Kingdom Monera (NCERT) - Free MCQ Practice Test with solutions, NEET Biology

A. N₂ fixation is the correct answer.

Cyanobacteria have specialised cells called heterocysts that convert atmospheric N₂ into nitrogenous compounds usable by plants; examples include Nostoc and Anabaena.

The nitrogen fixed by these organisms increases soil fertility and supplies plants with an essential nutrient, which is why they are deliberately used to improve agricultural fields-particularly in wetland crops like paddy.

Although cyanobacteria perform photosynthesis, this trait is not the primary reason for their use in soil improvement.

Algal blooms are typically harmful (they can cause oxygen depletion and ecological imbalance) and are not a beneficial reason for using cyanobacteria in agriculture.

Therefore, option A is the correct choice.

Correct answer: B

Bacteria do not produce gametes. Reproduction in bacteria is normally asexual, and gamete formation does not occur.

Most bacteria reproduce by binary fission, in which a single cell divides into two daughter cells with similar genetic material.

Many bacteria form endospores under unfavourable conditions; endospore formation is a survival strategy seen in genera such as Bacillus and Clostridium.

Bacteria can exchange genetic material by mechanisms such as conjugation, transformation and transduction. These processes involve transfer and recombination of DNA and are described as a primitive or "sort of" sexual reproduction, but they do not involve gamete formation.

Therefore only option B is incorrect; options A, C and D are correct in the intended sense.

The cyanobacteria are unicellular, colonial or filamentous, freshwater/marine or terrestrial algae. The colonies are generally surrounded by gelatinous sheath. They often form blooms in polluted water bodies. Some of these organisms can fix atmospheric nitrogen in specialised cells called heterocysts, e.g., Nostoc and Anabaena. Chemosynthetic autotrophic bacteria oxidise various inorganic substances such as nitrates, nitrites and ammonia and use the released energy for their ATP production. They play a great role in recycling nutrients like nitrogen, phosphorous, iron and sulphur.

Topic in NCERT: Eubacteria

Line in NCERT: "Some of these organisms can fix atmospheric nitrogen in specialised cells called heterocysts, e.g., Nostoc and Anabaena."

To identify the incorrect statement regarding bacteria:

- A: Bacteria are the most abundant microorganisms.
- B: Bacteria live in extreme habitats where very few other life forms can survive.
- C: Bacteria as a group show the most extensive metabolic diversity.
- D: They are obligate intracellular parasites.

The incorrect statement is: D: They are obligate intracellular parasites.

Explanation: Bacteria can be classified as intracellular or extracellular, with many living freely outside cells. While some bacteria can be obligate intracellular parasites, not all bacteria exhibit this behavior.