (transmembrane proteins)
• Nonpolar regions of the protein are embedded in
  the interior of the bilayer
• Polar regions of the protein protrude from both
  sides of the bilayer
• Transmembrane domain
• Spans the lipid bilayer
• Hydrophobic amino acids

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Membrane Proteins - Pores

• Extensive nonpolar regions within a transmembrane
  protein can create a pore through the membrane
• Cylinder of ? sheets in the protein secondary
  structure called a b-barrel
• Interior is polar and allows water and small
  polar molecules to pass through the membrane

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Passive Transport

• Passive transport is movement of molecules
  through the membrane in which
• No energy is required
• Molecules move in response to a concentration
  gradient
• Diffusion is movement of molecules from high
  concentration to low concentration down their
  concentration gradient
• Will continue until the concentration is the same
  in all regions . Animation

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Diffusion Rates

• Factors affecting diffusion rate through a
  membrane
• temperature - ? temp., ? motion of particles
• molecular weight - larger molecules move slower
• steepness of concentrated gradient - ?difference,
  ? rate
• membrane surface area - ? area, ? rate
• membrane permeability - ? permeability, ? rate

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The Nature of the Membrane and Molecule Transport

• Major barrier to crossing a biological membrane
  is the hydrophobic interior that repels polar
  molecules but not nonpolar molecules
• Nonpolar molecules will move until the
  concentration is equal on both sides
• Limited permeability to small polar molecules
• Very limited permeability to larger polar
  molecules and ions

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Transport of Polar Molecules

• Facilitated diffusion
• Molecules that cannot cross membrane easily may
  move through proteins
• Move from higher to lower concentration- down
  their concentration gradient animation
• Channel proteins
• Hydrophilic channel when open
• Carrier proteins
• Bind specifically to molecules they assist
• Membrane is selectively permeable

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Channel proteins

• Ion channels
• Allow the passage of ions
• Gated channels open or close in response to
  stimulus (chemical or electrical)
• 3 conditions determine direction
• Relative concentration on either side of membrane
• Voltage differences across membrane
• Gated channels channel open or closed

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Carrier proteins

• Can help transport both ions and other solutes,
  such as some sugars and amino acids
• Requires a concentration difference across the
  membrane
• Must bind to the molecule they transport
• Saturation rate of transport limited by number
  of transporters

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Osmosis

• Cytoplasm of the cell is an aqueous solution
• Water is solvent
• Dissolved substances are solutes
• Osmosis net diffusion of water across a
  membrane toward a higher solute concentration

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Osmotic concentration

• When 2 solutions have different osmotic
  concentrations
• Hypertonic solution has a higher solute
  concentration
• Hypotonic solution has a lower solute
  concentration
• When two solutions have the same osmotic
  concentration, the solutions are isotonic
• Aquaporins facilitate osmosis

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Osmotic pressure

• Force needed to stop osmotic flow
• Cell in a hypotonic solution gains water causing
  cell to swell creates pressure
• If membrane strong enough, cell reaches
  counterbalance of osmotic pressure driving water
  in with hydrostatic pressure driving water out
• Cell wall of prokaryotes, fungi, plants, protists
• If membrane is not strong, may burst
• Animal cells must be in isotonic environments

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Maintaining osmotic balance

• Some cells use extrusion in which water is
  ejected through contractile vacuoles
• Isosmotic regulation involves keeping cells
  isotonic with their environment
• Marine organisms adjust internal concentration to
  match sea water
• Terrestrial animals circulate isotonic fluid
• Plant cells use turgor pressure to push the cell
  membrane against the cell wall and keep the cell
  rigid

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Active Transport

• Requires energy ATP is used directly or
  indirectly to fuel active transport
• Moves substances from low to high
  concentration-up their concentration gradient.
• Requires the use of highly selective carrier
  proteins

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Carrier Protein and Active Transport

• Carrier proteins used in active transport include
• Uniporters move one molecule at a time
• Symporters move two molecules in the same
  direction
• Antiporters move two molecules in opposite
  directions
• Terms can also be used to describe facilitated
  diffusion carriers

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Sodiumpotassium (NaK) pump

• Direct use of ATP for active transport
• Uses an antiporter to move 3 Na out of the cell
  and 2 K into the cell
• Against their concentration gradient
• ATP energy is used to change the conformation of
  the carrier protein
• Affinity of the carrier protein for either Na or
  K changes so the ions can be carried across the
  membrane

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Coupled transport- Secondary Active Transport

• Uses ATP indirectly
• Uses the energy released when a molecule moves by
  diffusion to supply energy to active transport of
  a different molecule
• Symporter is used
• GlucoseNa symporter captures the energy from
  Na diffusion to move glucose against a
  concentration gradient

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Animation
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Bulk Transport

• Endocytosis
• Movement of substances into the cell
• Phagosytosis cell takes in particulate matter
• Pinocytosis cell takes in only fluid
• Receptor-mediated endocytosis specific
  molecules are taken in after they bind to a
  receptor
• Exocytosis
• Movement of substances out of cell
• Requires energy

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• In the human genetic disease familial
  hypercholesterolemia, the LDL receptors lack
  tails, so they are never fastened in the
  clathrin-coated pits and as a result, do not
  trigger vesicle formation. The cholesterol stays
  in the bloodstream of affected individuals,
  accumulating as plaques inside arteries and
  leading to heart attacks.

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• Exocytosis
• Movement of materials out of the cell
• Used in plants to export cell wall material
• Used in animals to secrete hormones,
  neurotransmitters, digestive enzymes