Chapter 13 Membrane Channels and Pumps

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Chapter 13Membrane Channels and Pumps

• Lipid bilayer impermeable to ions and polar
  molecules

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Transport of Molecules

• Two factors determine whether a molecule will
  cross a membrane
• (1) permeability of molecule in lipid bilayer
• Lipophilic molecules
• Simple diffusion High to Low
• Increase in entropy powers transport across the
  membrane
• (2) availability of an energy source
• Charged or polar molecules

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Membrane Proteins

• Membrane proteins confer permeability
• Protein Transporters to cross membrane
• Channels
• Pumps

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Types of Transport

• Passive Transport (Facilitated Diffusion)
• Channels
• Thermodynamically downhill transport of ions
• Active Transport
• Pumps
• Thermodynamically uphill transport of ions or
  molecules
• Requires an input of energy (ATP or light energy)

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ATP-Driven Pumps

• P-type ATPase
• ATP-Binding Cassette

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ATP-Driven Pumps

• P-type ATPase
• Ca2 -ATPase
• Na - K - ATPase
• Form key phosphorylated intermediate
• ATP hydrolysis coupled to orientation change and
  affinity change in ion-binding site

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ATP-Driven Pumps

• ATP-Binding Cassette
• Conformation change upon ATP binding and
  hydrolysis
• Bound ion transported across membrane

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Free Energy Stored in Concentration Gradients

• Z - electrical charge of transported species
• ?V potential across membrane
• F Faraday Constant (96.5 kJ/V-mol) or (23.1
  kcal/V)

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Ion Gradient

• Important energy storage forms
• Animal cells intracelllular concentrations
• High concentration of K
• Low concentration of Na
• Na-K ATPase / Na-K Pump
• Specific transport system
• Enzyme
• Energy provided from ATP hydrolysis

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Na-K Gradient

• Controls cell volume
• Renders neurons and muscle cells electrically
  excitable
• Drives the active transport of sugars and amino
  acids

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P-type ATPases

• Na-K ATPase
• H-K ATPase
• Gastric
• Pumps H into stomach to lower pH below 1.0
• Ca2 ATPase
• Pumps Ca2 from cytoplasm into sarcoplasmic
  reticulum of muscle cells

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P-type ATPases

• Forms key phosphorylated intermediate
• ATP hydrolysis
• Phosphoryl group linked to a specific aspartate
  residue in ATPase
• Aspartate residue conserved

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