Membrane Structure and Cellular Transportation Ch. 7

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Membrane Structure and Cellular
TransportationCh. 7

• AP Biology
• Ms. Haut

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Membrane Structure

• Made of phospholipid bilayer
• Polar (hydrophilic) heads of phospholipids
  oriented towards protein layers

• Nonpolar (hydrophobic) tails of phospholipids are
  oriented between polar heads

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Membrane Structure

• Proteins are individually embedded in the bilayer
• Hydrophilic portions exposed to water
• Hydrophobic portions in nonaqueous environment
  inside the bilayer

Phospholipid bilayer
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Fluid-Mosaic Model

• Membranes held together by weak hydrophobic
  interactions
• Lipids and some proteins can drift laterally
  within the membrane

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Mosaic of Different Molecules

• Integral proteinstransmembrane proteins that
  span the hydrophobic interior of the membrane
• Transport proteins

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Mosaic of Different Molecules

• Integral proteinstransmembrane proteins that
  span the hydrophobic interior of the membrane
• Peripheral proteinsattached t the membranes
  surface
• Transport proteins
• Carbohydratesfunction in cell-to-cell
  recognition (cell markers)
• Glycolipids, glycoproteins

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Traffic across Membranes

• Membranes molecular organization results in
  selective permeability
• Permits exchange of nutrients, waste products,
  oxygen, and inorganic ions.
• Allows some substances to cross more easily than
  others
• Hydrophobic moleculeshydrocarbons, CO2, and O2
  dissolve in and cross membrane
• Very small polar molecules, including H2O can
  cross easily

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Passive Transport Diffusion

• Diffusionmovement of a substance down its
  concentration gradient due to random thermal
  motion
• Spontaneous process that decreases free energy
  and increases entropy
• ? NO ENERGY EXPENDED

Diffusion of a gas?
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?Free energy Less stable
?Free energy More stable
A substance will diffuse from where it is more
concentrated to where it is less concentrated
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?Water
Selectively Permeable Membrane
Solute Molecules
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Passive Transport Osmosis

• Osmosisdiffusion of water across a selectively
  permeable membrane
• Water diffuses down its own concentration
  gradient (from hypotonic solution to hypertonic
  solution)

Hypotoniclower concentration of
solutes Hypertonichigher concentration of
solutes Isotonicequal solute concentration
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AP LAB EXERCISE 1A Diffusion
H2O/Lugols Solution (IKI)
15 glucose/ 1 starch
Initial Color
Initial contents
Bag
clear
15glucose and 1 starch
Let stand 30 minutes
Beaker
yellow
H2O IKI
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AP LAB EXERCISE 1A Diffusion
H2O/Lugols Solution (IKI)
15 glucose/ 1 starch
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Alternative Methods of Cellular
TransportationCh. 8

• AP Biology

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Facilitated Diffusion

• Diffusion of solutes across a membrane, with the
  help of transport proteins

• Is passive transport because solute is
  transported down its concentration gradient
• Aides transport of many polar molecules and ions
  that are inhibited by phospholipid bilayer

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Facilitated Diffusion

• Transport proteins share similar properties with
  enzymes

• They are specific for the solutes they transport
• They can be saturated with solutemaximum rate
  occurs when all binding sites are occupied
• They can be inhibited by molecules that resemble
  the solute (similar to competitive inhibition)

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Active Transport

• Energy-requiring process during which a transport
  protein pumps a molecule across a membrane,
  against its concentration gradient

• Is energetically uphill (?G) and requires the
  cell to expend energy
• Helps cells maintain steep ionic gradients across
  cell membrane (e.g., Na, K, Mg 2, Ca 2 , and
  Cl-)
• Transport proteins involved get energy from ATP
  to pump molecules against their concentration
  gradients

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Sodium/Potassium Pump
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Membrane Potential

• Gererated by some ion pumps
• Membrane potentialvoltage across membranes
• Ranges from 50 to 200 mv (the inside of the
  cell is negatively charged relative to outside
• Affects traffic of charged substances across
  membrane
• Favors diffusion of cations into cell anions out
  of cell (due to electrostatic attractionscytopla
  sm is negatively charged)

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Electrochemical Gradient

• Two forces drive the diffusion of ions across a
  membrane
• Chemical forceconcentration gradient
• Electrical forceeffect of membrane potential

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Electrochemical Gradient

• Electrogenic pumptransport protein that
  generates voltage across membranes

• Example is Na/K Pump
• 3 Na ions out/ 2 K ions in equals a net
  transfer of one positive charge from the
  cytoplasm to the extracellular fluid (a net loss
  of one positive charge), a process that stores
  energy in the form of voltage

• Stored energy can be trapped for cellular work

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Cotransport

• Process where a single ATP-powered pump actively
  transport one solute and indirectly drives the
  transport of other solutes against their
  concentration gradient

• ATP-powered pump actively transports one solute
  and creates potential energy in the gradient it
  creates
• Another transport protein couples the solutes
  downhill diffusion as it leaks back across the
  membrane with a second solutes uphill transport
  against its concentration gradient

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• Phagocytosisendocytosis of solid particles
• Forms food vacuoles that fuse with lysozome to be
  digested

• Pinocytosisendocytosis of fluid droplets
• Takes in solutes dissolved in the droplet

• Receptor-mediated endocytosisprocess of
  importing specific macromolecules into the cell
  by inward budding of vesicles formed from coated
  pits
• Occurs in response to binding specific ligands to
  receptors on cells surface

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