Membrane Structure and Function

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Membrane Structure and Function

• Chapter 8

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• Plasma membrane of cell selectively permeable
  (allows some substances to cross more easily than
  others)
• Made mostly of proteins and lipids
  (phospholipids).
• Phospholipids and proteins create unique physical
  environment (fluid mosaic model)

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Phospholipid
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• Membrane - bilayer - hydrophilic (water loving)
  heads pointing outwards, hydrophobic (water
  fearing) tails pointing inwards.
• Proteins help membrane to stick to water.

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• Fluid because lipids and proteins can move
  laterally.
• As temperatures drop, liquid membrane can
  solidify.
• Saturated fatty acid tails - more solid than
  unsaturated fatty acid tails.

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• Cholesterol found in membrane helps with fluidity
  of membrane.
• Membranes need to be fluid to work properly -
  systems in place to help keep it fluid.

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• Two different types of proteins are found in
  membrane.
• 1Peripheral proteins not in membrane, bound to
  surface of protein.
• 2Integral proteins in membrane often spanning
  entire membrane.

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http//users.rcn.com/jkimball.ma.ultranet/BiologyP
ages/M/MembraneProteins.gif
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• Membrane helps keep cells shape.
• Also aids in cell-to-cell recognition (ability of
  a cell to distinguish one type of neighboring
  cell from another)

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• Some substances move steadily across membrane
  (sugars, ions, and wastes like CO2)
• Hydrophobic molecules (i.e. hydrocarbons, CO2,
  and O2) can dissolve in lipid bilayer and cross
  easily.

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• Charged particles and polar molecules have more
  difficulty passing.
• Specific ions and polar molecules can cross lipid
  bilayer by passing through transport proteins
  that span membrane.

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• Diffusion - tendency for substance to spread out
  in open area.
• Permeable membrane separating a solution with dye
  molecules from pure water, dye molecules will
  cross barrier randomly.

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• No force acting upon it - substance will tend to
  move down its concentration gradient from where
  it is more concentrated to less concentrated
  (passive transport).

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• Diffusion of molecules with limited permeability
  through lipid bilayer may be assisted by
  transport proteins.

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http//library.thinkquest.org/27819/media/protein_
channel.gif
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• Difference in concentration - ions move from one
  area to other.
• Solution with higher solutes - hypertonic.
• Solution with lower solutes -hypotonic.
• equal - isotonic.

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http//www.biologycorner.com/resources/hypertonic.
gif
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http//www.biologycorner.com/resources/hypotonic.g
if
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• Solution hypertonic - higher solute but lower
  H2O .
• H2O moves into solution and solute moves out.

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• Movement of H2O across selectively permeable
  membrane - osmosis.
• 2 solutions isotonic, H2O molecules move at equal
  rates from one to the other, (no net osmosis)

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• Cell placed in hypertonic solution H20 rushes
  out of cell (cell shrinks).
• Cell placed in hypotonic solution H2O rushes
  into cell (cell swells).

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• Organism does not have rigid walls must have
  ability to osmoregulate and maintain internal
  environment.
• Plant cells expand when watered causing pressure
  to be exerted against cell wall.
• Allows plant to stand up against gravity (turgid
  cell) not watered, plant will begin to wilt
  (flaccid cell).

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• Plant loses enough water, plasma membrane will
  pull away from cell (plasmolysis).

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http//faculty.southwest.tn.edu/jiwilliams/plasmol
ysis.gif
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• Charged particles that cannot pass through
  membrane - proteins to pass through (facilitated
  diffusion - diffusion of substance down its
  gradient with help of transport protein)
• Some channel proteins (gated channels) open/close
  depending on presence/absence of physical or
  chemical stimulus.

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In this case, the protein actually rotates to
dump the materials to the inside of the cell.
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• Sometimes materials need to be moved against
  gradient (active transport)
• Active transport requires energy of cell to move
  substances from an area of low to an area of
  high (i.e. sodium-potassium pump in animal
  cells)

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http//www.sp.uconn.edu/terry/images/anim/antipor
t.gif
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• Sodium-potassium pump actively maintains gradient
  of sodium (Na) and potassium ions (K) across
  membrane.
• Sodium-potassium pump uses energy of 1 ATP to
  pump 3 Na ions out and 2 K ions in.

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• Cells maintain voltage across plasma membranes.
• Cytoplasm negative compared to opposite side of
  membrane (membrane potential - ranges from -50 to
  -200 millivolts)

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http//bioweb.wku.edu/courses/Biol131/images/neuro
nions.GIF
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• Membrane potential favors passive transport of
  cations (positive ions) into cell and anions
  (negative ions) out of cell.
• Creates an electrochemical gradient across
  membrane.

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• Some organisms have proton pumps that actively
  pump H out of cell (i.e. plants, bacteria, and
  fungi)

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• Materials leave membrane through lipid bilayer or
  through transport proteins.
• Exocytosis - transport vesicle buds from Golgi
  apparatus - moved by cytoskeleton to plasma
  membrane.
• When membranes meet - fuse - material is let out
  to outside of cell.

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• Endocytosis - cell brings in macromolecules and
  matter by forming new vesicles from plasma
  membrane.
• Membrane is inwardly pinched off and vesicle
  carries material to inside of cell.

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http//www.kscience.co.uk/as/module1/pictures/endo
exo.jpg
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• 1Phagocytosis (cell eating) - cell engulfs
  particle by extending pseudopodia around it,
  packaging it in a large vacuole.
• Contents of vacuole are digested when vacuole
  fuses with lysosome.

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• 2Pinocytosis (cell drinking) - cell creates
  vesicle around droplet of extracellular fluid.

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• 3Receptor-mediated endocytosis - specific in
  transported substances.
• Extracellular materials bind ligands (receptors)
  - causes vesicle to form.
• Allows materials to be engulfed in bulk (i.e.
  cholesterol in humans)

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