Membrane Transport Proteins

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Membrane Transport
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Permeability of Lipid Bilayer
Fig. 15-1
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Transmembrane forces acting upon solutes

• Uncharged solutes chemical (concentration)
  gradient
• Charged solutes electrochemical gradient, e.g.,
  affected by concentration gradient of solute and
  membrane potential

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Fig. 15-9
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Membrane Transport Proteins

• Integral, multipass transmembrane proteins
• Selective for specific small molecule(s)
• Types
• Channel proteins - mediate passive transport only
  of ions or water can be gated
• Carrier proteins - mediate passive, active or
  cotransport of ions and small hydrophilic
  molecules (e.g., amino acids, sugars,
  nucleotides, inorganic phosphate, etc.)

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Note large, relatively non-selective pores,
e.g., porins and gap junctions, are used only for
special cases of intracellular or direct
intercellular exchange, respectively chemical
gradients across the plasma membrane would be
destroyed if these structures were open to the
extracellular environment.
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Carrier Proteins

• Specific (and reversible) binding of solute
  similarities to enzyme kinetics
• Switching between cytoplasmic and outside faces
• Can mediate passive transport (facilitated
  diffusion), active transport or cotransport

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Fig. 15-5
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Importance of active transport used to establish
and maintain gradients, e.g., to create
differential between intra- and extracellular
concentrations of transported components role in
osmotic balance of cells ion gradients are
ultimately responsible for electrical polarity of
membranes
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• Cotransport one solute moves down its
  electrochemical gradient, providing the necessary
  free energy to transport the other solute against
  its electrochemical gradient (a.k.a. secondary
  active transport)
• Symport the two solutes move in the same
  direction, e.g. Na/glucose symport
• Antiport the two solutes move in opposite
  directions, e.g., HCO3-/Cl- antiport

Fig. 15-3(b)
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Specific Examples of Carrier Proteins

• GLUT1 uniport of mammalian cells
• AE1 protein of erythrocytes HCO3-/Cl- antiport
• Na/glucose symport (transcellular transport)
• Four classes of ATPases
• P-class ATPases Ca2-ATPases, H-ATPases in
  plasma membranes, Na/K-ATPase
• F-class ATPases we will discuss specific
  examples in mitochondria and chloroplast lectures
  
• V-class ATPases H-ATPase
• ABC superfamily bacterial permeases, MDR
  transport proteins and CFTR protein

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Uniport Transport

• How is Uniport Transport different from Passive
  Diffusion?
• The rate of facilitate transport by uniporters is
  higher than passive diffusion.
• Transport is specific. Each uniporter transports
  only a specific type of molecule.
• Transport occurs via a limited number of
  uniporter molecules, rather than throughout the
  phospholipid bilayer.

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Fig. 15-5
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GLUT1 UNIPORT
Fig. 15-7
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Anion Transport Across Erythrocyte Membrane
Fig. 15-20(a)
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Anion Transport Across Erythrocyte Membrane
Fig. 15-20(b)
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The Four Classes of ATP-Powered Transport Proteins
Fig. 15-10
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V-Class H-ATPases in Acid-Secreting Cell
Fig. 15-14
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Histidine Permease (ABC Transporter) of
Gram-Negative Bacterium
Fig. 15-15
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Structural Model for Mammalian MDR1 Protein
Fig. 15-16
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Figure 15-17
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Mechanism of Action of Muscle Ca2-ATPase
Fig. 15-11
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Catalytic Subunit of Muscle Ca2-ATPase
Fig. 15-12
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Structure and Net Functional Activity of
Na/K-ATPase
Fig. 15-13(a)
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Detailed Mechanism of Action of Na/K-ATPase
Fig. 15-13(b)
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Importance of Na/K-ATPase

• Establishes large chemical concentration gradient
  of Na across plasma membrane of animal cells,
  which is used for
• Osmotic regulation of animal cells
• Cotransport of metabolites nutrient uptake by
  animal cells
• Contributes (10) to membrane potential pump is
  electrogenic (3 positive charges go out but only
  two positive charges come in)
• Responsible for quick, active regeneration of
  resting membrane potential in excitable cells
  (neurons, muscle) these cells generate action
  potentials, which are produced by rapid changes
  in membrane potential caused by the rapid flow of
  Na and K ions across the membrane through ion
  channels

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Fig. 15-23
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Nutrient uptake (transcellular transport) by
intestinal epithelial cells of mammals

• Na/K-ATPase in basolateral membrane keeps
  intracellular concentration of Na low.
• Na/glucose symport in apical membrane
  cotransports glucose from lumen of intestine into
  epithelial cell.
• Glucose uniport in basolateral membrane passively
  transports glucose out of the intestinal cell
  into the extracellular fluid glucose then
  diffuses into nearby capillary.
• Tight junctions between epithelial cells prevent
  glucose from diffusing back into gut lumen tight
  junctions also maintain membrane segregation of
  different transporters to apical and basolateral
  membranes, respectively.
• The basolateral location of the Na/K-ATPase in
  the epithelial cells ensures that Na is pumped
  into the extracellular space, instead of back
  into the gut lumen. What is the advantage of
  this directionality of Na transport?

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Fig. 15-25
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Fig.s 15-18 15-19
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Fig. 15-28
Fig. 15-29
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Osmotic Regulation of Animal and Plant Cells

• Animal cells minimize difference between the
  total solute concentration of the extracellular
  and intracellular fluids
• Plant cells function best when they are
  hypoosmotic with respect to the extracellular
  fluid and are in a turgid state (note importance
  of cell wall and vacuole in
  this process).

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Fig. 15-22
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Major Plant/Animal Differences

• Na/K ATPase and Na-symport in animals
• H (proton) ATPase and H-symport in plants
• Osmolarity
• animal cells are isoosmotic with respect to the
  extracellular fluid (ECF)
• plant cells are hyperosmotic with respect to the
  ECF and are turgid

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