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Membranes Chapter 5 Membrane Structure Phospholipids arranged in a bilayer Globular proteins inserted in the lipid bilayer Fluid mosiac model mosaic of proteins ... – PowerPoint PPT presentation

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Title: Membranes

  • Chapter 5

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

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

  • Transmission electron microscope (TEM) of the
    cell membrane

  • 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

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

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

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

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

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

Passive Transport
  • Passive transport is movement of molecules
    through the membrane in which
  • No energy is required
  • Molecules move in response to a concentration
  • 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

Diffusion Rates
  • Factors affecting diffusion rate through a
  • 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

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

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

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

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

  • 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

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

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

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

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
  • Antiporters move two molecules in opposite
  • Terms can also be used to describe facilitated
    diffusion carriers

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

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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

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
  • 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.

  • 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
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