AcornPrep

AP Biology Unit 2: Cell Structure and Function — Worked Examples

Identifying Transport Mechanisms from Experimental Data

Hard

Researchers measured the uptake rate of molecule X into cells under four experimental conditions shown in the table. Based on these results, what is the most likely mechanism by which molecule X enters the cell?

  1. Simple diffusion across the lipid bilayer
  2. Facilitated diffusion through a channel or carrier protein ✓ Correct
  3. Active transport via an ATP-dependent pump
  4. Endocytosis via vesicle formation
Solution

The data show three key findings: (1) uptake is independent of ATP (oligomycin has no effect), ruling out active transport and endocytosis; (2) uptake requires a transport protein (denaturation drops rate to near baseline); (3) uptake requires a concentration gradient (equalized concentration stops transport). Together, these indicate facilitated diffusion. Choice A is wrong because simple diffusion does not require a protein. Choice C is wrong because ATP inhibition had no effect. Choice D is wrong because endocytosis requires ATP for vesicle formation.

Signal Sequence Mutation and Protein Targeting

Hard

A researcher creates a mutation in the gene encoding insulin, a secretory protein. The mutation deletes the signal peptide sequence at the N-terminus. When the mutant mRNA is translated in cells, where will the resulting protein accumulate?

  1. In the rough endoplasmic reticulum, because translation always begins on the rough ER
  2. In the cytoplasm, because the ribosome-mRNA complex will not be directed to the ER membrane ✓ Correct
  3. In the Golgi apparatus, because proteins are sorted there regardless of signal sequence
  4. Outside the cell, because the protein will still be secreted through an alternative pathway
Solution

The signal peptide is recognized by the signal recognition particle (SRP), which directs the ribosome-mRNA complex to the rough ER membrane. Without the signal peptide, the SRP cannot bind, so translation completes on free ribosomes in the cytoplasm and the protein accumulates there. Choice A incorrectly assumes all translation starts on rough ER — translation begins on free ribosomes and is redirected to ER only if a signal peptide is present. Choice C assumes the Golgi receives proteins independently of the ER pathway. Choice D assumes alternative secretion pathways exist for all proteins.