Photosynthesis Reaction Stages 3 — Questions and Answers
Question 1: Which molecule acts as the mobile electron carrier between Photosystem II and the cytochrome b6f complex?
- Plastocyanin
- Ferredoxin
- Plastoquinone (Correct answer)
- NADPH
Correct answer: Plastoquinone
Plastoquinone (PQ) is a lipid-soluble molecule that picks up electrons and protons from PSII and delivers them to the cytochrome b6f complex.
Question 2: In the light-independent reactions, what is the immediate product after CO2 is fixed by RuBisCO onto RuBP?
- Two molecules of 3-phosphoglycerate (Correct answer)
- One molecule of glucose-6-phosphate
- One molecule of G3P
- Two molecules of NADPH
Correct answer: Two molecules of 3-phosphoglycerate
RuBisCO catalyzes the addition of CO2 to RuBP, forming an unstable 6-carbon intermediate that immediately splits into two 3-phosphoglycerate (3-PGA) molecules.
Question 3: Which wavelengths of light are LEAST effectively absorbed by chlorophyll a?
- Red (650-700 nm)
- Violet (400-430 nm)
- Green (500-560 nm) (Correct answer)
- Blue (430-480 nm)
Correct answer: Green (500-560 nm)
Chlorophyll a reflects green wavelengths, making it the least efficient at absorbing green light and giving plants their green color.
Question 4: What structural feature of the chloroplast maximizes the surface area for the light reactions?
- The outer membrane
- The stroma lamellae
- The grana stacks (Correct answer)
- The intermembrane space
Correct answer: The grana stacks
Grana are stacks of thylakoid discs that greatly increase membrane surface area, packing more photosystems into a small volume.
Question 5: How many ATPs and NADPHs does the Calvin cycle require to reduce three CO2 molecules to one G3P?
- 3 ATP and 2 NADPH
- 6 ATP and 6 NADPH
- 9 ATP and 6 NADPH (Correct answer)
- 6 ATP and 3 NADPH
Correct answer: 9 ATP and 6 NADPH
Three turns of the Calvin cycle (fixing 3 CO2) require 9 ATP and 6 NADPH to produce one net G3P molecule.
Question 6: What is photorespiration, and which enzyme is responsible for it?
- CO2 fixation by RuBisCO in low light conditions
- O2 fixation by RuBisCO competing with CO2 fixation (Correct answer)
- Respiration that only occurs in the light using photosystems
- ATP synthesis driven by light in the absence of CO2
Correct answer: O2 fixation by RuBisCO competing with CO2 fixation
Photorespiration occurs when RuBisCO binds O2 instead of CO2, producing phosphoglycolate and wasting energy.
Question 7: In which region of the chloroplast does the Calvin cycle take place?
- Thylakoid lumen
- Thylakoid membrane
- Stroma (Correct answer)
- Outer membrane
Correct answer: Stroma
The Calvin cycle enzymes, including RuBisCO, are dissolved in the stroma, the fluid-filled space surrounding the thylakoids.
Photosynthesis Reaction Stages 3
Which molecule acts as the mobile electron carrier between Photosystem II and the cytochrome b6f complex?