Photosynthesis

1. Which of the following accurately describes the events in Photosystem II (PSII) during the light reactions? Select all that apply.

• A. Water is split to replace lost electrons in P680

• B. ATP is directly synthesized at PSII
  

• C. The primary electron acceptor captures an excited electron from chlorophyll a

• D. P680 becomes oxidized after donating an electron

• E. NADPH is produced at PSII

Answer explanation

Explanation:

A. True – Water is split in PSII to replace the electron lost by P680.

B. False – ATP is synthesized downstream via chemiosmosis, not directly at PSII.

C. True – The excited electron is captured by the primary electron acceptor in PSII.

D. True – After donating an electron, P680 becomes P680⁺ (oxidized).

E. False – NADPH is formed later in PSI, not in PSII.

2. Which of the following conditions would most likely increase photorespiration in a C3 plant? Select all that apply.

• A. High light intensity and low CO₂ concentration

• B. Low oxygen levels in the leaf

• C. High oxygen levels and hot, dry conditions

• D. Open stomata during midday

• E. Rubisco binding to O₂ instead of CO₂

Answer explanation

A. True – Low CO₂ and high light can enhance photorespiration.

B. False – Low O₂ reduces photorespiration.

C. True – Hot, dry conditions lead to stomatal closure, reducing CO₂ and increasing O₂.

D. False – Open stomata promote gas exchange and reduce photorespiration.

E. True – Photorespiration begins when rubisco fixes O₂ instead of CO₂.

3. Which components are directly involved in the formation of NADPH during photosynthesis?

Select all that apply.

• A. Photosystem II

• B. Ferredoxin (Fd)

• C. NADP⁺ reductase

• D. Photosystem I

• E. ATP synthase

Answer explanation

A. False – PSII initiates the chain but does not directly contribute to NADPH formation.

B. True – Ferredoxin transfers electrons to NADP⁺ reductase.

C. True – NADP⁺ reductase catalyzes the formation of NADPH.

D. True – PSI provides the electrons used to reduce NADP⁺.

E. False – ATP synthase is involved in ATP, not NADPH production.

4. Which adaptations help C4 and CAM plants minimize photorespiration and conserve water?

Select all that apply.

• A. Temporal separation of CO₂ uptake and fixation in CAM plants

• B. Spatial separation of CO₂ fixation and the Calvin cycle in C4 plants

• C. Use of rubisco in mesophyll cells for initial CO₂ fixation in C4 plants

• D. Storage of CO₂ as malic acid in CAM plants

• E. Initial CO₂ fixation by PEP carboxylase in both CAM and C4 plants

Answer explanation

A. True – CAM plants fix CO₂ at night and run the Calvin cycle during the day.

B. True – C4 plants fix CO₂ in mesophyll cells, Calvin cycle occurs in bundle-sheath cells.

C. False – PEP carboxylase, not rubisco, fixes CO₂ in C4 mesophyll cells.

D. True – CAM plants store CO₂ as malic acid in vacuoles.

E. True – PEP carboxylase initiates CO₂ fixation in both adaptations.

5. During the Calvin cycle, which of the following are true regarding energy and carbon flow?

Select all that apply.

• A. Each CO₂ molecule fixed requires 2 ATP and 2 NADPH

• B. G3P is an intermediate used to form glucose

• C. 6 CO₂ molecules are required to produce one G3P

• D. The regeneration of RuBP requires ATP

• E. The Calvin cycle operates in the thylakoid lumen

Answer explanation

A. False – Each CO₂ requires 3 ATP and 2 NADPH.

B. True – G3P is a key output that can be used to form glucose.

C. False – 3 CO₂ molecules are needed to form one G3P; 6 are needed for one glucose.

D. True – ATP is used in regenerating RuBP.

E. False – The Calvin cycle takes place in the stroma, not the thylakoid lumen.

6. 6. Which of the following correctly describe differences between C3, C4, and CAM photosynthesis?

• A. C3 plants fix CO₂ directly into a 3-carbon compound using rubisco in mesophyll cells.

• B. C4 plants separate carbon fixation and the Calvin cycle into different types of cells.

• C. CAM plants open their stomata during the day to fix CO₂ and minimize water loss.

• D. C4 plants initially fix CO₂ into a 4-carbon compound in bundle-sheath cells.

• E. CAM plants use temporal separation, fixing CO₂ at night and running the Calvin cycle during the day.

Answer explanation

A. True – C3 plants use rubisco to directly fix CO₂ into 3-PGA in mesophyll cells.

B. True – C4 plants use spatial separation: CO₂ is fixed in mesophyll cells and Calvin cycle occurs in bundle-sheath cells.

C. False – CAM plants open stomata at night, not during the day.

D. False – The initial CO₂ fixation in C4 plants occurs in mesophyll cells, not bundle-sheath cells.

E. True – CAM plants perform temporal separation: fix CO₂ at night, Calvin cycle by day.

7. 7. The enzyme responsible for fixing carbon dioxide in the Calvin cycle is called _____________

Answer explanation

Answer:
Rubisco (Ribulose-1,5-bisphosphate carboxylase/oxygenase)

Explanation:
Rubisco catalyzes the first major step of carbon fixation, attaching CO₂ to RuBP to form 3-phosphoglycerate (3-PGA), a three-carbon compound

8. 8. In C₄ plants, CO₂ is initially fixed into a four-carbon compound called __________ in mesophyll cells.

Answer explanation

Answer:
Oxaloacetate

Explanation:
C₄ plants like maize and sugarcane use the enzyme PEP carboxylase to fix CO₂ into oxaloacetate in mesophyll cells. This compound is then shuttled to bundle-sheath cells where CO₂ is released for the Calvin cycle, reducing photorespiration

9. 9. The process by which ATP is produced in the thylakoid membrane during the light reactions is called __________.

Answer explanation

Answer:
Photophosphorylation

Explanation:
Photophosphorylation involves using light energy to pump protons across the thylakoid membrane, creating a gradient that drives ATP synthesis via ATP synthase

10. 10. The splitting of water in Photosystem II releases __________, protons (H⁺), and oxygen.

Answer explanation

Answer:
Electrons

Explanation:
In Photosystem II (PSII), light energy drives the splitting of water molecules—a process known as photolysis. This reaction yields electrons (which replace those lost by chlorophyll), protons (contributing to the proton gradient used in ATP synthesis), and molecular oxygen (O₂), which is released as a byproduct.