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General Biology: Membrane Proteins, Transport, and Protein Structure

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  • Effect of replacing hydrophobic amino acids in transmembrane regions with charged amino acids
    Protein A will have difficulty embedding in the membrane because the hydrophobic bilayer interior disfavors charged residues, making insertion energetically unfavorable.
  • Why can Protein B with charged amino acids on cytoplasmic surface still insert normally?
    The cytoplasmic surface is aqueous and tolerates charged or nonpolar substitutions, so membrane insertion is less affected.
  • What does a plateau in solute transport rate at high concentrations indicate?
    It indicates transport protein saturation, characteristic of facilitated diffusion with finite transport capacity.
  • How does simple diffusion differ from facilitated diffusion in transport rate vs. concentration?
    Simple diffusion shows a linear increase in transport rate with concentration, while facilitated diffusion plateaus due to protein saturation.
  • Why does enzyme activity decrease at low pH without change in molecular mass?
    Ionization of amino acid side chains alters tertiary structure by disrupting ionic and hydrogen bonds, changing protein shape.
  • Mutation causing receptor to bind ligand but fail to activate signaling likely affects which domain?
    The intracellular signaling domain, as ligand binding occurs but downstream activation fails.
  • Effect of membranes composed mostly of saturated phospholipids at low temperatures
    Reduced membrane fluidity causes rigidity, impairing transport and signaling, leading to decreased cell growth.
  • What does a mutation with no change in amino acid sequence suggest?
    The mutation is likely synonymous or in a noncoding region due to genetic code redundancy.
  • Why can two proteins with identical primary structures differ in activity?
    Differences in tertiary structure affect folding and function despite identical amino acid sequences.
  • Why does Cell A transport glucose faster than Cell B despite equal transporter amounts?
    Cell A likely has a larger glucose concentration gradient, driving faster facilitated diffusion.
  • What indicates successful Golgi inhibition in cells?
    Accumulation of secreted proteins in the ER, as transport from ER to Golgi is blocked.
  • Effect of mutating a membrane channel pore lining from polar to nonpolar amino acids
    Water transport decreases because polar residues stabilize water passage; nonpolar residues reduce permeability.
  • Why does replacing a buried hydrophobic leucine with charged arginine reduce enzyme activity?
    Charged residue destabilizes the hydrophobic core, disrupting tertiary structure and proper folding.
  • How does increasing temperature affect membrane permeability?
    Higher temperature increases phospholipid movement, enhancing membrane fluidity and permeability.
  • Which process is impaired if hydrogen bonds between DNA strands cannot form?
    DNA replication, as base pairing depends on hydrogen bonds.
  • What cellular defect causes accumulation of proteins inside the ER?
    Failure of Golgi transport, blocking protein movement from ER to Golgi.
  • What happens to cells placed in a hypertonic solution?
    Water leaves the cells due to osmosis toward higher solute concentration outside.
  • Effect of reducing pore diameter in a membrane channel protein
    Reduced transport rate because smaller pores restrict molecular passage.
  • Structural feature contributing to protein stability at high temperatures
    Additional ionic interactions increase stability in thermophilic proteins.
  • Result of removing the ER signal sequence from a protein
    Protein remains in the cytosol, failing to be targeted to the ER.
  • Membrane change causing decreased fluidity despite constant temperature
    Increased saturated fatty acids, which pack tightly and reduce fluidity.
  • Which molecule crosses membranes most efficiently without transport proteins?
    Oxygen, a small nonpolar molecule that diffuses directly through the bilayer.
  • What supports facilitated diffusion when transport plateaus at high solute concentration?
    ATP consumption remains unchanged, indicating passive transport via proteins.
  • Effect of disrupting protein quaternary structure
    Loss of interactions among subunits, impairing overall protein function.
  • Effect of phospholipids with shorter fatty acid tails on membrane fluidity
    Increased membrane fluidity due to less tail length and weaker van der Waals forces.
  • Organelle involved if receptor abundance on plasma membrane decreases
    Golgi apparatus, responsible for protein processing and trafficking.
  • Effect of lacking cholesterol in membranes at cold temperatures
    Excessive membrane rigidity due to loss of cholesterol's fluidity-maintaining role.
  • Why might a membrane protein function normally despite a mutation?
    Conservative amino acid substitution preserves protein structure and function.
  • Effect of higher GC content in DNA molecules
    Higher melting temperature due to increased hydrogen bonding and stability.
  • Property explaining selective water passage but exclusion of sodium ions in a channel
    Channel pore chemistry, including polarity and size, determines selectivity.