Quiz: Photosynthesis — 18 questions

1. What is the primary purpose of photosynthesis?

A. To produce ATP by transferring electrons to oxygen

B. To convert light energy into chemical energy stored in sugars

C. To convert chemical energy in sugars into heat energy

D. To break down glucose into carbon dioxide and water

Answer: (B)

Explanation

Photosynthesis synthesizes sugars from carbon dioxide, water, and light, converting light energy into chemical energy. The Calvin cycle uses energy from the light reactions to fix carbon.

2. Which equation correctly represents the overall reaction of photosynthesis?

A. C₆H₁₂O₆ + 6H₂O → 6CO₂ + 6O₂

B. 6CO₂ + 6O₂ → C₆H₁₂O₆ + 6H₂O

C. 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂

D. C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O

Answer: (C)

Explanation

The overall photosynthesis reaction uses carbon dioxide and water as reactants and produces glucose and oxygen. The reverse equation represents cellular respiration.

3. A plant cell has generated ATP and NADPH using light energy. What role do these molecules play next in photosynthesis?

A. They are converted directly into carbon dioxide and water

B. They are released as waste products from the chloroplast

C. They absorb light in place of chlorophyll during the light reactions

D. They are used by the Calvin cycle to fix carbon and produce sugar

Answer: (D)

Explanation

The light reactions produce ATP and NADPH, which the Calvin cycle uses to make sugar by fixing carbon. They are not waste products or replacements for light-absorbing pigments.

4. Where does photosynthesis take place in a plant cell?

A. In the cytoplasm, outside membrane-bound organelles

B. In the mitochondrion, an organelle surrounded by one membrane

C. In the nucleus, where genetic information is stored

D. In the chloroplast, an organelle surrounded by two membranes

Answer: (D)

Explanation

Photosynthesis takes place in chloroplasts, which are organelles with two membranes. Mitochondria are primarily associated with cellular respiration.

5. What are grana within a chloroplast?

A. Stacks of thylakoids

B. Stacks of stromal fluid

C. Layers of chloroplast membranes

D. Clusters of carbon-fixing enzymes

Answer: (A)

Explanation

Grana are stacks of thylakoids within the chloroplast. The stroma is the surrounding fluid rather than a stack of membranes.

6. Which sequence correctly lists the three phases of the Calvin cycle?

A. Fixation, reduction, and regeneration

B. Absorption, transport, and oxidation

C. Excitation, electron transfer, and photolysis

D. Glycolysis, fermentation, and respiration

Answer: (A)

Explanation

The Calvin cycle proceeds through carbon fixation, reduction, and regeneration. The other sequences describe different processes or aspects of the light reactions.

7. What happens during carbon fixation in the Calvin cycle?

A. G3P accepts CO₂, leading to the formation of RuBP

B. RuMP is converted into glucose before carbon dioxide enters

C. 3PG is reduced to G3P using light directly

D. RuBP accepts CO₂, leading to the formation of 3PG

Answer: (D)

Explanation

During fixation, RuBP accepts carbon dioxide and 3PG is formed. G3P is produced later during the reduction phase.

8. Which conversion occurs during the reduction phase of the Calvin cycle?

A. RuMP is reduced to RuBP

B. G3P is reduced to 3PG

C. RuBP is reduced to carbon dioxide

D. 3PG is reduced to G3P

Answer: (D)

Explanation

During reduction, 3PG is converted into G3P. Regeneration instead involves restoring RuBP from RuMP.

9. What role does Rubisco play during carbon fixation?

A. It splits water to release oxygen and hydrogen

B. It transfers electrons from chlorophyll to NADPH

C. It removes carbon from CO2 and attaches it to another molecule

D. It removes oxygen from CO2 and attaches it to another molecule

Answer: (C)

Explanation

Rubisco fixes carbon by removing carbon from CO2 and connecting it to another molecule. It does not fix oxygen by removing oxygen from CO2.

10. Which statement correctly describes Rubisco’s binding properties?

A. It has two active sites and a tenfold greater affinity for O2 than for CO2

B. It has eight active sites and equal affinity for CO2 and O2

C. It has eight active sites and a tenfold greater affinity for CO2 than for O2

D. It has ten active sites and a tenfold greater affinity for O2 than for CO2

Answer: (C)

Explanation

Rubisco has eight active sites and binds CO2 with an affinity ten times greater than its affinity for O2.

11. What distinguishes the antenna from the reaction center in a photosystem?

A. The antenna splits water, while the reaction center produces chlorophyll a

B. The antenna produces ATP, while the reaction center produces carbon dioxide

C. The antenna collects light energy, while the reaction center transfers energy to an electron acceptor

D. The antenna transfers energy to an electron acceptor, while the reaction center collects light energy

Answer: (C)

Explanation

Pigments in the antenna collect light and direct its energy to the reaction center, where energy is transferred to an electron acceptor.

12. How many kinds of photosystems do plants have, and what are they called?

A. Two: Photosystem II and Photosystem I

B. Two: Photosystem A and Photosystem B

C. Three: Photosystems A, B, and C

D. Three: Photosystems I, II, and III

Answer: (A)

Explanation

Plants contain two kinds of photosystems: Photosystem II and Photosystem I.

13. How does electron transport contribute to ATP production in the light reactions?

A. It establishes a proton gradient that ATP synthase uses to produce ATP

B. It directly joins ADP and phosphate without requiring a gradient

C. It transfers carbon dioxide into the Calvin cycle to produce ATP

D. It splits water, and the released oxygen is converted directly into ATP

Answer: (A)

Explanation

Light excites electrons, and their transport establishes a proton gradient. ATP synthase uses that gradient to produce ATP.

14. Which pairing correctly identifies the products associated with Photosystems II and I?

A. Photosystem II produces glucose, while Photosystem I produces oxygen

B. Photosystem II produces NADPH, while Photosystem I produces ATP

C. Photosystem II produces ATP, while Photosystem I produces NADPH

D. Photosystem II produces oxygen, while Photosystem I produces carbon dioxide

Answer: (C)

Explanation

Photosystem II is associated with ATP production, whereas Photosystem I produces NADPH used in the Calvin cycle.

15. What replaces the electron lost by chlorophyll a in Photosystem II?

A. An electron transferred from NADPH

B. An electron released by ATP synthase

C. An electron obtained by splitting water

D. An electron obtained by splitting carbon dioxide

Answer: (C)

Explanation

After Photosystem II chlorophyll a loses an electron, it receives replacement electrons from water that has been split.

16. What is the source of the oxygen released during the light reactions?

A. The reduction of NADPH molecules

B. The oxidation of glucose molecules

C. The breakdown of carbon dioxide molecules

D. The splitting of water molecules

Answer: (D)

Explanation

The oxygen we breathe is produced when water is split during the light reactions.

17. What consequence of Rubisco’s competition between O2 and CO2 is particularly relevant to C4 and CAM plants?

A. They have adaptations related to managing photorespiration caused by O2 competing with CO2

B. They use entirely different enzymes instead of Rubisco to fix carbon dioxide

C. They represent separate phases within the Calvin cycle

D. They avoid the need for carbon dioxide during photosynthesis

Answer: (A)

Explanation

O2 and CO2 compete for Rubisco’s binding sites, creating consequences that are especially important for C4 and CAM plants because of their adaptations to this competition. C4 and CAM plants are not separate phases of the Calvin cycle.

18. A student claims that C4 and CAM plants are two distinct phases of the Calvin cycle because both are associated with Rubisco’s binding of O2 and CO2. What is the best correction?

A. C4 and CAM plants are Calvin-cycle phases that occur after Rubisco binds oxygen

B. C4 and CAM plants are forms of oxygen competition that prevent Rubisco from binding carbon dioxide

C. C4 and CAM plants are enzymes that replace Rubisco during carbon fixation

D. C4 and CAM plants are plant types associated with consequences of Rubisco’s O2–CO2 competition

Answer: (D)

Explanation

C4 and CAM plants are linked to the consequences of competition between O2 and CO2 for Rubisco binding sites. They should not be classified as separate phases of the Calvin cycle.