Q.69 The site of photosynthesis in cyanobacteria is ____________.
(A) Chloroplast
(B) Chromatophores
(C) Thylakoids
(D) Chlorosomes
The site of photosynthesis in cyanobacteria is thylakoids.
This holds true for competitive exams like CSIR NET Life Sciences, where precise terminology distinguishes prokaryotic photosynthesis from eukaryotic processes. Thylakoids host the light-dependent reactions in cyanobacteria, unlike plant chloroplasts.
Option Analysis
Cyanobacteria, prokaryotic oxygenic phototrophs, lack membrane-bound organelles, so photosynthesis occurs in specialized internal membranes.
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(A) Chloroplast: Incorrect. Chloroplasts are double-membrane organelles exclusive to eukaryotic algae and plants, containing stacked thylakoids (grana). Cyanobacteria, being prokaryotes, do not possess chloroplasts.
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(B) Chromatophores: Partially misleading. Chromatophores refer to invaginated plasma membrane systems with pigments in photosynthetic prokaryotes, often used broadly for cyanobacterial thylakoids in older texts. However, in cyanobacteria, these are specifically organized as thylakoids, not vesicular like in purple bacteria.
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(C) Thylakoids: Correct. Thylakoid membranes, distinct from the plasma membrane, contain photosystems I and II, ATP synthase, and phycobilisomes for light harvesting. They drive oxygenic photosynthesis by hosting electron transport chains.
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(D) Chlorosomes: Incorrect. Chlorosomes are antenna complexes in green sulfur bacteria (anoxygenic photosynthesis), aggregating bacteriochlorophylls for low-light efficiency. Absent in cyanobacteria.
The site of photosynthesis in cyanobacteria refers to their unique thylakoid membranes, enabling oxygenic photosynthesis without chloroplasts. As prokaryotes pivotal to Earth’s oxygenation event, cyanobacteria power global primary production through these structures.
Thylakoids: Core Photosynthetic Site
Thylakoids form extensive, unstacked membrane networks in the cyanobacterial cytoplasm, separate from the plasma membrane. They embed photosystems I (PSI) and II (PSII), cytochrome b6f, and ATP synthase for light-driven ATP/NADPH production. Phycobilisomes attach externally, harvesting 450-660 nm light for energy transfer.
Why Not Other Structures?
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Chloroplasts: Eukaryotic innovation from cyanobacterial endosymbiosis; absent in free-living cyanobacteria.
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Chromatophores: Term for pigment vesicles in anoxygenic bacteria (e.g., Rhodobacter); in cyanobacteria, synonymous with thylakoids but less precise for exams.
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Chlorosomes: Green bacteria’s self-assembled antennae for anoxygenic, low-light photosynthesis; irrelevant to oxygenic cyanobacterial processes.
CSIR NET Relevance
Questions like “The site of photosynthesis in cyanobacteria is ________” test thylakoid specificity. Sources confirm thylakoids as the functional locus, with chromatophore usage varying by context but thylakoids standard in advanced biology.
Primary Key Phrase: site of photosynthesis in cyanobacteria
This phrase targets CSIR NET searches, emphasizing thylakoids’ role in molecular biology and biotechnology units.
