Question 1

A researcher observes that a membrane protein with several hydrophobic amino acids in its transmembrane region replaced by charged amino acids fails to insert into the membrane efficiently, while a similar protein with charged amino acids on its cytoplasmic surface replaced by hydrophobic amino acids still inserts normally. Which principle best explains this observation?

Accepted Answer

Charged residues in the transmembrane region disrupt favorable interactions with the hydrophobic core of the membrane.

Question 2

A scientist measures the rate of solute transport across a membrane and observes that as solute concentration increases, the transport rate rises linearly at first but then plateaus. What is the most likely explanation for this observation?

Accepted Answer

The transport protein has become saturated, indicating facilitated diffusion.

Question 3

An enzyme that functions optimally at pH 7 loses 95% of its activity at pH 3, but its molecular mass remains unchanged. What is the most likely explanation?

Accepted Answer

Ionization of amino acid side chains at low pH alters the enzyme's tertiary structure.

Question 4

A membrane protein facilitating glucose transport is present in equal amounts in two cells. Cell A transports glucose twice as rapidly as Cell B. Which factor most likely accounts for this difference?

Accepted Answer

Cell A has a larger glucose concentration gradient across its membrane.

Question 5

A cell line is engineered to have membranes composed almost entirely of saturated phospholipids. At low temperatures, researchers observe decreased growth. Which mechanism best explains this result?

Accepted Answer

Reduced membrane fluidity impairs transport and signaling.

Question 6

A mutation alters one nucleotide in a gene, but protein sequencing reveals no change in the amino acid sequence. Which conclusion is most justified?

Accepted Answer

The mutation occurred in a noncoding region or produced a synonymous codon.

Question 7

A researcher introduces a mutation that prevents formation of hydrogen bonds between complementary DNA strands. Which process would be most directly impaired?

Accepted Answer

DNA replication

Question 8

A cell line lacks functional lysosomes. Which observation would most strongly support lysosomal dysfunction?

Accepted Answer

Accumulation of damaged organelles

Question 9

A scientist removes cholesterol from animal cell membranes. Which effect would most likely occur at high temperatures?

Accepted Answer

Increased membrane fluidity

Question 10

A researcher observes that a membrane transport protein reaches saturation at high solute concentrations. Which conclusion is best supported?

Accepted Answer

The protein mediates facilitated diffusion.

Question 11

A mutation changes a codon from GAA to GAG, but both codons encode glutamic acid. What is the most likely outcome?

Accepted Answer

Protein function remains unchanged.

Question 12

A scientist inhibits ribosome function in a eukaryotic cell. Which organelle would be affected most immediately?

Accepted Answer

Rough ER

Question 13

Cells are placed into a hypertonic solution. Which outcome is expected?

Accepted Answer

Water leaves cells.

Question 14

A membrane channel protein is mutated so that its pore becomes lined with nonpolar amino acids rather than polar amino acids. Which prediction is most reasonable?

Accepted Answer

Water transport through the channel will likely decrease.

Question 15

A researcher observes increased transport as concentration increases until transport plateaus. What evidence would most strongly support facilitated diffusion?

Accepted Answer

ATP consumption remains unchanged.

Question 16

A scientist engineers a membrane containing phospholipids with shorter fatty acid tails. What is the likely effect?

Accepted Answer

Increased membrane fluidity

Question 17

A membrane channel allows passage of water but excludes sodium ions. Which property best explains this selectivity?

Accepted Answer

Channel pore chemistry

Question 18

A protein's quaternary structure is disrupted. Which observation is most likely?

Accepted Answer

Loss of interactions among subunits

Question 19

A cell is moved from an isotonic solution into a hypotonic solution. What occurs immediately?

Accepted Answer

Net water influx

Question 20

Scientists remove all transport proteins from an artificial membrane. Which molecule would still cross most efficiently?

Accepted Answer

Oxygen

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