Biology,  Zoology


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  • Includes the bacteria and cyanobacteria (commonly called blue-green algae).
  • Since only bacteria are prokaryotic (lacking a true nucleus), that is without nuclear membrane), monera is the only prokaryotic kingdom.
  • Bacteria were the first organisms to evolve on planet earth afterlife originated around 3.5 billion years ago and were the only organisms on earth for almost the next two billion years.
  • All bacteria and cyanobacteria are single-celled or unicellular (monere : single)
  • Monerans are also the most numerous of all living organisms.

Structure of a bacterial cell

The single-celled bacterium has a cell wall made of compound peptidoglycan
covering the cell membrane; a single chromosome. The cell has ribosomes but no
membrane-bound organelles. Let us get to know some details of these parts

Structure of a bacterium

Cell wall

All prokaryotes have a rigid cell wall, which protects and gives shape to the cell. The cell wall is made up of a chemical, peptidoglycan, unique to bacteria.

Pili (Singular: pilus)

Pili are short and thin thread like structures projecting out from the cell wall in some bacteria.


Some bacteria move with the help of one or two flagella. Flagella are longer and thicker than pili. Their structure is different from flagella of eukaryotes.

Plasma Membrane

Plasma membrane, present below the cell wall, encloses the cytoplasm and other cell contents. It is made up of lipids and proteins, as in eukaryotes.

Genetic Material

One circular chromosome made of a double helical molecule of DNA is located in a region of the cytoplasm called nucleoid. Since the chromosome is not lodged within a true nucleus, bacteria are termed prokaryotes. Hence Monera is the prokaryotic kingdom. Apart from the chromosome, many species of bacteria possess rings of DNA called plasmids, which replicate along with bacterial chromosome and bear genes for antibiotic resistance, sex factor etc.

Cell Organelles

Membrane-bound organelles like endoplasmic reticulum, mitochondria, chloroplast, Golgi complex are absent. Only ribosomes are present, which are different from those of eukaryotes.

Prokaryotes have no nuclear membrane around the genetic material and no cell organelles. They have only the ribosomes.

Monera – General body functions

A. Nutrition:

The four nutritional categories found in bacteria are :

  1.  Autotrophs – synthesize their own organic food.
  2. Saprotrophs – feed on the dead and decaying matter.
  3. Symbionts – use food from other living organisms with which they are associated for mutual benefit.
  4. Parasites – absorb food from living organisms and cause harm to them.

B. Respiration

Respiration in bacteria may be either

  1. aerobic i.e. using oxygen for respiration or
  2. anaerobic i.e. respiration in the absence of oxygen.

Cellular respiration or breakdown of food to release energy occurs in mesosomes
which are the inner extensions of the cell membrane.

C. Reproduction

Asexual Reproduction
Bacteria reproduce asexually by binary fission under favourable conditions it takes about 20 minutes for one bacteria to divide into two by binary fission.

Binary Fission in Bacteria

Sexual Reproduction
Some bacteria show a primitive mode of sexual reproduction. It is different from sexual reproduction in a higher form. The steps are:
(a) Two conjugating (lie very close for the exchange of genes) bacteria are held together by pili.
(b) A segment of DNA strand is transferred from one bacterium to another bacterium.

Conjugation in Bacteria

Beneficial and harmful bacteria

They harm us by causing many diseases. On the other hand, some bacteria are very useful.

Diseases Caused By Bacteria

Name of Bacterium

Diseases Caused

Vibrio cholera


Salmonella typhi


Clostridium tetani


Corynebacterium diptheriae


Mycobacterium tuberculosis


Beneficial Activities of Bacteria

Name of bacterium



Found in roots of legumes, (Peas, grams, Pulses etc) fixes atmospheric nitrogen as ammonia, which is then converted into useful amino acid.


Makes the soil fertile. It fixes atmospheric nitrogen in the soil.


Produces Streptomycin antibiotic.


Ferments lactose (milk sugar) to lactic acid. This helps in setting of milk into curd

Methanogenic bacteria

Sewage treatment


These were earlier called the blue-green algae. A very successful group on primitive earth. They could carry out photosynthesis and the oxygen releases during the process changed the earth’s atmosphere and gradually the level of oxygen increased in the earth’s atmosphere.
Cyanobacteria (blue green algae)

Differences between Bacteria and Cyanobacteria



Smaller cells

Comparatively larger cells

May have flagella

Do not have flagella.

Some bacteria (green) carry out photosynthesis in a different way and do not release oxygen

They all carry out photosynthesis in the usual manner as in green plants and release oxygen

Sexual reproduction by conjugation.

Conjugation has not been observed.


Kingdom Monera includes two groups
1. Archaebacteria and
2. Eubacteria
Archaebacteria includes bacteria that live in unusual environments particularly at low levels of oxygen. Main types of Archaebacteria are

  • Methanogenic bacteria that live in sewage and intestinal tracts of animals
  • Thermoacidophilic bacteria that live in hot springs.
  • Halophilic bacteria live in salty conditions e.g. where hot sun concentrates sea water Eubacteria include cyanobacteria and all other bacteria.
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