BIOL 304: MOLECULAR CELL BIOLOGY EXAM
1 – VERSION A QUESTIONS AND ANSWERS
WITH RATIONALES/ GRADED A+/2026
UPDATE /100% PASS
Section I: Multiple Choice (50 Questions)
Select the best Answer for each question.
1. Which of the following bonds is primarily responsible for the base-pairing specificity in DNA, but
is also critical for the tertiary structure of proteins?
a) Peptide bonds
b) Disulfide bridges
c) Hydrogen bonds
d) Hydrophobic interactions
Answer: c) Hydrogen bonds
Rationale: While peptide bonds link amino acids, hydrogen bonds provide the specificity for DNA
base pairing (A-T, G-C). In proteins, hydrogen bonds stabilize alpha-helices and beta-sheets,
dictating secondary structure. Hydrophobic interactions drive folding, but specificity often comes
from hydrogen bonding.
2. A mutation in a protein kinase renders it unable to hydrolyze ATP. Which function is directly
impaired?
a) Substrate binding
b) Phosphate transfer to a target protein
c) Allosteric regulation
d) Protein scaffolding
Answer: b) Phosphate transfer to a target protein
Rationale: Protein kinases catalyze the transfer of the gamma-phosphate from ATP to a
substrate (phosphorylation). If the enzyme cannot hydrolyze (or rather, utilize) ATP, it cannot
perform this transfer. The kinase may still bind the substrate, but the catalytic activity is lost.
, 3. Which of the following is NOT a feature of the fluid mosaic model of membrane structure?
a) Lipids can undergo lateral diffusion.
b) Transmembrane proteins are embedded in the lipid bilayer.
c) Flip-flop (transverse diffusion) occurs rapidly for all lipids.
d) The membrane is asymmetric.
Answer: c) Flip-flop (transverse diffusion) occurs rapidly for all lipids.
Rationale: Flip-flop (moving from one leaflet to the other) is energetically unfavorable for
phospholipids due to the polar head group passing through the hydrophobic core. It is very slow
unless catalyzed by flippases. Lateral diffusion is rapid and a key feature of the fluid mosaic
model.
4. Which organelle is the primary site of oxidative phosphorylation in animal cells?
a) Nucleus
b) Endoplasmic reticulum
c) Golgi apparatus
d) Mitochondrion
Answer: d) Mitochondrion
Rationale: The mitochondrion houses the electron transport chain and ATP synthase in its inner
membrane, which together perform oxidative phosphorylation to generate ATP. The ER is for
protein synthesis/lipid synthesis, and the Golgi is for modification and sorting.
5. The Km of an enzyme is a measure of:
a) The maximum reaction velocity.
b) The affinity of the enzyme for its substrate.
c) The turnover number.
d) The enzyme's inhibition constant.
Answer: b) The affinity of the enzyme for its substrate.
Rationale: Km (Michaelis constant) is the substrate concentration at which the reaction velocity
is half of Vmax. It is inversely related to affinity; a low Km indicates high affinity.
6. Which type of transport requires direct hydrolysis of ATP to move solutes against their
concentration gradient?
a) Facilitated diffusion
b) Secondary active transport
c) Primary active transport
d) Simple diffusion
Answer: c) Primary active transport
Rationale: Primary active transport (e.g., Na+/K+ ATPase) uses ATP directly to pump solutes
against their gradient. Secondary active transport uses the gradient established by primary
transport (e.g., symporters/antiporters) but does not hydrolyze ATP itself.
1 – VERSION A QUESTIONS AND ANSWERS
WITH RATIONALES/ GRADED A+/2026
UPDATE /100% PASS
Section I: Multiple Choice (50 Questions)
Select the best Answer for each question.
1. Which of the following bonds is primarily responsible for the base-pairing specificity in DNA, but
is also critical for the tertiary structure of proteins?
a) Peptide bonds
b) Disulfide bridges
c) Hydrogen bonds
d) Hydrophobic interactions
Answer: c) Hydrogen bonds
Rationale: While peptide bonds link amino acids, hydrogen bonds provide the specificity for DNA
base pairing (A-T, G-C). In proteins, hydrogen bonds stabilize alpha-helices and beta-sheets,
dictating secondary structure. Hydrophobic interactions drive folding, but specificity often comes
from hydrogen bonding.
2. A mutation in a protein kinase renders it unable to hydrolyze ATP. Which function is directly
impaired?
a) Substrate binding
b) Phosphate transfer to a target protein
c) Allosteric regulation
d) Protein scaffolding
Answer: b) Phosphate transfer to a target protein
Rationale: Protein kinases catalyze the transfer of the gamma-phosphate from ATP to a
substrate (phosphorylation). If the enzyme cannot hydrolyze (or rather, utilize) ATP, it cannot
perform this transfer. The kinase may still bind the substrate, but the catalytic activity is lost.
, 3. Which of the following is NOT a feature of the fluid mosaic model of membrane structure?
a) Lipids can undergo lateral diffusion.
b) Transmembrane proteins are embedded in the lipid bilayer.
c) Flip-flop (transverse diffusion) occurs rapidly for all lipids.
d) The membrane is asymmetric.
Answer: c) Flip-flop (transverse diffusion) occurs rapidly for all lipids.
Rationale: Flip-flop (moving from one leaflet to the other) is energetically unfavorable for
phospholipids due to the polar head group passing through the hydrophobic core. It is very slow
unless catalyzed by flippases. Lateral diffusion is rapid and a key feature of the fluid mosaic
model.
4. Which organelle is the primary site of oxidative phosphorylation in animal cells?
a) Nucleus
b) Endoplasmic reticulum
c) Golgi apparatus
d) Mitochondrion
Answer: d) Mitochondrion
Rationale: The mitochondrion houses the electron transport chain and ATP synthase in its inner
membrane, which together perform oxidative phosphorylation to generate ATP. The ER is for
protein synthesis/lipid synthesis, and the Golgi is for modification and sorting.
5. The Km of an enzyme is a measure of:
a) The maximum reaction velocity.
b) The affinity of the enzyme for its substrate.
c) The turnover number.
d) The enzyme's inhibition constant.
Answer: b) The affinity of the enzyme for its substrate.
Rationale: Km (Michaelis constant) is the substrate concentration at which the reaction velocity
is half of Vmax. It is inversely related to affinity; a low Km indicates high affinity.
6. Which type of transport requires direct hydrolysis of ATP to move solutes against their
concentration gradient?
a) Facilitated diffusion
b) Secondary active transport
c) Primary active transport
d) Simple diffusion
Answer: c) Primary active transport
Rationale: Primary active transport (e.g., Na+/K+ ATPase) uses ATP directly to pump solutes
against their gradient. Secondary active transport uses the gradient established by primary
transport (e.g., symporters/antiporters) but does not hydrolyze ATP itself.
