Kingdom Monera

Kingdom Monera is the kingdom of bacteria, the only prokaryotes in R.H. Whittaker's five kingdom classification. These notes on Kingdom Monera first trace how classification grew from Aristotle and the two kingdom system to five kingdoms. They then cover bacterial shapes, modes of nutrition, archaebacteria, eubacteria, cyanobacteria, reproduction and Mycoplasma. Every NCERT point is written as short points with tables and memory tricks, and the practice uses the match-list, statement and sequence formats that NEET asks most.

1. How Biological Classification Developed

• Early classification was instinctive. It was based on using organisms for food, shelter and clothing, not on scientific criteria.
• ★ Exam imp Aristotle made the earliest attempt at a scientific basis. He used simple morphological characters.
• Aristotle divided plants into trees, shrubs and herbs.
• He divided animals into two groups: those with red blood and those without it.
• In Linnaeus' time, a Two Kingdom system was developed: Plantae for all plants and Animalia for all animals.

1.1 Why the two kingdom system was found inadequate

• It did not separate eukaryotes (cells with a true nucleus) from prokaryotes (cells without a membrane-bound nucleus).
• It did not separate unicellular from multicellular organisms.
• It did not separate photosynthetic organisms (green algae) from non-photosynthetic ones (fungi).
• Sorting into plants and animals was easy, but a large number of organisms fitted neither group.
• ★ Exam imp So the two kingdom classification, though used for a long time, was found inadequate.
• Besides gross morphology, other characters were needed: cell structure, nature of wall, mode of nutrition, habitat, methods of reproduction and evolutionary relationships.
• Plant and animal kingdoms have remained constant in every system. What changed is which organisms they include, and the number and nature of the other kingdoms.

2. Whittaker's Five Kingdom Classification

• ★ Exam imp R.H. Whittaker (1969) proposed the Five Kingdom Classification.
• His five kingdoms: Monera, Protista, Fungi, Plantae and Animalia.
• His main criteria: cell structure, body organisation, mode of nutrition, reproduction and phylogenetic relationships (evolutionary relationships).

2.1 Characteristics of the five kingdoms

2.2 The three-domain system

• A three-domain system has also been proposed.
• It divides Kingdom Monera into two domains; all the eukaryotic kingdoms form the third domain.
• This gives a six kingdom classification.

2.3 What changed from the old grouping

• Earlier systems placed bacteria, blue green algae, fungi, mosses, ferns, gymnosperms and angiosperms together under 'Plants'.
• ★ Exam imp The one character uniting all of them was the presence of a cell wall.
• This joined groups that differed widely in other characters. Prokaryotic bacteria and blue green algae (cyanobacteria) sat with eukaryotic groups.
• Unicellular and multicellular forms sat together: Chlamydomonas and Spirogyra were both placed under algae.
• Heterotrophic fungi were not separated from autotrophic green plants, though their walls differed: chitin in fungi, cellulose in green plants.
• Once such characters were considered, fungi got a separate kingdom: Kingdom Fungi.
• All prokaryotic organisms were grouped under Kingdom Monera.
• The unicellular eukaryotes were placed in Kingdom Protista.
• Protista brought Chlamydomonas and Chlorella (earlier algae within plants, with cell walls) together with Paramoecium and Amoeba (earlier animals, without cell walls).
• Organisms once in different kingdoms came together because the criteria for classification changed. Such changes will continue as understanding improves.
• The aim is a system that reflects morphological, physiological and reproductive similarities and is also phylogenetic (based on evolutionary relationships).
• Plantae and Animalia, the plant and animal kingdoms, are studied separately in detail.

3. Kingdom Monera: Bacteria

• ★ Exam imp Bacteria are the sole members of Kingdom Monera.
• They are the most abundant micro-organisms and occur almost everywhere.
• Hundreds of bacteria are present in a handful of soil.
• They live in extreme habitats too: hot springs, deserts, snow and deep oceans, where very few other life forms survive.
• Many live in or on other organisms as parasites.

3.1 Shapes of bacteria

Bacteria are grouped into four categories on the basis of shape.

3.2 Nutrition in bacteria

• Bacterial structure is very simple, but their behaviour is very complex.
• ★ Exam imp As a group, bacteria show the most extensive metabolic diversity compared to many other organisms.
• Autotrophic bacteria synthesise their own food from inorganic substrates.
• Autotrophs are either photosynthetic autotrophs or chemosynthetic autotrophs.
• The vast majority of bacteria are heterotrophs: they depend on other organisms or on dead organic matter for food.

4. Archaebacteria

• Archaebacteria are special because they live in some of the harshest habitats.
• ★ Exam imp Archaebacteria differ from other bacteria in having a different cell wall structure. This feature lets them survive extreme conditions.
• Methanogens are present in the gut of several ruminant animals such as cows and buffaloes.
• Methanogens produce methane (biogas) from the dung of these animals.

5. Eubacteria

• There are thousands of different eubacteria, the 'true bacteria'.
• ★ Exam imp They have a rigid cell wall and, if motile, a flagellum.

5.1 Cyanobacteria (blue-green algae)

• ★ Exam imp Cyanobacteria (also called blue-green algae) have chlorophyll a like green plants.
• They are photosynthetic autotrophs.
• They are unicellular, colonial or filamentous; they live in fresh water, the sea or on land (terrestrial).
• Their colonies are generally surrounded by a gelatinous sheath.
• They often form blooms in polluted water bodies.
• Some fix atmospheric nitrogen in specialised cells called heterocysts, e.g., Nostoc and Anabaena.

5.2 Chemosynthetic autotrophic bacteria

• Chemosynthetic autotrophic bacteria oxidise inorganic substances such as nitrates, nitrites and ammonia.
• They use the energy released for their ATP production.
• They play a great role in recycling nutrients such as nitrogen, phosphorus, iron and sulphur.

5.3 Heterotrophic bacteria

• ★ Exam imp Heterotrophic bacteria are the most abundant in nature.
• The majority are important decomposers.
• Helpful uses: making curd from milk, production of antibiotics, and fixing nitrogen in legume roots.
• Some are pathogens that harm human beings, crops, farm animals and pets.
• Well-known bacterial diseases: cholera, typhoid, tetanus and citrus canker.

6. Reproduction in Bacteria

• ★ Exam imp Bacteria reproduce mainly by fission.
• Under unfavourable conditions, they sometimes produce spores.
• They also show a sort of sexual reproduction, by a primitive type of DNA transfer from one bacterium to another.

7. Mycoplasma

• ★ Exam imp Mycoplasma are organisms that completely lack a cell wall.
• They are the smallest living cells known.
• They can survive without oxygen.
• Many Mycoplasma are pathogenic in animals and plants.

8. Exam Essentials: Traps, Exceptions, Numbers and Pairs

Numbers to Remember

• 1969: Whittaker's Five Kingdom Classification.
• 2 kingdoms in Linnaeus' time; 5 kingdoms in Whittaker's system.
• 3 domains in the three-domain system, giving a 6 kingdom classification.
• 4 bacterial shapes; 3 groups of archaebacteria.
• Aristotle: 3 plant groups and 2 animal groups.
• Hundreds of bacteria in a handful of soil; thousands of kinds of eubacteria.

Pairs to Match

Examples to Remember

9. Quick Revision

• Early classification was instinctive and use-based; Aristotle first used morphology.
• Linnaeus' time: two kingdoms, Plantae and Animalia; found inadequate.
• Whittaker (1969): five kingdoms by cell structure, body organisation, nutrition, reproduction and phylogeny.
• Three-domain system splits Monera into two domains, giving six kingdoms.
• The old 'Plants' were united only by the cell wall.
• Prokaryotes went to Monera, unicellular eukaryotes to Protista, and fungi to Fungi.
• Bacteria are the sole members of Monera, the most abundant micro-organisms.
• Shapes: coccus, bacillus, vibrium, spirillum.
• Bacteria show the most extensive metabolic diversity; most are heterotrophs.
• Archaebacteria: halophiles, thermoacidophiles, methanogens; different cell wall.
• Eubacteria: rigid wall, flagellum if motile.
• Cyanobacteria: chlorophyll a, blooms, heterocysts in Nostoc and Anabaena.
• Bacterial diseases: cholera, typhoid, tetanus, citrus canker.
• Reproduction: fission, spores, primitive DNA transfer.
• Mycoplasma: no wall, smallest cells, survive without oxygen.

10. Solved Examples

11. Practice Questions

Common Mistakes to Avoid