BIO 105

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BIO 105

• Cell Membranes

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Cell Membranes
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Cell Membranes

• The eukaryotic cell possesses many membranes
• The outer membrane is called the plasma membrane.
• Many organelles are bounded by membranes..ER,
  mitochondria, Golgi, nucleus
• All membranes have similar properties.

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Plasma Membrane

• The interface between the cell and the external
  environment
• Physical and chemical barrier
• Selective
• Responsive to signals
• Only 2 molecules thick
• No plasma membrane.no cell
• Comprised of 2 layers of lipid molecules and
  specific proteins.the cells personality

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Membrane Structure

• 10,000 membranes thickness of sheet of paper
• Lipids form the foundation of the membrane
• Lipids are phospholipids

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Phospholipids

• Consist of
• 3 carbon backbone derived from glycerol
• 2 fatty acids
• Phosphorylated alcohol group (charged)

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Structure of Phospholipids
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Some Definitions

• Hydrophilic (water-loving) molecules that are
  charged (polar) and will attract H2O .
• Hydrophobic (water-hating) molecules that are
  uncharged (non-polar)and will shun water.

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Structure of Phospholipids
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So, what happens when you place a phospholipid
into H2O?

• The hydrophilic (charged) region is just fine and
  attracts H2O.
• But what happens to the hydrophobic region (the
  fatty acids).no place to run
• A single phospholipid molecule in H2O never
  happens in nature.
• But multiple phospholipid molecules do occur in
  H2O .what happens then?
• They form a bilayer.

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Phospholipid Bilayer
OUTSIDE INSIDE
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Phospholipid Bilayer

• Forms spontaneously
• Polar (hydrophilic) portion faces outside H2O and
  inside H2O
• Non-polar (hydrophobic) fatty acids face each
  other and form the middle of the sandwich.
• Phospholipids are free to float laterally within
  each of the bilayers.
• About the same viscosity (flow) as olive oil.

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Phospholipid Bilayer
OUTSIDE INSIDE
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The Fluid Mosaic Model
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Features of the Model

• Cell membranes comprised of
• A phospholipid bilayer
• Proteins
• Peripheral on exterior or interior surface
• Transmembrane spanning the entire width

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The Fluid Mosaic Model
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Transmembrane Proteins

• Two types
• Free to float laterally within the lipid bilayers
• Fixed in a specific place of the membrane by the
  cytoskeleton

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The Fluid Mosaic Model
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Free FloatingTransmembrane Proteins
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Fixed Transmembrane Proteins
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Membrane Protein Functions

• Transporter inside ? outside

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Membrane Protein Functions

• 2. Enzyme substrate ? product

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Membrane Protein Functions

• 3. Cell Surface Receptors hormones, etc.

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Membrane Protein Functions

• 4. Cell Surface identity marker -- glycoprotein

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Membrane Protein Functions

• 5. Cell Adhesion shared molecules

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Membrane Protein Functions

• 6. Attachment to cytoskeleton

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Structure of Transmembrane Proteins
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Structure of Transmembrane Proteins

• Portions of protein exposed to outside H2O and
  inside H2O (both are polar and hydrophilic)
• Portions of protein that spans the width of the
  non-polar, hydrophobic, fatty acid width of
  membrane several times

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Structure of Transmembrane Proteins
Based on neutral and charged AA sequence
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Diffusion

• Molecules in water are in constant random motion.
• This motion causes these molecules to move from
  regions of high ? low concentration.
• This process is called diffusion.
• When molecules are in equal concentration in all
  regions, the substance is said to be in
  equilibrium.

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Diffusion
Equilibrium

• Solvent
• Solute
• Aqueous solution

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Diffusion Across a Membrane

• Both water and solutes will diffuse down their
  concentration gradients.
• But what happens when we consider the biological
  world and consider the plasma membrane (PM).
• Only H2O and hydrophobic (non-polar) molecules
  can freely pass across the lipophilic (fatty
  acids) portion of the PM.
• Most solutes (ions, AA, sugars) are not lipid
  soluble and cant diffuse across H2O the PM.
• H2O can diffuse down its gradient across the PM.

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Diffusion Across a Membrane
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Osmosis

• Osmosis is the movement of H2O across a membrane
  that permits H2O flow but not that of one or more
  solutes.
• Osmotic concentration the concentration of all
  solutes on one side of a selective membrane.
• If the concentration of a solution is higher on
  one side of a membrane, the solution is said to
  be hyperosmotic.
• If the concentration of a solution is lower on
  one side of a membrane, the solution is said to
  be hypoosmotic.

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Osmosis (cont.)

• If the osmotic concentration of solutions on both
  sides is equal, the solutions are said to be
  isosmotic.
• Water will flow across a membrane in the
  direction of the hyperosmotic solution in order
  to dilute the solution.

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Osmosis Demonstration
hydrostatic (osmotic) pressure
How do polar molecules get across the PM?
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Bulk Passage In and Out of Cells

• Endocytosis
• PM engulfment of surrounding fluids or large
  particles such as bacteria. Three types are

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Endocytosis

• Pinocytosis ingestion of surrounding
  extracellular fluids non-selective
• Phagocytosis ingestion of particulate matter
  and bacteria

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3. Receptor-mediated
3.
Selective engulfment
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Bulk Passage In and Out of Cells

• Exocytosis
• Reverse of endocytosis
• Extrusion of bulk materials via membrane-bound
  vesicles
• Adds
• membrane
• to PM

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Question

• Bulk transport aside, what about hydrophilic
  (polar) molecules?
• How do they get across the PM?
• Transport proteins
• They are trans-membrane proteins within the PM

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1. Ion Channels

• These proteins have a hollow channel that spans
  the entire width of the PM.
• Channel is filled with H2O.
• Ion channels are specific for specific ions
  Ca, Cl-, Na, K
• Direction of ion flow (in or out of cell) is
  determined by concentration gradient.
• These channels are ! In nerve and muscle cells

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1. Ion Channels (cont.)
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2. Facilitative Diffusion

• Proteins that physically bind specific solutes
• Transport solutes from one side of membrane
• Release them on other side of membrane
• Direction depends on solute gradient
• Passive diffusion vs. facilitative diffusion

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2. Facilitative Diffusion (cont.)

• Can be saturatedlimited by the number of
  transport proteins

rate of transport of solute (mol/sec)
solute
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2. Facilitative Diffusion (cont.)
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Passive Diffusion

• Ion channel and facilitative transport are
  examples of passive diffusion.
• They do not require energy.
• Solutes are driven by concentration gradient
  (high ? low)
• But what about transporting solutes against the
  concentration gradient?

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Active Transport

• Ability of cell membrane to move solutes against
  their concentration gradient
• Can occur in either direction (in or out)
• Requires significant expenditure of cells energy
  (ATP)

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3. Na/K Pump

• Most animal cells have low internal Na and
  high internal K.
• They do this by actively pumping Na out of the
  cell and actively pumping K into the cell.
• This process requires 1/3 of a cells total
  energy (ATP) output.

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Na/K Pump
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Na/K Pump
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3. Na/K Pump (cont.)

• Three Na ions leave the cell and two K ions
  enter the cell every cycle.
• 100 cycles/sec

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4. Proton Pump

• Transports protons against concentration gradient
• Requires energy
• Generation of ATP by chemiosmosis

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5. Cotransport

• Cotransport 2 substances pumped in same
  direction
• One goes down gradient and the other goes up
  gradient
• Requires energy

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6. Countertransport

• Countertransport similar to cotransport except
  2 substances go in opposite directions