Kingdom Monera — Bacteria & Archaea

NCERT grounding

NCERT Class 11 Biology, Chapter 2 (Biological Classification), opens the five-kingdom survey with §2.1 Kingdom Monera. It states plainly that "Bacteria are the sole members of the Kingdom Monera" and that they are "the most abundant micro-organisms," occurring everywhere from hot springs and deserts to snow and deep oceans. The chapter groups bacteria into four shapes, splits the kingdom into archaebacteria (§2.1.1) and eubacteria (§2.1.2), and closes with the wall-less Mycoplasma. NIOS Chapter 2 adds that Monera is the only prokaryotic kingdom and the most primitive, with bacteria the first cellular life around 3.5 billion years ago.

"Compared to many other organisms, bacteria as a group show the most extensive metabolic diversity."

NCERT Class 11 · Biological Classification §2.1

The prokaryotic cell & bacterial shapes

What unifies every moneran is the prokaryotic grade of organisation. The genetic material is a single circular chromosome of double-helical DNA lying free in a cytoplasmic region called the nucleoid — there is no nuclear membrane, so the cell is "pro-karyote" (before-nucleus). Around the cytoplasm sits a plasma membrane of lipids and proteins, and outside that a rigid cell wall. NCERT records the moneran wall as non-cellulosic, built of polysaccharide plus amino acid; NIOS names the unique compound peptidoglycan. The cell carries only 70S ribosomes and no membrane-bound organelles — no mitochondria, no chloroplast, no endoplasmic reticulum or Golgi.

Several accessory features matter for NEET. Many bacteria carry one or more plasmids — small extra rings of DNA bearing antibiotic-resistance genes and acting as the sex (F) factor. Inward folds of the plasma membrane called mesosomes host cellular respiration. Short, thin pili project from the wall and hold cells together during conjugation, while longer, thicker flagella — made of a single coiled protein, flagellin — drive movement and differ structurally from eukaryotic flagella.

On shape, NCERT groups bacteria into four categories you must recall by both singular and plural form: the spherical Coccus (pl. cocci), the rod-shaped Bacillus (pl. bacilli), the comma-shaped Vibrium/Vibrio (pl. vibrio) and the spiral Spirillum (pl. spirilla). Shape is a morphological label only — it carries no information about metabolism, habitat or pathogenicity. A coccus may be harmless or lethal; the diversity that defines Monera lies in chemistry, not outline.

Archaebacteria — life in extremes

Archaebacteria are the special monerans of harsh habitats. NCERT defines three signature groups by environment: halophiles in extremely salty areas, thermoacidophiles in hot springs, and methanogens in marshy areas. The defining structural trait is a different cell wall — distinct from the peptidoglycan wall of other bacteria — and this difference is precisely what lets archaea survive conditions that would destroy ordinary cells.

Methanogens are the most exam-relevant. They live in the gut of ruminants such as cows and buffaloes and produce methane (biogas) from dung; NIOS adds that they also work in sewage treatment. This dual role — gut symbiont and biogas producer — is a frequent matching-question target.

Eubacteria, cyanobacteria & nutrition

The eubacteria — "true bacteria" — number in the thousands and are marked by a rigid cell wall and, if motile, a flagellum. NCERT places three functionally distinct groups under eubacteria: cyanobacteria, chemosynthetic autotrophs, and heterotrophic bacteria.

Cyanobacteria (blue-green algae)

Cyanobacteria, also called blue-green algae, have chlorophyll a similar to green plants and are photosynthetic autotrophs. They may be unicellular, colonial or filamentous, occur in fresh water, marine and terrestrial habitats, and their colonies are often wrapped in a gelatinous sheath. In polluted water bodies they form blooms. Crucially, some — notably Nostoc and Anabaena — fix atmospheric nitrogen in specialised thick-walled cells called heterocysts. NIOS stresses that despite the "algae" name they perform oxygenic photosynthesis like green plants, releasing oxygen, and that their early activity raised Earth's atmospheric oxygen.

Chemosynthetic & heterotrophic bacteria

Chemosynthetic autotrophic bacteria oxidise inorganic substances — nitrates, nitrites and ammonia — and use the released energy to make ATP. They are central to recycling nitrogen, phosphorus, iron and sulphur. Heterotrophic bacteria are the most abundant of all; most are decomposers. They make curd from milk, produce antibiotics and fix nitrogen in legume roots, while pathogens cause cholera, typhoid, tetanus and citrus canker.

Reproduction & Mycoplasma

Bacteria reproduce mainly by fission — asexual binary fission in which one cell copies its circular chromosome and splits into two. NIOS notes that under favourable conditions one cell divides in about 20 minutes, so a single bacterium can yield about eight cells in an hour. Under unfavourable conditions some bacteria produce resistant spores.

Bacteria also show a primitive, sexual-like genetic exchange: a DNA segment is transferred from one bacterium to another. In conjugation, two cells are held together by pili and a strand of DNA — or the F-factor from a donor (male) to a recipient (female) cell — passes across. This is recombination, not gamete fusion; no zygote forms.

Finally, Mycoplasma are the monerans that completely lack a cell wall. NCERT names them the smallest living cells known; the PYQ record fixes the size at about 0.3 µm, small enough to pass through filters under 1 µm. They can survive without oxygen and many are pathogenic in animals and plants. The wall-less state is their single most tested fact, and it directly explains both their tiny filterable size and their resistance to wall-targeting antibiotics like penicillin.

Worked examples

Common confusion & NEET traps