Membranes

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Membranes
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Membranes

• Phospholipids
• Fluid Mosaic Model
• Passage and Transport

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Phospholipids
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Fluid-Mosaic Model

• Membrane molecules are in continuous motion (
  fluidity).
• Type of phospholipid determines fluidity.
• Cholesterol and proteins are embedded in
  bilayer.
• Carbohydrates are attached to some of the
  lipids and proteins

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Transmembrane Proteins
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Membrane Proteins

• Anchor cells together (cell adhesion molecules)
• Bind chemical messengers (receptors)
• Catalyze reactions (enzymes)
• Transport hydrophilic molecules (channels and
  pumps)
• Identify type of cell (self identification in
  immunity)

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Membrane Transport

• Diffusion
• Osmosis
• Facilitated Diffusion
• Active Transport
• Primary active transport
• Secondary active transport
• Endocytosis
• Phagocytosis
• Exocytosis

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Diffusion

• Solutes dissolved in a solvent will move down
  their concentration gradient.
• Rate influenced by
• Steepness of gradient
• Temperature
• Mass of solute
• Surface area
• Diffusion distance

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Osmosis

• Movement of water either across the lipid bilayer
  or through aquaporins.
• Water also moves from high to low concentration.
• Governed by hydrostatic pressure (due to water)
  and osmotic pressure (due to impassible solutes).

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Osmosis
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Tonicity

• Isotonic
• Hypotonic
• Hypertonic

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Facilitated Diffusion

• Passive transport
• Solute binds to a transporter on one side and
  released on the other
• What regulates the rate of facilitated diffusion?

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Active Transport

• Primary active transport
• Energy derived from ATP hydrolysis
• Typical of pumps that maintain concentrations
  against the gradient

• Secondary active transport
• Energy formed from either a Na or H gradient.
• By allowing primary gradient to leak, the
  energy of that movement can move another solute
  against its own gradient.

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Active Transport
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Endocytosis

• Use of vesicles to take up substances
• Receptor-mediated endocytosis happens by
• Binding
• Vesicle formation
• Uncoating
• Fusion with endosome
• Recycling of receptors to plasma membrane
• Degradation in lysosomes

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Phagocytosis

• Type of endocytosis were cell englufs large
  solids.
• A variant of this is bulk-phase endocytosis, or
  pinocytosis (taking in droplets of water)

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Exocytosis

• Release or secretion of material from the cell
• Form secretory vesicle inside cell.
• Fuse with plasma membrane
• Release contents into extracellular fluid.

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Summary of Membrane Transport
Table 3.1 in text
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Transport by Proteins
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• Proteins change shape to move molecules across
  membranes.
• Proteins can cooperate to move molecules across
  membranes.
• (e.g., facilitated transport linked to active
  transport.

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Model of a Cell Membrane
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Summary

• Membranes are formed by the amphiphilic nature of
  phospholipids.
• Membrane molecules are in constant flux,
  creating a fluid and dynamic layer.
• Membrane proteins serve a number of functions,
  including regulating the passage of molecules
  into and out of the cell.