Photosynthesis in C4 plants: In the bundle sheath chloroplasts, which photosystem is present at a much lower level than in mesophyll chloroplasts?

Introduction / Context:
C4 photosynthesis separates initial CO2 fixation and the Calvin cycle into mesophyll and bundle sheath cells. The distribution of photosystems between these tissues is specialized to minimize photorespiration and maximize efficiency. Recognizing which photosystem is reduced in bundle sheath chloroplasts reveals how C4 plants tailor light reactions for carbon concentration mechanisms.

Given Data / Assumptions:

• C4 anatomy includes mesophyll and bundle sheath cells.
• Bundle sheath chloroplasts typically exhibit reduced grana.
• C4 pathway aims to concentrate CO2 around Rubisco in bundle sheath cells.

Concept / Approach:
Grana stacking is associated with PSII-rich thylakoids and linear electron flow. Bundle sheath chloroplasts in many C4 species are agranal or have reduced grana, indicating reduced PSII activity. This shift favors cyclic electron flow around PSI to generate ATP needed for C4 metabolism without excessive oxygen evolution in the bundle sheath compartment.

Step-by-Step Solution:

Verification / Alternative check:
Anatomical and biochemical studies in NADP-ME type C4 plants document agranal chloroplasts and diminished PSII complexes in the bundle sheath relative to mesophyll, aligning with the carbon concentrating strategy.

Why Other Options Are Wrong:

• PSI: generally maintained for cyclic flow; not the one specifically reduced.
• PSI or PSII depending on species: while variations exist, the classic and tested concept is reduced PSII.
• Both equal: contradicts observed differentiation.
• Neither: bundle sheath chloroplasts clearly retain photosynthetic capacity.

Common Pitfalls:
Confusing C3 and C4 chloroplast structure or assuming uniform photosystem distribution across all leaf tissues.

Final Answer:
PSII (Photosystem II)