COMSAE PHASE 1 CELLULAR BIOLOGY
PRACTICE EXAM WITH ACTUAL
QUESTIONS AND VERIFIED ANSWERS,
PLUS EXPLAINED RATIONALES/EXPERT
VERIFIED FOR GUARANTEED 100% PASS
2026/LATEST UPDATE/INSTANT
DOWNLOAD PDF
1. A researcher isolates a newly synthesized membrane protein from a
eukaryotic cell and determines that the protein contains several long
stretches of hydrophobic amino acids, each approximately 20 amino
acids in length. The protein is subsequently demonstrated to span the
plasma membrane multiple times. Which structural feature most directly
permits these regions to remain stable within the hydrophobic core of the
lipid bilayer?
A. Covalent attachment of carbohydrates to the cytosolic domain
B. Exposure of polar amino acid side chains toward the membrane
interior
C. Formation of α-helical transmembrane segments with hydrophobic
side chains facing outward
D. Extensive disulfide bonding between cytosolic cysteine residues
Answer: C. Formation of α-helical transmembrane segments with
hydrophobic side chains facing outward
Rationale: Transmembrane proteins commonly contain α-helices
approximately 20–25 amino acids long, allowing hydrophobic side
chains to interact favorably with the hydrocarbon tails of membrane
phospholipids. The peptide backbone hydrogen bonds stabilize the helix
internally, while hydrophobic side chains face the lipid environment.
Carbohydrates are generally attached to extracellular domains, and
1
,disulfide bonds are favored in the oxidizing extracellular environment
rather than the cytosol.
2. A patient has a genetic defect that markedly reduces the activity of the
Na⁺/K⁺-ATPase in skeletal muscle cells. Which immediate alteration
would most directly result from inhibition of this transporter?
A. Increased intracellular Na⁺ and decreased intracellular K⁺
B. Increased intracellular K⁺ and decreased intracellular Na⁺
C. Increased intracellular Ca²⁺ exclusively because calcium directly
enters through the pump
D. Complete elimination of membrane potential
Answer: A. Increased intracellular Na⁺ and decreased intracellular
K⁺
Rationale: The Na⁺/K⁺-ATPase uses ATP to transport three Na⁺ ions out
of the cell and two K⁺ ions into the cell. Inhibition therefore causes
intracellular Na⁺ to accumulate while intracellular K⁺ decreases. The
resulting disruption of ionic gradients eventually affects membrane
potential, cell volume, and secondary active transport. The pump does
not directly transport calcium.
3. A red blood cell is placed into a solution with substantially lower
effective solute concentration than the cytoplasm. Water rapidly enters
the cell, causing it to swell and eventually rupture. Which process best
explains this observation?
A. Facilitated diffusion of water through glucose transporters
B. Osmosis through the plasma membrane and aquaporins
C. Primary active transport of water into the cell
D. Endocytosis of extracellular fluid
Answer: B. Osmosis through the plasma membrane and aquaporins
2
,Rationale: Osmosis is the net movement of water across a selectively
permeable membrane toward the compartment with greater effective
osmolarity. A hypotonic extracellular solution causes water to enter the
red blood cell. Aquaporins facilitate rapid transmembrane water
movement. This can lead to cellular swelling and hemolysis if the
osmotic gradient is sufficiently large.
4. A mutation prevents a cell from properly assembling its nuclear pore
complexes. Which cellular process would be most directly impaired?
A. β-oxidation of fatty acids
B. Oxidative phosphorylation
C. Exchange of proteins and RNA between nucleus and cytoplasm
D. Synthesis of membrane phospholipids in the smooth endoplasmic
reticulum
Answer: C. Exchange of proteins and RNA between nucleus and
cytoplasm
Rationale: Nuclear pore complexes regulate transport between the
nucleus and cytoplasm. Small molecules can pass relatively freely,
whereas larger proteins and RNAs require regulated transport
mechanisms involving nuclear localization or export signals and
transport receptors. Nuclear pore dysfunction therefore directly
compromises nucleocytoplasmic trafficking.
5. A secreted protein is synthesized by a pancreatic acinar cell. Which
sequence most accurately describes the major intracellular pathway
followed by this protein?
A. Nucleus → lysosome → Golgi apparatus → plasma membrane
B. Rough ER → Golgi apparatus → secretory vesicles → plasma
membrane
3
, C. Smooth ER → mitochondria → lysosome → plasma membrane
D. Golgi apparatus → rough ER → nucleus → secretory vesicle
Answer: B. Rough ER → Golgi apparatus → secretory vesicles →
plasma membrane
Rationale: Proteins destined for secretion are synthesized on ribosomes
attached to the rough endoplasmic reticulum. They enter the ER during
translation, undergo folding and initial modification, and are transported
in vesicles to the Golgi apparatus. Following additional processing and
sorting, they enter secretory vesicles that fuse with the plasma
membrane through exocytosis.
6. A cell produces a mutant protein containing a defective signal peptide
that prevents its normal targeting to the rough endoplasmic reticulum.
Where would this protein most likely be synthesized?
A. On free cytosolic ribosomes
B. Inside the lysosomal lumen
C. Within the Golgi apparatus
D. Inside the mitochondrial intermembrane space
Answer: A. On free cytosolic ribosomes
Rationale: Ribosomes begin translation in the cytosol. A functional ER
signal sequence directs the translating ribosome to the rough ER.
Without appropriate targeting information, the protein remains
synthesized on free ribosomes and generally remains in the cytosol
unless another targeting sequence directs it elsewhere.
7. A lysosomal storage disorder is caused by deficiency of an enzyme
required to degrade a particular sphingolipid. Which cellular
compartment would most directly accumulate the undegraded substrate?
4
PRACTICE EXAM WITH ACTUAL
QUESTIONS AND VERIFIED ANSWERS,
PLUS EXPLAINED RATIONALES/EXPERT
VERIFIED FOR GUARANTEED 100% PASS
2026/LATEST UPDATE/INSTANT
DOWNLOAD PDF
1. A researcher isolates a newly synthesized membrane protein from a
eukaryotic cell and determines that the protein contains several long
stretches of hydrophobic amino acids, each approximately 20 amino
acids in length. The protein is subsequently demonstrated to span the
plasma membrane multiple times. Which structural feature most directly
permits these regions to remain stable within the hydrophobic core of the
lipid bilayer?
A. Covalent attachment of carbohydrates to the cytosolic domain
B. Exposure of polar amino acid side chains toward the membrane
interior
C. Formation of α-helical transmembrane segments with hydrophobic
side chains facing outward
D. Extensive disulfide bonding between cytosolic cysteine residues
Answer: C. Formation of α-helical transmembrane segments with
hydrophobic side chains facing outward
Rationale: Transmembrane proteins commonly contain α-helices
approximately 20–25 amino acids long, allowing hydrophobic side
chains to interact favorably with the hydrocarbon tails of membrane
phospholipids. The peptide backbone hydrogen bonds stabilize the helix
internally, while hydrophobic side chains face the lipid environment.
Carbohydrates are generally attached to extracellular domains, and
1
,disulfide bonds are favored in the oxidizing extracellular environment
rather than the cytosol.
2. A patient has a genetic defect that markedly reduces the activity of the
Na⁺/K⁺-ATPase in skeletal muscle cells. Which immediate alteration
would most directly result from inhibition of this transporter?
A. Increased intracellular Na⁺ and decreased intracellular K⁺
B. Increased intracellular K⁺ and decreased intracellular Na⁺
C. Increased intracellular Ca²⁺ exclusively because calcium directly
enters through the pump
D. Complete elimination of membrane potential
Answer: A. Increased intracellular Na⁺ and decreased intracellular
K⁺
Rationale: The Na⁺/K⁺-ATPase uses ATP to transport three Na⁺ ions out
of the cell and two K⁺ ions into the cell. Inhibition therefore causes
intracellular Na⁺ to accumulate while intracellular K⁺ decreases. The
resulting disruption of ionic gradients eventually affects membrane
potential, cell volume, and secondary active transport. The pump does
not directly transport calcium.
3. A red blood cell is placed into a solution with substantially lower
effective solute concentration than the cytoplasm. Water rapidly enters
the cell, causing it to swell and eventually rupture. Which process best
explains this observation?
A. Facilitated diffusion of water through glucose transporters
B. Osmosis through the plasma membrane and aquaporins
C. Primary active transport of water into the cell
D. Endocytosis of extracellular fluid
Answer: B. Osmosis through the plasma membrane and aquaporins
2
,Rationale: Osmosis is the net movement of water across a selectively
permeable membrane toward the compartment with greater effective
osmolarity. A hypotonic extracellular solution causes water to enter the
red blood cell. Aquaporins facilitate rapid transmembrane water
movement. This can lead to cellular swelling and hemolysis if the
osmotic gradient is sufficiently large.
4. A mutation prevents a cell from properly assembling its nuclear pore
complexes. Which cellular process would be most directly impaired?
A. β-oxidation of fatty acids
B. Oxidative phosphorylation
C. Exchange of proteins and RNA between nucleus and cytoplasm
D. Synthesis of membrane phospholipids in the smooth endoplasmic
reticulum
Answer: C. Exchange of proteins and RNA between nucleus and
cytoplasm
Rationale: Nuclear pore complexes regulate transport between the
nucleus and cytoplasm. Small molecules can pass relatively freely,
whereas larger proteins and RNAs require regulated transport
mechanisms involving nuclear localization or export signals and
transport receptors. Nuclear pore dysfunction therefore directly
compromises nucleocytoplasmic trafficking.
5. A secreted protein is synthesized by a pancreatic acinar cell. Which
sequence most accurately describes the major intracellular pathway
followed by this protein?
A. Nucleus → lysosome → Golgi apparatus → plasma membrane
B. Rough ER → Golgi apparatus → secretory vesicles → plasma
membrane
3
, C. Smooth ER → mitochondria → lysosome → plasma membrane
D. Golgi apparatus → rough ER → nucleus → secretory vesicle
Answer: B. Rough ER → Golgi apparatus → secretory vesicles →
plasma membrane
Rationale: Proteins destined for secretion are synthesized on ribosomes
attached to the rough endoplasmic reticulum. They enter the ER during
translation, undergo folding and initial modification, and are transported
in vesicles to the Golgi apparatus. Following additional processing and
sorting, they enter secretory vesicles that fuse with the plasma
membrane through exocytosis.
6. A cell produces a mutant protein containing a defective signal peptide
that prevents its normal targeting to the rough endoplasmic reticulum.
Where would this protein most likely be synthesized?
A. On free cytosolic ribosomes
B. Inside the lysosomal lumen
C. Within the Golgi apparatus
D. Inside the mitochondrial intermembrane space
Answer: A. On free cytosolic ribosomes
Rationale: Ribosomes begin translation in the cytosol. A functional ER
signal sequence directs the translating ribosome to the rough ER.
Without appropriate targeting information, the protein remains
synthesized on free ribosomes and generally remains in the cytosol
unless another targeting sequence directs it elsewhere.
7. A lysosomal storage disorder is caused by deficiency of an enzyme
required to degrade a particular sphingolipid. Which cellular
compartment would most directly accumulate the undegraded substrate?
4
