BSC 108 Exam 3 Updated Questions And Correct Answers
Table of Contents
Section I: Cellular Respiration & Energy Metabolism ........................................ 4
Section II: Photosynthesis & Light Energy Conversion ........................................ 6
Section III: Cell Division & Genetic Continuity ........................................ 8
Section IV: Mendelian Genetics & Inheritance Patterns ........................................ 10
Section V: DNA Structure, Replication & Gene Expression ........................................ 12
Section VI: Mutations & Genetic Variation ........................................ 14
Section VII: Scenario-Based Application ........................................ 16
Answer Key ........................................ 17
,Q1. During glycolysis, glucose is split into two molecules of pyruvate. What is the net
ATP yield per glucose molecule during this cytoplasmic pathway?
A. 0 ATP (all ATP produced is consumed in the investment phase)
B. 2 ATP (4 produced minus 2 consumed in the investment phase)
C. 4 ATP (gross yield with no consumption)
D. 36 ATP (total aerobic yield from one glucose)
Correct Answer: B. 2 ATP (4 produced minus 2 consumed in the investment phase)
Rationale: Glycolysis uses 2 ATP in the investment phase (hexokinase and phosphofructokinase
steps) and produces 4 ATP in the payoff phase, resulting in a net gain of 2 ATP per glucose molecule.
Option A is incorrect because the payoff phase does yield a net positive. Option C describes the gross
yield, ignoring the investment phase. Option D refers to the total aerobic yield, not glycolysis alone.
Q2. Which of the following correctly describes the role of NAD+ in glycolysis?
A. NAD+ is reduced to NADH when glyceraldehyde-3-phosphate is oxidized, capturing
highenergy electrons
B. NAD+ is oxidized to NADH during substrate-level phosphorylation
C. NAD+ donates electrons to the electron transport chain directly from glycolysis
D. NAD+ is synthesized from NADH during the energy investment phase
Correct Answer: A. NAD+ is reduced to NADH when glyceraldehyde-3-phosphate is
oxidized, capturing high-energy electrons
Rationale: In the glyceraldehyde-3-phosphate dehydrogenase step, NAD+ acts as an oxidizing
agent and is reduced to NADH, capturing electrons released when G3P is oxidized to
1,3bisphosphoglycerate. Option B is incorrect because NAD+ is reduced, not oxidized, and this occurs
during an oxidation-reduction step, not substrate-level phosphorylation. Option C is wrong because
NADH from glycolysis must first be shuttled into the mitochondria. Option D is incorrect because
NADH is produced from NAD+, not the reverse.
Q3. During pyruvate oxidation, each pyruvate molecule is converted to acetyl-CoA in the
mitochondrial matrix. Which enzyme catalyzes this reaction, and what byproduct is
released?
A. Pyruvate kinase; H2O is released
B. Pyruvate dehydrogenase complex; CO2 is released
C. Citrate synthase; ATP is released
D. Lactate dehydrogenase; NADH is released
, Correct Answer: B. Pyruvate dehydrogenase complex; CO2 is released
Rationale: The pyruvate dehydrogenase complex (PDC) catalyzes the oxidative decarboxylation of
pyruvate, producing acetyl-CoA, NADH, and CO2. Option A is incorrect because pyruvate kinase
functions in glycolysis, not pyruvate oxidation, and no water is released. Option C is wrong because
citrate synthase acts in the citric acid cycle, and no ATP is produced during pyruvate oxidation.
Option D is incorrect because lactate dehydrogenase functions in lactic acid fermentation, not
pyruvate oxidation, and NADH is a product, not a byproduct released.
Q4. In the citric acid cycle, substrate-level phosphorylation directly produces GTP (or
ATP). Which step of the cycle is responsible for this energy harvest?
A. Isocitrate to alpha-ketoglutarate (catalyzed by isocitrate dehydrogenase)
B. Succinyl-CoA to succinate (catalyzed by succinyl-CoA synthetase)
C. Succinate to fumarate (catalyzed by succinate dehydrogenase) D. Malate to oxaloacetate
(catalyzed by malate dehydrogenase)
Correct Answer: B. Succinyl-CoA to succinate (catalyzed by succinyl-CoA synthetase)
Rationale: Succinyl-CoA synthetase catalyzes the conversion of succinyl-CoA to succinate, coupling
the released energy to phosphorylate GDP to GTP (or ADP to ATP in some organisms). This is the
only substrate-level phosphorylation step in the citric acid cycle. Options A, C, and D involve redox
reactions that produce NADH or FADH2, not direct substrate-level phosphorylation of GTP/ATP.
Q5. The electron transport chain (ETC) in the inner mitochondrial membrane
establishes a proton gradient through chemiosmosis. Which of the following best
describes how ATP synthase utilizes this gradient?
A. Protons flow through the F0 subunit from the matrix to the intermembrane space,
driving rotation that catalyzes ADP phosphorylation in the F1 subunit
B. Protons flow through the F0 subunit from the intermembrane space into the matrix,
driving rotation of the gamma subunit that catalyzes ADP + Pi → ATP in the F1 subunit
C. ATP synthase pumps protons from the matrix to the intermembrane space using
energy from ATP hydrolysis
D. Protons diffuse passively across the inner membrane without ATP synthase
involvement, and ATP is produced by substrate-level phosphorylation
Correct Answer: B. Protons flow through the F0 subunit from the intermembrane space
into the matrix, driving rotation of the gamma subunit that catalyzes ADP + Pi → ATP in
the F1 subunit
Rationale: ATP synthase uses the exergonic flow of protons (H+) from the intermembrane space
(high H+ concentration) back into the matrix (low H+ concentration) through the F0 channel. This
proton flow drives rotation of the gamma subunit, which causes conformational changes in the F1
catalytic subunit, catalyzing the phosphorylation of ADP to ATP. Option A reverses the direction of
Table of Contents
Section I: Cellular Respiration & Energy Metabolism ........................................ 4
Section II: Photosynthesis & Light Energy Conversion ........................................ 6
Section III: Cell Division & Genetic Continuity ........................................ 8
Section IV: Mendelian Genetics & Inheritance Patterns ........................................ 10
Section V: DNA Structure, Replication & Gene Expression ........................................ 12
Section VI: Mutations & Genetic Variation ........................................ 14
Section VII: Scenario-Based Application ........................................ 16
Answer Key ........................................ 17
,Q1. During glycolysis, glucose is split into two molecules of pyruvate. What is the net
ATP yield per glucose molecule during this cytoplasmic pathway?
A. 0 ATP (all ATP produced is consumed in the investment phase)
B. 2 ATP (4 produced minus 2 consumed in the investment phase)
C. 4 ATP (gross yield with no consumption)
D. 36 ATP (total aerobic yield from one glucose)
Correct Answer: B. 2 ATP (4 produced minus 2 consumed in the investment phase)
Rationale: Glycolysis uses 2 ATP in the investment phase (hexokinase and phosphofructokinase
steps) and produces 4 ATP in the payoff phase, resulting in a net gain of 2 ATP per glucose molecule.
Option A is incorrect because the payoff phase does yield a net positive. Option C describes the gross
yield, ignoring the investment phase. Option D refers to the total aerobic yield, not glycolysis alone.
Q2. Which of the following correctly describes the role of NAD+ in glycolysis?
A. NAD+ is reduced to NADH when glyceraldehyde-3-phosphate is oxidized, capturing
highenergy electrons
B. NAD+ is oxidized to NADH during substrate-level phosphorylation
C. NAD+ donates electrons to the electron transport chain directly from glycolysis
D. NAD+ is synthesized from NADH during the energy investment phase
Correct Answer: A. NAD+ is reduced to NADH when glyceraldehyde-3-phosphate is
oxidized, capturing high-energy electrons
Rationale: In the glyceraldehyde-3-phosphate dehydrogenase step, NAD+ acts as an oxidizing
agent and is reduced to NADH, capturing electrons released when G3P is oxidized to
1,3bisphosphoglycerate. Option B is incorrect because NAD+ is reduced, not oxidized, and this occurs
during an oxidation-reduction step, not substrate-level phosphorylation. Option C is wrong because
NADH from glycolysis must first be shuttled into the mitochondria. Option D is incorrect because
NADH is produced from NAD+, not the reverse.
Q3. During pyruvate oxidation, each pyruvate molecule is converted to acetyl-CoA in the
mitochondrial matrix. Which enzyme catalyzes this reaction, and what byproduct is
released?
A. Pyruvate kinase; H2O is released
B. Pyruvate dehydrogenase complex; CO2 is released
C. Citrate synthase; ATP is released
D. Lactate dehydrogenase; NADH is released
, Correct Answer: B. Pyruvate dehydrogenase complex; CO2 is released
Rationale: The pyruvate dehydrogenase complex (PDC) catalyzes the oxidative decarboxylation of
pyruvate, producing acetyl-CoA, NADH, and CO2. Option A is incorrect because pyruvate kinase
functions in glycolysis, not pyruvate oxidation, and no water is released. Option C is wrong because
citrate synthase acts in the citric acid cycle, and no ATP is produced during pyruvate oxidation.
Option D is incorrect because lactate dehydrogenase functions in lactic acid fermentation, not
pyruvate oxidation, and NADH is a product, not a byproduct released.
Q4. In the citric acid cycle, substrate-level phosphorylation directly produces GTP (or
ATP). Which step of the cycle is responsible for this energy harvest?
A. Isocitrate to alpha-ketoglutarate (catalyzed by isocitrate dehydrogenase)
B. Succinyl-CoA to succinate (catalyzed by succinyl-CoA synthetase)
C. Succinate to fumarate (catalyzed by succinate dehydrogenase) D. Malate to oxaloacetate
(catalyzed by malate dehydrogenase)
Correct Answer: B. Succinyl-CoA to succinate (catalyzed by succinyl-CoA synthetase)
Rationale: Succinyl-CoA synthetase catalyzes the conversion of succinyl-CoA to succinate, coupling
the released energy to phosphorylate GDP to GTP (or ADP to ATP in some organisms). This is the
only substrate-level phosphorylation step in the citric acid cycle. Options A, C, and D involve redox
reactions that produce NADH or FADH2, not direct substrate-level phosphorylation of GTP/ATP.
Q5. The electron transport chain (ETC) in the inner mitochondrial membrane
establishes a proton gradient through chemiosmosis. Which of the following best
describes how ATP synthase utilizes this gradient?
A. Protons flow through the F0 subunit from the matrix to the intermembrane space,
driving rotation that catalyzes ADP phosphorylation in the F1 subunit
B. Protons flow through the F0 subunit from the intermembrane space into the matrix,
driving rotation of the gamma subunit that catalyzes ADP + Pi → ATP in the F1 subunit
C. ATP synthase pumps protons from the matrix to the intermembrane space using
energy from ATP hydrolysis
D. Protons diffuse passively across the inner membrane without ATP synthase
involvement, and ATP is produced by substrate-level phosphorylation
Correct Answer: B. Protons flow through the F0 subunit from the intermembrane space
into the matrix, driving rotation of the gamma subunit that catalyzes ADP + Pi → ATP in
the F1 subunit
Rationale: ATP synthase uses the exergonic flow of protons (H+) from the intermembrane space
(high H+ concentration) back into the matrix (low H+ concentration) through the F0 channel. This
proton flow drives rotation of the gamma subunit, which causes conformational changes in the F1
catalytic subunit, catalyzing the phosphorylation of ADP to ATP. Option A reverses the direction of
