Answer: Heterocysts are nitrogen-fixing cells with thick walls that exclude O 2 , protecting the oxygen-sensitive nitrogenase enzyme.
- A Heterocysts are specialized photosynthetic cells that primarily function to generate extra oxygen for the surrounding filament
- B Heterocysts are nitrogen-fixing cells with thick walls that exclude O<sub>2</sub>, protecting the oxygen-sensitive nitrogenase enzyme
- C Heterocysts store starch reserves specifically to sustain the cyanobacterial organism through periods of drought stress
- D Heterocysts secrete specific growth-regulating hormones that control the overall growth rate of the cyanobacterial filament
Correct answer: B. Heterocysts are nitrogen-fixing cells with thick walls that exclude O<sub>2</sub>, protecting the oxygen-sensitive nitrogenase enzyme
Explanation: Heterocysts are thick-walled, differentiated cells in cyanobacteria that maintain an anaerobic microenvironment essential for nitrogen fixation, since the enzyme nitrogenase is irreversibly inactivated by oxygen.
Sewage passes through physical removal (primary), then aerobic biological treatment where microbes form flocs that consume organic matter (secondary); the settled activated sludge is then digested anaerobically to produce biogas - a deliberate switch from aerobic to anaerobic conditions between stages.
Concept context
Microorganisms in food, medicine, sewage treatment, biocontrol, and biofertilizers