BIOC 385 METABOLIC
BIOCHEMISTRY FINAL EXAM | 300
PRACTICE QUESTIONS WITH
DETAILED ANSWERS & RATIONALES
| UNIVERSITY OF ARIZONA
Photosynthesis & Light Reactions
1. Why did the Biosphere 2 project have to be interrupted after only a few
months?
a) The humans ran out of food
b) Levels of CO2 rose to dangerous levels
c) The plants stopped producing oxygen
d) The rate of photosynthesis was too high
Answer: b) Levels of CO2 rose to dangerous levels. Rationale: Microbial respiration in
the soil produced CO2 faster than the plants could fix it, leading to unsafe atmospheric
CO2 concentrations.
2. The __________ is driven directly by photons from the sun in the
photosynthesis light reactions.
a) pumping of protons
b) synthesis of ATP
c) transfer of electrons
d) synthesis of glucose
Answer: c) transfer of electrons. Rationale: Photons excite chlorophyll electrons, which
are then transferred through the electron transport chain; proton pumping and ATP
synthesis are downstream consequences.
3. Heme and chlorophyll have similar overall structures. What is common
between the two molecules?
a) Color
b) Hydrophobic tail
c) Polycyclic planar structure
d) Metal ion found at their center
Answer: c) Polycyclic planar structure. Rationale: Both contain a porphyrin ring, a
planar cyclic tetrapyrrole; the central metal differs (Fe in heme, Mg in chlorophyll).
,4. Chlorophyll absorbs light energy efficiently because of the
a) metal ion in the center of the molecule
b) ring structure
c) planar structure
d) alternating double and single bonds
Answer: d) alternating double and single bonds. Rationale: The conjugated system of
alternating double and single bonds allows delocalized π electrons to absorb visible
light.
5. Which of the following is a photosynthetic light-harvesting pigment?
a) FeS cluster
b) NADPH
c) Plastoquinone
d) β-carotene
Answer: d) β-carotene. Rationale: β-carotene is an accessory pigment that absorbs light
in the blue-green region and transfers energy to chlorophyll.
6. Approximately how many light-harvesting chromophores are found per
photosystem enzyme, such as in PSI or PSII?
a) 2
b) About a dozen
c) About 100 to 200
d) About 1000 to 2000
Answer: c) About 100 to 200. Rationale: Each photosystem contains a large antenna
complex of roughly 100–200 pigment molecules that funnel excitation energy to the
reaction center.
7. Resonance energy transfer in photosynthesis refers to the
a) transfer of electrons between photosystems
b) transfer of excitation energy between pigment molecules
c) pumping of protons across the thylakoid membrane
d) splitting of water to release oxygen
Answer: b) transfer of excitation energy between pigment molecules. Rationale:
Resonance energy transfer (FRET) allows excitation energy to hop between antenna
pigments until it reaches the reaction center.
8. Which of the following is an electron transfer molecule in photosynthesis
light reactions?
a) Plastoquinone
b) Photon
c) Paraquat
d) ATP
Answer: a) Plastoquinone. Rationale: Plastoquinone (PQ) is a mobile electron carrier
that shuttles electrons from PSII to the cytochrome b6f complex.
,9. The photosynthetic Z scheme describes the
a) synthesis of glucose from CO2
b) movement of electrons driven by the absorption of light energy
c) production of O2
d) reduction of NADPH
Answer: b) movement of electrons driven by the absorption of light energy. Rationale:
The Z scheme traces the redox potentials of electron carriers from water to NADP+,
shaped by the two photoexcitation events at PSII and PSI.
10. The primary difference between ATP synthesis during the photosynthetic
light reactions and ATP synthesis in the mitochondrial electron transport chain
is the
a) cellular location of the proton motive force
b) enzyme catalyzing the ATP synthesis
c) number of ATP produced per 360° rotation of the F1 subunit of ATP synthase
d) number of NADPH required to produce ATP
Answer: a) cellular location of the proton motive force. Rationale: In chloroplasts the
proton gradient is across the thylakoid membrane (lumen vs. stroma), whereas in
mitochondria it is across the inner mitochondrial membrane (matrix vs. intermembrane
space).
11. In chloroplasts, PSI generally localizes in the __________, whereas PSII localizes
in the __________.
a) thylakoid lumen; thylakoid grana
b) thylakoid lamellae; thylakoid grana
c) stroma; thylakoid lumen
d) thylakoid grana; thylakoid lamellae
Answer: b) thylakoid lamellae; thylakoid grana. Rationale: PSI is enriched in unstacked
lamellae, while PSII is concentrated in stacked grana regions.
12. In what location of the cell does the Calvin cycle occur?
a) Chloroplast grana
b) Thylakoid lumen
c) Chloroplast stroma
d) Cytoplasm
Answer: c) Chloroplast stroma. Rationale: The Calvin cycle enzymes are soluble in the
stroma, where ATP and NADPH from the light reactions are available.
13. The Calvin cycle requires which species from the light reactions?
a) ATP and NADPH
b) Reducing electrons
c) CO2 and ATP
d) NADH and FADH2
, Answer: a) ATP and NADPH. Rationale: ATP provides energy and NADPH provides
reducing power for CO2 fixation.
14. The first stage of the three-stage Calvin cycle is catalyzed by the enzyme
a) rubisco
b) phosphoglycerate kinase
c) glyceraldehyde-3-phosphate dehydrogenase
d) aldolase
Answer: a) rubisco. Rationale: Ribulose-1,5-bisphosphate carboxylase/oxygenase
(rubisco) catalyzes CO2 fixation, the first committed step of the Calvin cycle.
15. The overall balanced reaction for the light reactions of photosynthesis is
a) 2 H2O + 8 photons + 2 NADP+ + 3 ADP + 3 Pi → O2 + 2 NADPH + 3 ATP
b) CO2 + H2O → glucose + O2
c) 2 H2O + 2 NADP+ → O2 + 2 NADPH + 2 H+
d) 6 CO2 + 6 H2O → C6H12O6 + 6 O2
Answer: a) 2 H2O + 8 photons + 2 NADP+ + 3 ADP + 3 Pi → O2 + 2 NADPH + 3 ATP.
Rationale: This equation accounts for the stoichiometry of water splitting, NADP+
reduction, and ATP synthesis driven by 8 photons.
16. The O2 generated from photosynthesis is derived from
a) CO2
b) water molecules that directly reduce PSII
c) glucose
d) bicarbonate
Answer: b) water molecules that directly reduce PSII. Rationale: The oxygen-evolving
complex of PSII splits water, releasing O2 as a byproduct.
17. Paraquat is an effective herbicide that kills plants by disrupting the
photosynthetic electron transport system. It operates by
a) blocking PSII
b) stealing electrons from PSI
c) inhibiting rubisco
d) uncoupling the proton gradient
Answer: b) stealing electrons from PSI. Rationale: Paraquat accepts electrons from PSI,
diverting them from NADP+ reduction and generating toxic superoxide radicals.
18. In C4 and CAM plants, CO2 is first incorporated into __________, which is then
converted to malate.
a) oxaloacetate
b) phosphoenolpyruvate (PEP)
c) ribulose-1,5-bisphosphate
d) pyruvate
BIOCHEMISTRY FINAL EXAM | 300
PRACTICE QUESTIONS WITH
DETAILED ANSWERS & RATIONALES
| UNIVERSITY OF ARIZONA
Photosynthesis & Light Reactions
1. Why did the Biosphere 2 project have to be interrupted after only a few
months?
a) The humans ran out of food
b) Levels of CO2 rose to dangerous levels
c) The plants stopped producing oxygen
d) The rate of photosynthesis was too high
Answer: b) Levels of CO2 rose to dangerous levels. Rationale: Microbial respiration in
the soil produced CO2 faster than the plants could fix it, leading to unsafe atmospheric
CO2 concentrations.
2. The __________ is driven directly by photons from the sun in the
photosynthesis light reactions.
a) pumping of protons
b) synthesis of ATP
c) transfer of electrons
d) synthesis of glucose
Answer: c) transfer of electrons. Rationale: Photons excite chlorophyll electrons, which
are then transferred through the electron transport chain; proton pumping and ATP
synthesis are downstream consequences.
3. Heme and chlorophyll have similar overall structures. What is common
between the two molecules?
a) Color
b) Hydrophobic tail
c) Polycyclic planar structure
d) Metal ion found at their center
Answer: c) Polycyclic planar structure. Rationale: Both contain a porphyrin ring, a
planar cyclic tetrapyrrole; the central metal differs (Fe in heme, Mg in chlorophyll).
,4. Chlorophyll absorbs light energy efficiently because of the
a) metal ion in the center of the molecule
b) ring structure
c) planar structure
d) alternating double and single bonds
Answer: d) alternating double and single bonds. Rationale: The conjugated system of
alternating double and single bonds allows delocalized π electrons to absorb visible
light.
5. Which of the following is a photosynthetic light-harvesting pigment?
a) FeS cluster
b) NADPH
c) Plastoquinone
d) β-carotene
Answer: d) β-carotene. Rationale: β-carotene is an accessory pigment that absorbs light
in the blue-green region and transfers energy to chlorophyll.
6. Approximately how many light-harvesting chromophores are found per
photosystem enzyme, such as in PSI or PSII?
a) 2
b) About a dozen
c) About 100 to 200
d) About 1000 to 2000
Answer: c) About 100 to 200. Rationale: Each photosystem contains a large antenna
complex of roughly 100–200 pigment molecules that funnel excitation energy to the
reaction center.
7. Resonance energy transfer in photosynthesis refers to the
a) transfer of electrons between photosystems
b) transfer of excitation energy between pigment molecules
c) pumping of protons across the thylakoid membrane
d) splitting of water to release oxygen
Answer: b) transfer of excitation energy between pigment molecules. Rationale:
Resonance energy transfer (FRET) allows excitation energy to hop between antenna
pigments until it reaches the reaction center.
8. Which of the following is an electron transfer molecule in photosynthesis
light reactions?
a) Plastoquinone
b) Photon
c) Paraquat
d) ATP
Answer: a) Plastoquinone. Rationale: Plastoquinone (PQ) is a mobile electron carrier
that shuttles electrons from PSII to the cytochrome b6f complex.
,9. The photosynthetic Z scheme describes the
a) synthesis of glucose from CO2
b) movement of electrons driven by the absorption of light energy
c) production of O2
d) reduction of NADPH
Answer: b) movement of electrons driven by the absorption of light energy. Rationale:
The Z scheme traces the redox potentials of electron carriers from water to NADP+,
shaped by the two photoexcitation events at PSII and PSI.
10. The primary difference between ATP synthesis during the photosynthetic
light reactions and ATP synthesis in the mitochondrial electron transport chain
is the
a) cellular location of the proton motive force
b) enzyme catalyzing the ATP synthesis
c) number of ATP produced per 360° rotation of the F1 subunit of ATP synthase
d) number of NADPH required to produce ATP
Answer: a) cellular location of the proton motive force. Rationale: In chloroplasts the
proton gradient is across the thylakoid membrane (lumen vs. stroma), whereas in
mitochondria it is across the inner mitochondrial membrane (matrix vs. intermembrane
space).
11. In chloroplasts, PSI generally localizes in the __________, whereas PSII localizes
in the __________.
a) thylakoid lumen; thylakoid grana
b) thylakoid lamellae; thylakoid grana
c) stroma; thylakoid lumen
d) thylakoid grana; thylakoid lamellae
Answer: b) thylakoid lamellae; thylakoid grana. Rationale: PSI is enriched in unstacked
lamellae, while PSII is concentrated in stacked grana regions.
12. In what location of the cell does the Calvin cycle occur?
a) Chloroplast grana
b) Thylakoid lumen
c) Chloroplast stroma
d) Cytoplasm
Answer: c) Chloroplast stroma. Rationale: The Calvin cycle enzymes are soluble in the
stroma, where ATP and NADPH from the light reactions are available.
13. The Calvin cycle requires which species from the light reactions?
a) ATP and NADPH
b) Reducing electrons
c) CO2 and ATP
d) NADH and FADH2
, Answer: a) ATP and NADPH. Rationale: ATP provides energy and NADPH provides
reducing power for CO2 fixation.
14. The first stage of the three-stage Calvin cycle is catalyzed by the enzyme
a) rubisco
b) phosphoglycerate kinase
c) glyceraldehyde-3-phosphate dehydrogenase
d) aldolase
Answer: a) rubisco. Rationale: Ribulose-1,5-bisphosphate carboxylase/oxygenase
(rubisco) catalyzes CO2 fixation, the first committed step of the Calvin cycle.
15. The overall balanced reaction for the light reactions of photosynthesis is
a) 2 H2O + 8 photons + 2 NADP+ + 3 ADP + 3 Pi → O2 + 2 NADPH + 3 ATP
b) CO2 + H2O → glucose + O2
c) 2 H2O + 2 NADP+ → O2 + 2 NADPH + 2 H+
d) 6 CO2 + 6 H2O → C6H12O6 + 6 O2
Answer: a) 2 H2O + 8 photons + 2 NADP+ + 3 ADP + 3 Pi → O2 + 2 NADPH + 3 ATP.
Rationale: This equation accounts for the stoichiometry of water splitting, NADP+
reduction, and ATP synthesis driven by 8 photons.
16. The O2 generated from photosynthesis is derived from
a) CO2
b) water molecules that directly reduce PSII
c) glucose
d) bicarbonate
Answer: b) water molecules that directly reduce PSII. Rationale: The oxygen-evolving
complex of PSII splits water, releasing O2 as a byproduct.
17. Paraquat is an effective herbicide that kills plants by disrupting the
photosynthetic electron transport system. It operates by
a) blocking PSII
b) stealing electrons from PSI
c) inhibiting rubisco
d) uncoupling the proton gradient
Answer: b) stealing electrons from PSI. Rationale: Paraquat accepts electrons from PSI,
diverting them from NADP+ reduction and generating toxic superoxide radicals.
18. In C4 and CAM plants, CO2 is first incorporated into __________, which is then
converted to malate.
a) oxaloacetate
b) phosphoenolpyruvate (PEP)
c) ribulose-1,5-bisphosphate
d) pyruvate
