Ch 7: Membrane Structure and Function

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Ch 7 Membrane Structure and Function
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Fluid Mosaic Model

• Cell membrane
• Selectively permeable allows some substances to
  cross more easily than others
• 8 nm thick
• Fluid structure of phospholipids w/ a mosaic of
  various proteins embedded in it
• Cholesterol maintains fluidity acts as
  temperature buffer
• Less fluid as temp increases by restricting
  movement
• More fluid as temp decreases by maintaining space

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Structure of Cell Membrane
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Membrane Proteins

• Different types of cells contain different types
  of membrane proteins
• Integral proteins embedded into the hydrophobic
  core of lipid bilayer, can go all the way through
  membrane
• Peripheral proteins
• on surface of membrane

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Types of Membrane Proteins

• Transport
• Channel/ Carrier proteins (hydrophilic tunnel)
  allow polar molecules and ions to pass through
  hydrophobic layer
• Channel and Carrier proteins are specific for the
  substance they transport
• Enzymatic
• Signal Transduction (chemical messages)
• Receptor proteins transmit information from
  outside of the cell to inside of the cell
• Cell Recognition
• ID tags glycolipids or glycoproteins
• Intercellular joining
• Join together adjacent cells
• Cell Support
• Attach to cytoskeleton or extra-cellular matrix
  for stability

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Passive Transport

• Passive Transport - Movement across the cell
  membrane that does not require energy
• Diffusion movement of particles from an area of
  high concentration to an area of low
  concentration until an equilibrium is reached (go
  with/ down its concentration gradient)
• Concentration gradient the difference in the
  concentration of a substance across a space
• Equilibrium conc. of a substance
• is equal throughout a space
• (doesnt stop moving)

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Osmosis

• Osmosis diffusion of free water across a
  selectively permeable membrane
• Water diffuses across the cell membrane from the
  region of low solute conc. to that of a higher
  solute
• conc. until it
• reaches equilibrium

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Osmoregulation in Cells w/o cell walls

• Osmoregulation control of water balance
• Tonicity ability of a solution to cause a cell
  to gain /lose water
• Isotonic solution no net movement of water
  across the cell membrane
• Hypertonic solution more free water inside the
  cell . cell will lose water to its environment,
  and shrivel
• Hypotonic solution less free water inside the
  cell . water will enter cell, it will swell and
  lyse
• Some cells have a contractile vacuole to pump
  water out of cell

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Osmoregulation of Cells w/ walls

• Plant cells are healthiest in a hypotonic
  solution, osmotic pressure keeps cell wall turgid
  (very firm)
• Plant cells are flaccid (limp) in an isotonic
  solution
• In a hypertonic solution, the cell membrane will
  shrink and pull away from the cell wallcalled
  plasmolysis (wilt)

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Facilitated Diffusion Passive Transport aided
by proteins

• Channel Proteins hydrophilic passageways
• Some are always open for diffusion
• Rate of movement is determined by conc. Gradient
• () charged ions more likely to diffuse INTO cell
• (-) charged ions morel likely to diffuse out of
  the cell
• Some ion channels have gates and can be opened
  by
• Stretching of cell membrane
• Change in electrical charge
• Binding of specific molecule

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Voltage Gated Channels

• Nerve Cells transmit electrical signals by
  opening a series of Na gated channels
• Channel is closed
• Area changes voltage
• Channel opens briefly
• Na flood into cell voltage changes
• Channel closes and electrical signal passes on to
  the next voltage channel

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Chemically Gated Channels

• Nerve cells send out a neurotransmitter called
  acetylcholine (ACh) to nearby muscle cells to
  signal muscle to contract
• ACh binds to ACh receptor protein
• Receptor gate opens for a microsecond to allow
  Na in
• Na sets off muscle contraction

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Facilitated Diffusion Passive Transport aided
by proteins

• Carrier Proteins
• Specific substance binds to carrier protein
• Protein changes shape and transports substance
  across cell membrane
• Molecules is released into the cell, and carrier
  protein returns to its original shape

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Active Transport

• Active Transport - uses energy to move solutes
  AGAINST conc. gradient
• Carrier proteins act as pumps powered by ATP
• Examples
• Sodium Potassium pump
• Proton Pump
• Cotransport

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Sodium Potassium pump

• Pumps 3 Na out of the cell and 2 K into the
  cell
• Actively transports both ions against their conc.
  Gradient, powered by ATP
• Prevents Na from accumulating in the cell

1. 3 Na and a P (from ATP) bind to inside protein
   pump
2. Pump changes shape transporting 3 Na across
   membrane and out
3. 2 K bind to pump and are transported across
   membrane
4. 2 K and P are released inside of cell

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Proton Pump

• Actively transports protons (H) through the
  internal membranes of mitochondria and
  chloroplasts

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Cotransport (Coupled Transport)

• Cotransport an ATP powered pump that transports
  a specific solute, can indirectly drive the
  active transport of several other solutes
• Ex. As proton pump pumps H out, H diffuse back
  into the cell pulling sucrose molecules into the
  cell with it!

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Bulk Transport Substances that are too large to
be transported by carrier proteins

• Exocytosis (export)
• Secretion of macromolecules by fusion of vesicle
  with membrane, releasing the contents outside of
  cell

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Bulk Transport (contd)

• Endocytosis
• Cell membrane engulfs particles and pinches off
  to form vesicle inside the cell. Vesicle may fuse
  w/ lysosomes or other organelles
• 3 Types
• Phagocytosis
• Pinocytosis
• Receptor-mediated