Kingdom Monera: Bacteria
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Direct answer
Monera comprises all prokaryotes — bacteria, cyanobacteria and mycoplasma — organisms without a true nucleus or membrane-bound organelles. Bacteria are the most abundant microorganisms and occur in four basic shapes: coccus (spherical), bacillus (rod-shaped), vibrio (comma-shaped) and spirillum (spiral). Archaebacteria survive the harshest habitats as methanogens, halophiles and thermoacidophiles, while eubacteria include photosynthetic cyanobacteria such as Nostoc and Anabaena that fix nitrogen, and mycoplasma, the smallest living cells known, which completely lack a cell wall.
What you must remember
- Shapes: coccus, bacillus, vibrio and spirillum; many bacteria possess flagella for motility.
- Archaebacteria: methanogens live in marshes and the rumen of cattle and produce methane (biogas); halophiles inhabit salty areas; thermoacidophiles thrive in hot sulphur springs.
- Photosynthetic autotrophic eubacteria: cyanobacteria (blue-green algae) such as Nostoc and Anabaena have chlorophyll a comparable to plants and many fix nitrogen in specialised cells called heterocysts.
- Chemoautotrophic bacteria oxidise inorganic chemicals such as nitrates, nitrites and ammonia to release energy; heterotrophic bacteria are mostly decomposers.
- Mycoplasma: organisms without a cell wall, the smallest living cells known, unaffected by many antibiotics that target wall synthesis.
- Reproduction: mainly by fission; under unfavourable conditions some produce spores, and genetic recombination occurs through transformation, transduction and conjugation.
- Uses: enormous importance in nitrogen fixation, antibiotic production (Streptomyces), and as decomposers recycling nutrients.
Common confusion
The classic exam trap is treating all Monerans as one uniform group. Archaebacteria differ from eubacteria in cell-wall structure and membrane lipids and are the most ancient; they live where nothing else survives — hot springs (thermoacidophiles), brine (halophiles) and anaerobic marshes and animal guts (methanogens). A second frequent error is calling mycoplasma a virus: mycoplasma are living cells that can survive without oxygen, lack only a wall, whereas viruses are inert outside a host.
Exam-focused takeaway
NEET draws direct MCQs on bacterial shapes, on matching archaebacterial groups with habitats, and on the examples Nostoc and Anabaena for nitrogen-fixing cyanobacteria with heterocysts. Statement questions test that Streptomyces yields antibiotics and that methanogens in the rumen generate methane used as biogas. Assertion-reason items probe why mycoplasma escape cell-wall-acting antibiotics. Learn habitat-example pairs verbatim, because the exam almost never goes beyond NCERT wording here.
Frequently asked questions
What are the four shapes of bacteria?
Coccus (spherical), bacillus (rod), vibrio (comma) and spirillum (spiral); this morphological classification is the most quoted in NEET.
Where are methanogens found and what do they produce?
Methanogens live in anaerobic environments such as marshes and the rumen of cattle and produce methane gas, the basis of biogas.
Which Monerans fix atmospheric nitrogen?
Cyanobacteria such as Nostoc and Anabaena fix nitrogen, with fixation concentrated in specialised thick-walled cells called heterocysts.
Why is mycoplasma unique among bacteria?
Mycoplasma completely lack a cell wall, are the smallest living cells known, and can survive without oxygen.
How do bacteria reproduce?
Bacteria reproduce mainly by fission; some form spores under stress, and genetic exchange occurs by transformation, transduction and conjugation.