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Difficulty: MediumKingdom Monera: Bacteria and Cyanobacteria

A microscopic examination of a filamentous cyanobacterium reveals specialized, thick-walled cells called heterocysts distributed at regular intervals along the filament. What is the primary metabolic function performed by these specialized cells?

  1. Fixation of atmospheric nitrogen into ammonia under anaerobic conditionsAnswer
  2. B
    Oxygenic photosynthesis occurring inside membrane-bound chloroplasts
  3. C
    Secretion of digestive enzymes for saprophytic organic absorption
  4. D
    Conversion of soil nitrates into free nitrogen gas during denitrification

Answer

Fixation of atmospheric nitrogen into ammonia under anaerobic conditions
The correct answer accurately identifies the role of heterocysts in fixing atmospheric nitrogen. The enzyme responsible for nitrogen fixation, nitrogenase, is inactivated by oxygen gas. Heterocysts adapt to this constraint by forming thick walls impermeable to oxygen, inactivating oxygen-producing Photosystem II, and relying on adjacent vegetative cells for photosynthetic carbohydrates.

Step-by-Step Solution

1
Identify the biological nature and group of cyanobacteria
Cyanobacteria are photosynthetic prokaryotes belonging to Kingdom Monera.
Understanding prokaryotic cellular architecture helps distinguish moneran structures from eukaryotic organelles.
2
Analyze the structural adaptations of heterocysts
Heterocysts develop thick cell walls and lack Photosystem II.
The enzyme nitrogenase, which catalyzes nitrogen fixation, is irreversibly inactivated by molecular oxygen (O2O_2).
3
Deduce the primary function of heterocysts
Heterocysts fix atmospheric nitrogen gas (N2N_2) into ammonia (NH3NH_3).
By spatially separating nitrogen fixation in heterocysts from oxygenic photosynthesis in vegetative cells, cyanobacteria perform both processes effectively.

Key Concept

Specialization of heterocysts for nitrogen fixation in Cyanobacteria
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Topics

Kingdom Monera: Bacteria and Cyanobacteria

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