Chapter 21 - Membrane Transport

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Yasham
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Chapter 21 - Membrane Transport
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2011-12-01 21:21:15
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Chapter 21 Membrane Transport
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Chapter 21 - Membrane Transport Biochemistry
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  1. What are the two forms of transport that allow for molecules to cross the biological membranes by random diffusion?
    • Nonmediated transport
    • Passive transport.

    The ability of the compound to spontaneously cross the biological membrane depends mainly on the electric charge of the molecules and to a lesser extent on the molar mass.
  2. (T/F) Electrically neutral and small molecules pass through the membrane more easily than charged large particles.
    True.

    Hence the ability of the biological membrane to prevent water diffusion
  3. (T/F) Glucose can enter cells by a nonmediated pathway.
    True. The flux of glucose into a cell increases if the external glucose concentration rises.
  4. The majority of biologically significant molecules and ions are transported through ______ transport.
    Mediate transport.

    There are specialize proteins that mediate transmembrane exchange of H2O, Na+, K+, Ca+, Cl-, amino acids, carbohydrates, nucleotides etc.
  5. What are the transport systems in biological membranes?
    • Uniporter - Transport one type of molecule
    • Symporter - Transport two or more different molecules in same direction.
    • Anitporter - Involved in the movement of two or more different molecules across a membrane in opposite directions.
  6. What is an example of a uniporter?
    Voltage-gated K+ channel
  7. What is an example of a symport?
    SGLT1, which co-transports Na+ and glucose across luminal membrane of the epithelial cells so that glucose can be absorbed into the bloodstream.
  8. What is an example of an antiporter?
    Na+/Ca2+ ATPase, which exchange one calcium ion for three sodium ions to remove cytoplasmic calcium
  9. What is the equation for free energy of a neutral substance transporting across two sides of a biological membrane?

    From out to in.
    DeltaG = RT ln(S-in/S-out)
  10. What is the equation for free energy of a charged ion transporting across two sides of a biological membrane?
    DeltaG = RT ln(S-in/S-out) + zFdeltaY

    • Where z is the ionic charge of the ion
    • DeltaY is the electric potential difference between the interior and exterior of a cell (membrane potential)
    • F is faraday's constant
  11. What are the concentrations of ions Na+ and K+ on the inside and
    • Outside the cell
    • ~150 mM Na+
    • 4mM K+

    • Inside the cell
    • ~12 mM Na+
    • ~140 mM K+
  12. What are the categories of mediate transport?
    • Passive transport (facilitated diffusion)
    • (No ATP - with gradient)

    • Active transport
    • (ATP required - against gradient)
  13. What are aquaporins?
    Water transport proteins
  14. What section of the aquaporins interrupt the chain of hydrogen bonds from forming in water?
    Two Asn residues. The central water molecule cannot accept hydrogens from the neighboring H2O molecules and disrupts the hydrogen bond chain required for proton jumping.
  15. What are porins? What are they typically composed of?
    Transmembrane proteins that form channels specific for the transportation of different molecules across the outer membranes of cells.

    They are typically composed of alternating polar and non-polar residues.
  16. What are ionophores? What are the two subcategories of these?
    Ionophores are lipid-soluble molecules that transport ions across the lipid bilayer of the cell membrame.

    Carrier ionophores are mobile ion carries that specifically bind to a particular ion. They facilitate the ion's passage through the hydrophobic interior of the lipid membrane by shielding its charge from the surrounding environment.

    Channel-forming ionophores introduce a hydrophilic pore into the membrane. This disrupts the transmembrane ion concentration gradients, thus causing an antibiotic effect.
  17. What are carrier ionophores?
    Carrier ionophores are mobile ion carries that specifically bind to a particular ion. They facilitate the ion's passage through the hydrophobic interior of the lipid membrane by shielding its charge from the surrounding environment.

  18. What are channel-forming ionophores?
    Channel-forming ionophores introduce a hydrophilic pore into the membrane. This disrupts the transmembrane ion concentration gradients, thus causing an antibiotic effect.

  19. What is a depsipeptide?
    A molecule with both peptide and ester bonds.
  20. The radius of valinomycin (a carrier ionophore) is the perfect size for K+ ions but is too large for Na+ or Li+. What does this indicate?
    This indicates that the protein has greater affinity of K+ ions rather than Na+ or Li+
  21. What are potassium channels?
    • Passive mediators for allowing potassium to pass through.
    • The channels are composed of four idenitcal or related protein subunits that are arranged around a central ion-conducting pore.

    The loop may form a selectivity filter, a portion of the pore that is responsible for the highly impressive selectivity of the channel.

    The distinctive pore-loop structure of potassium is responsible for selectivity of the channel toward K+.
  22. What is unique about the selectivity filter for potassium channels?
    The filter does not allow for Na+ or Ca2+.

    The arrangement of TVGYG creates four potential ion binding sites for a K+ ion in the dehydrated state.

    The diameter is strictly maintained, which results in the high energy of a dehydrated Na+ in the selectivty filter and therefore accounts for the channel's high selectivity for K+ ions.

    The filter usually contains two K+ ions separated by one water molecule.
  23. What are voltage-gated Kv channels?
    Channels that are sensitive to changes in membrane potential such as the nerve impulse
  24. What is the electrochemical poential of membranes?
    • -40 to -80 mV with the negatively charged inside
    • Resting potential is near -60 mV
  25. What are mecanosensitive channels?
    Transduce physical deformation at a cell membrane into an electrochemical response.
  26. What are the five times of active transporters?
    P-type ATPases: Transmembrane transporters of Na+, K+, and Ca2+ - Undergo phosphorylation during transport process.

    F-type ATPases: Transport proteons across the membrane. They synthesize ATP from ADP and Pi using the H+ gradient during oxidative phosphorylation.

    V-type ATPases: Function in plant vacuoles and animal lysosomes. Resemble F-type ATPases.

    A-type ATPases: Transmembrane anion-transporters

    ATP-binding cassette (ABC) transporters: Translocate across membranes a wide variety of substances (ie ions, metabolic products, lipids and drugs)
  27. What are P-type ATPases?
    P-type ATPases: Transmembrane transporters of Na+, K+, and Ca2+ - Undergo phosphorylation during transport process.
  28. What are F-type ATPases?
    F-type ATPases: Transport proteons across the membrane. They synthesize ATP from ADP and Pi using the H+ gradient during oxidative phosphorylation.
  29. What are V-type ATPases?
    Function in plant vacuoles and animal lysosomes. Resemble F-type ATPases.
  30. What are A-type ATPases?
    Transmembrane anion-transporters
  31. What are ATP-binding cassette (ABC) transporters?
    Translocate across membranes a wide variety of substances (ions, metabolic products, lipids drugs etc.)
  32. What are Na+/K+ ATPases?
    P-type ATPases that are also known as sodium-potassium pumps.

    Responsible for estabilishing the transmembrane gradient of K+ and Na+ concentrations and maintaining the electrochemical potential of the membrane. It is an antiport.
  33. In Na+/K+ ATPases, one ATP molecule is required for the transport of ___ K+ ____ and ____ Na+ to the _____ of the cell.
    • Two K+ inside the cell
    • Three Na+ to the outside of the cell
  34. Na+/K+ ATPases has two conformational states: E1 and E2. What does each state correspond to?
    E1: Opened to the intracellular environment; releases K+ and binds ATP and Na+.

    E2: Interacts with extracellular environment, releases Na+ and binds K+
  35. What is a primary active transporter?
    What is a secondary active transporter?
    • Primary: Directly uses the energy of ATP hydrolysis for transmembrane transport.
    • Na+/K+ ATPase

    Secondary: Free energy of the membrane potential is used to transport other molecules against concentration gradient.
  36. What are the 4 sections of the action potential?
    • Depolarization
    • Action potential
    • Depolarization
    • Hyperpolarization
  37. What is depolarization?
    The stimulation of a neuron causes Na+ channels to open allowing Na+ to rush inside the cell along the electrochemical gradient.

    The migration of Na+ inside the cell changes the electric potential of the membrane
  38. What occurs after depolarization and the action potential?
    The voltage-gated K+ channels open. K+ ions spontaneously flow out of the cell along the gradient concentration.

    Reduces the membrane potential leading to repolarization.
  39. What occurs after repolarization?
    Both Na+ and K+ channels close. But the potassium channels remain open and cause an undershoot until it reaches the resting potential again.

    The depolarization of the membrane segment induces the opening of nearby voltage-gated Na2+ channels for more action potentials to occur near by.
  40. What do Sodium-dependent glucose co-transports do?
    They do not hydrolyze ATP to fuel the endergonic process of transporting glucose against the gradient but rather use the electrochemical gradient created by Na+/K+ ATPase.
  41. What is the energy of ATP hydrolysis?
    30.5 kJ/mol
  42. Give an example of each:
    Symporter
    Antiporter
    Uniporter
    • Symporter: Lactose permease
    • Antiporter: Na+/K+ ATPase
    • Uniporter: K+ channel

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