Cell structure and membrane transport sample question 1

Colchicine blocks microtubule polymerization and taxol stabilizes microtubules against depolymerization, so the spindle cannot work normally.

Cell structure and membrane transport sample question 2

Cytokinesis, which depends on a contractile ring that pinches the cell in two

Cell structure and membrane transport sample question 3

Without the pump, the Na⁺ gradient slowly runs down, and the symporter loses the energy that drives glucose uptake.

Cell structure and membrane transport sample question 4

the protein is synthesized in full on free cytosolic ribosomes and does not enter the ER, so it is not secreted.

Cell structure and membrane transport sample question 5

Na⁺–glucose cotransport is driven by the Na⁺ gradient and does not depend on cAMP, so Na⁺, glucose and water are still absorbed.

MCAT · Biological and Biochemical Foundations · Biology

Cell structure and membrane transport

Cell structure and membrane transport is part of Biology in the MCAT Biological and Biochemical Foundations section. The questions below are written in the style of the real test, each with the answer and a worked explanation.

Question types you will see here include identify the transport mode, osmosis and tonicity, na+/K+ pump and ion gradients and saturable versus linear uptake.

Common traps: calling any protein-mediated transport 'active'; missing that a symporter's energy is the Na+ gradient, not ATP at the carrier; reading the gradient direction backwards.

Sample questions

Question 1
Colchicine and paclitaxel (taxol) both arrest cells in mitosis. Which statement best describes their actions?
- A.
  Colchicine stabilizes microtubules and taxol blocks their assembly, so the spindle cannot form, and the cell arrest occurs because the nuclear envelope fails to reform after division.
- B.
  Both drugs depolymerize actin filaments, so the contractile ring cannot form.
- C.
  Colchicine blocks microtubule polymerization and taxol stabilizes microtubules against depolymerization, so the spindle cannot work normally.
- D.
  Both drugs cut DNA, which causes the cells to stop at the spindle checkpoint.
Answer: C. Colchicine blocks microtubule polymerization and taxol stabilizes microtubules against depolymerization, so the spindle cannot work normally.
Colchicine binds tubulin and prevents polymerization, whereas taxol prevents disassembly; both disrupt the dynamic behavior of spindle microtubules, so chromosomes cannot segregate and cells arrest in mitosis. A reverses the two drugs, and B and D name the wrong targets (actin or DNA).
Question 2
A cultured fibroblast is treated with a drug that depolymerizes actin filaments but leaves microtubules intact. Which process is most directly impaired?
- A.
  Separation of the sister chromatids toward the opposite spindle poles at anaphase
- B.
  Transport of vesicles along the length of a long cell extension
- C.
  Cytokinesis, which depends on a contractile ring that pinches the cell in two
- D.
  Beating of cilia on the apical surface of the cell
Answer: C. Cytokinesis, which depends on a contractile ring that pinches the cell in two
The cleavage furrow is produced by a ring of actin filaments and myosin II that tightens beneath the plasma membrane, so without actin the cytoplasm cannot divide and binucleate cells form. Chromosome movement, long-range vesicle transport by kinesin and dynein, and ciliary beating all depend on microtubules, which the drug leaves intact.
Question 3
Intestinal cells take up glucose through SGLT1, a Na⁺-glucose symporter. When ouabain, an inhibitor of the Na⁺/K⁺ ATPase, is added, glucose uptake declines gradually over several minutes. What is the best explanation?
- A.
  Without the pump, the Na⁺ gradient slowly runs down, and the symporter loses the energy that drives glucose uptake.
- B.
  Ouabain blocks SGLT1 directly, because the symporter hydrolyzes ATP to move glucose and ouabain binds that ATP site.
- C.
  Ouabain blocks GLUT2 in the opposite membrane, so glucose cannot cross the cell at all.
- D.
  Ouabain makes the cell membrane impermeable to glucose, so uptake stops within seconds.
Answer: A. Without the pump, the Na⁺ gradient slowly runs down, and the symporter loses the energy that drives glucose uptake.
SGLT1 does not use ATP; it is driven by the inward Na⁺ gradient, which the Na⁺/K⁺ pump maintains. Blocking the pump lets the gradient dissipate gradually, so uptake falls with delay. B treats the symporter as an ATPase; C would not give a gradual decline; D would give an immediate, complete stop.
Question 4
A secreted protein has an N-terminal signal sequence. In a cell that lacks functional signal recognition particle, the most likely result is that
- A.
  the protein is synthesized in full on free cytosolic ribosomes and does not enter the ER, so it is not secreted.
- B.
  the protein enters the ER lumen after translation is complete, and secretion proceeds normally.
- C.
  the protein enters the Golgi apparatus directly from the cytosol and is secreted without any ER processing or glycosylation.
- D.
  the protein is synthesized on mitochondrial ribosomes and imported into the matrix.
Answer: A. the protein is synthesized in full on free cytosolic ribosomes and does not enter the ER, so it is not secreted.
A is correct: SRP binds the signal peptide and brings the ribosome to the ER translocon, and without it the chain is not targeted to the rough ER. B claims normal post-translational entry; C skips the ER, which is the entry point to the secretory pathway; D gives the protein mitochondrial ribosomes without any basis.
Question 5
Cholera toxin locks Gsα in its active form, so cAMP rises in intestinal epithelial cells and CFTR-mediated Cl⁻ secretion causes severe watery diarrhea. Oral rehydration solution containing glucose and sodium still helps because
- A.
  glucose lowers cAMP in the epithelial cells, which reverses the effect of the toxin on Gsα and stops the secretion of Cl⁻ and water.
- B.
  glucose blocks the CFTR channel directly, which stops the secretion of Cl⁻ into the lumen.
- C.
  Na⁺ and glucose cross the apical membrane by simple diffusion, which the toxin does not affect.
- D.
  Na⁺–glucose cotransport is driven by the Na⁺ gradient and does not depend on cAMP, so Na⁺, glucose and water are still absorbed.
Answer: D. Na⁺–glucose cotransport is driven by the Na⁺ gradient and does not depend on cAMP, so Na⁺, glucose and water are still absorbed.
The apical Na⁺–glucose symporter uses the Na⁺ gradient maintained by the basolateral Na⁺/K⁺ ATPase (secondary active transport), is not controlled by cAMP and still works, and water follows the absorbed solutes. Glucose does not act on Gsα (A) or on CFTR (B), and glucose is too polar to enter cells by simple diffusion (C).

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