PCB 3023C | PCB3023C Final Exam: Cell Biology
Updated and Latest Questions and Correct
Answers with Rationale - Florida Gulf Coast
University
1. Which component of the plasma membrane is primarily responsible for preventing the
tight packing of phospholipids at low temperatures?
A. Integral membrane proteins
B. Cholesterol
C. Glycoproteins
D. Peripheral proteins
Correct Answer: B
Expert Explanation: Cholesterol acts as a fluidity buffer in animal cell membranes to
maintain structural integrity. At low temperatures, it wedges between phospholipids to
prevent them from crystallizing or packing too tightly. Conversely, at high temperatures, it
helps stabilize the membrane by reducing excessive movement. It is an amphipathic
molecule that aligns with the hydrophobic tails of phospholipids. This dual role is essential
for maintaining membrane functionality across varying environmental conditions.
2. In the sodium-potassium pump (Na+/K+-ATPase) cycle, what event directly triggers the
release of three sodium ions to the extracellular space?
A. Binding of two potassium ions
B. Phosphorylation of the pump by ATP
C. Dephosphorylation of the pump
D. Binding of a new ATP molecule
Correct Answer: B
Expert Explanation: The sodium-potassium pump is a primary active transport
mechanism that moves ions against their concentration gradients. When three sodium ions
bind internally, the pump is phosphorylated by ATP, inducing a conformational change.
This change reduces the affinity for sodium, causing their release into the extracellular
fluid. The pump then gains a high affinity for potassium ions found outside the cell. This
process is vital for maintaining the resting membrane potential in neurons and muscle
cells.
3. During glycolysis, which enzyme is responsible for the ‘committed step’ that is highly
regulated by ATP and citrate levels?
A. Hexokinase
,B. Phosphofructokinase-1 (PFK-1)
C. Pyruvate kinase
D. Aldolase
Correct Answer: B
Expert Explanation: Phosphofructokinase-1 catalyzes the conversion of fructose-6-
phosphate to fructose-1,6-bisphosphate. This step is considered the primary rate-limiting
step of glycolysis because it is irreversible under cellular conditions. High levels of ATP act
as an allosteric inhibitor, signaling that the cell has sufficient energy. Citrate also inhibits
PFK-1, linking the activity of the citric acid cycle to glycolysis rates. This sophisticated
feedback loop ensures that glucose is only metabolized when metabolic demand is high.
4. Which of the following best describes the source of energy that directly powers ATP
synthase in the mitochondria?
A. The hydrolysis of GTP
B. The reduction of NAD+ to NADH
C. The flow of protons down their electrochemical gradient
D. The transfer of electrons between cytochromes
Correct Answer: C
Expert Explanation: ATP synthase functions like a molecular motor driven by the proton
motive force. During the electron transport chain, protons are pumped into the
mitochondrial intermembrane space, creating a steep gradient. As these protons flow back
into the matrix through the Fo subunit, the enzyme rotates. This rotational energy is used
by the F1 subunit to catalyze the phosphorylation of ADP into ATP. This process, known as
chemiosmosis, is the primary method of ATP production in aerobic organisms.
5. A cell biologist treats cells with a drug that prevents the acidification of lysosomes. Which
cellular process would be most directly inhibited?
A. Protein synthesis in the rough ER
B. Lipid synthesis in the smooth ER
C. Exocytosis of secretory vesicles
D. Degradation of macromolecules by acid hydrolases
Correct Answer: D
Expert Explanation: Lysosomes contain acid hydrolases that require a low pH
environment, typically around 4.5 to 5.0, to function effectively. The acidic environment is
maintained by V-type ATPases that pump protons into the lysosomal lumen. If the pH
increases, these enzymes become denatured or inactive, halting the breakdown of cellular
, waste. This disruption would lead to the accumulation of undigested materials, similar to
what is seen in lysosomal storage diseases. Proper acidification is therefore critical for
cellular ‘recycling’ and metabolic homeostasis.
6. Microtubules exhibit ‘dynamic instability.’ Which molecular event is associated with the
transition from growth to shrinkage (catastrophe)?
A. Loss of the GTP cap at the plus end
B. Binding of Taxol to the tubulin dimers
C. Phosphorylation of the alpha-tubulin subunit
D. Increased concentration of free tubulin dimers
Correct Answer: A
Expert Explanation: Dynamic instability allows microtubules to rapidly explore the three-
dimensional space of the cell. Growth occurs as long as GTP-bound tubulin dimers are
added faster than the GTP is hydrolyzed. The ‘GTP cap’ stabilizes the microtubule end and
promotes continued polymerization. If hydrolysis catches up to the tip, the cap is lost, and
the GDP-tubulin structure becomes unstable. This leads to a rapid depolymerization phase
known as catastrophe, where the microtubule shrinks quickly.
7. Where would you expect to find a protein that contains a C-terminal KDEL sequence?
A. Mitochondrial matrix
B. Nucleoplasm
C. Lysosomal lumen
D. Endoplasmic Reticulum (ER) lumen
Correct Answer: D
Expert Explanation: The KDEL sequence (Lys-Asp-Glu-Leu) is a specific retrieval signal
for soluble proteins meant to reside in the ER. If these proteins are accidentally transported
to the Golgi apparatus, KDEL receptors recognize them. The receptors then package the
proteins into COPI-coated vesicles for retrograde transport back to the ER. This mechanism
ensures that ER-resident enzymes, such as chaperones, remain in their correct location.
Without this signal, these essential proteins would be secreted or lost from the cell.
8. Activation of a G-protein coupled receptor (GPCR) by a signaling molecule leads to which
immediate downstream effect?
A. Dimerization of the receptor subunits
B. Exchange of GDP for GTP on the alpha subunit
C. Auto-phosphorylation of the cytoplasmic tail
D. Phosphorylation of the ligand
Updated and Latest Questions and Correct
Answers with Rationale - Florida Gulf Coast
University
1. Which component of the plasma membrane is primarily responsible for preventing the
tight packing of phospholipids at low temperatures?
A. Integral membrane proteins
B. Cholesterol
C. Glycoproteins
D. Peripheral proteins
Correct Answer: B
Expert Explanation: Cholesterol acts as a fluidity buffer in animal cell membranes to
maintain structural integrity. At low temperatures, it wedges between phospholipids to
prevent them from crystallizing or packing too tightly. Conversely, at high temperatures, it
helps stabilize the membrane by reducing excessive movement. It is an amphipathic
molecule that aligns with the hydrophobic tails of phospholipids. This dual role is essential
for maintaining membrane functionality across varying environmental conditions.
2. In the sodium-potassium pump (Na+/K+-ATPase) cycle, what event directly triggers the
release of three sodium ions to the extracellular space?
A. Binding of two potassium ions
B. Phosphorylation of the pump by ATP
C. Dephosphorylation of the pump
D. Binding of a new ATP molecule
Correct Answer: B
Expert Explanation: The sodium-potassium pump is a primary active transport
mechanism that moves ions against their concentration gradients. When three sodium ions
bind internally, the pump is phosphorylated by ATP, inducing a conformational change.
This change reduces the affinity for sodium, causing their release into the extracellular
fluid. The pump then gains a high affinity for potassium ions found outside the cell. This
process is vital for maintaining the resting membrane potential in neurons and muscle
cells.
3. During glycolysis, which enzyme is responsible for the ‘committed step’ that is highly
regulated by ATP and citrate levels?
A. Hexokinase
,B. Phosphofructokinase-1 (PFK-1)
C. Pyruvate kinase
D. Aldolase
Correct Answer: B
Expert Explanation: Phosphofructokinase-1 catalyzes the conversion of fructose-6-
phosphate to fructose-1,6-bisphosphate. This step is considered the primary rate-limiting
step of glycolysis because it is irreversible under cellular conditions. High levels of ATP act
as an allosteric inhibitor, signaling that the cell has sufficient energy. Citrate also inhibits
PFK-1, linking the activity of the citric acid cycle to glycolysis rates. This sophisticated
feedback loop ensures that glucose is only metabolized when metabolic demand is high.
4. Which of the following best describes the source of energy that directly powers ATP
synthase in the mitochondria?
A. The hydrolysis of GTP
B. The reduction of NAD+ to NADH
C. The flow of protons down their electrochemical gradient
D. The transfer of electrons between cytochromes
Correct Answer: C
Expert Explanation: ATP synthase functions like a molecular motor driven by the proton
motive force. During the electron transport chain, protons are pumped into the
mitochondrial intermembrane space, creating a steep gradient. As these protons flow back
into the matrix through the Fo subunit, the enzyme rotates. This rotational energy is used
by the F1 subunit to catalyze the phosphorylation of ADP into ATP. This process, known as
chemiosmosis, is the primary method of ATP production in aerobic organisms.
5. A cell biologist treats cells with a drug that prevents the acidification of lysosomes. Which
cellular process would be most directly inhibited?
A. Protein synthesis in the rough ER
B. Lipid synthesis in the smooth ER
C. Exocytosis of secretory vesicles
D. Degradation of macromolecules by acid hydrolases
Correct Answer: D
Expert Explanation: Lysosomes contain acid hydrolases that require a low pH
environment, typically around 4.5 to 5.0, to function effectively. The acidic environment is
maintained by V-type ATPases that pump protons into the lysosomal lumen. If the pH
increases, these enzymes become denatured or inactive, halting the breakdown of cellular
, waste. This disruption would lead to the accumulation of undigested materials, similar to
what is seen in lysosomal storage diseases. Proper acidification is therefore critical for
cellular ‘recycling’ and metabolic homeostasis.
6. Microtubules exhibit ‘dynamic instability.’ Which molecular event is associated with the
transition from growth to shrinkage (catastrophe)?
A. Loss of the GTP cap at the plus end
B. Binding of Taxol to the tubulin dimers
C. Phosphorylation of the alpha-tubulin subunit
D. Increased concentration of free tubulin dimers
Correct Answer: A
Expert Explanation: Dynamic instability allows microtubules to rapidly explore the three-
dimensional space of the cell. Growth occurs as long as GTP-bound tubulin dimers are
added faster than the GTP is hydrolyzed. The ‘GTP cap’ stabilizes the microtubule end and
promotes continued polymerization. If hydrolysis catches up to the tip, the cap is lost, and
the GDP-tubulin structure becomes unstable. This leads to a rapid depolymerization phase
known as catastrophe, where the microtubule shrinks quickly.
7. Where would you expect to find a protein that contains a C-terminal KDEL sequence?
A. Mitochondrial matrix
B. Nucleoplasm
C. Lysosomal lumen
D. Endoplasmic Reticulum (ER) lumen
Correct Answer: D
Expert Explanation: The KDEL sequence (Lys-Asp-Glu-Leu) is a specific retrieval signal
for soluble proteins meant to reside in the ER. If these proteins are accidentally transported
to the Golgi apparatus, KDEL receptors recognize them. The receptors then package the
proteins into COPI-coated vesicles for retrograde transport back to the ER. This mechanism
ensures that ER-resident enzymes, such as chaperones, remain in their correct location.
Without this signal, these essential proteins would be secreted or lost from the cell.
8. Activation of a G-protein coupled receptor (GPCR) by a signaling molecule leads to which
immediate downstream effect?
A. Dimerization of the receptor subunits
B. Exchange of GDP for GTP on the alpha subunit
C. Auto-phosphorylation of the cytoplasmic tail
D. Phosphorylation of the ligand
