Kingdom Monera: The Realm of Prokaryotes

Kingdom Monera includes all bacteria and cyanobacteria (commonly called blue-green algae). Because only bacteria are prokaryotic—lacking a true nucleus and a nuclear membrane—Monera stands as the sole completely prokaryotic kingdom.

Bacteria were the first cellular organisms to evolve on Earth after life originated around 3.5 billion years ago, remaining the sole cellular life forms for nearly two billion years. Most bacteria are single-celled (unicellular), though actinomycetes and some cyanobacteria are multicellular and filamentous (with possible branching). Monerans are also the most numerous of all living cellular organisms.

Structure of a Bacterial Cell

A single-celled bacterium features a protective cell wall made of peptidoglycan covering the plasma membrane, alongside a single circular chromosome, ribosomes, and the absence of membrane-bound organelles.

Cell Wall: Rigid exterior layer made of peptidoglycan (unique to bacteria), lipids, polysaccharides, and proteins, providing protection and shape.

Pili: Short, thin, thread-like tubular structures projecting from the cell wall in some bacteria.

Flagella: Longer and thicker than pili, helping certain bacteria move. Their structure differs from eukaryotic flagella.

Plasma Membrane: Located beneath the cell wall, enclosing cytoplasm and contents; composed of lipids and proteins like eukaryotes.

Genetic Material: A single circular double-helical DNA molecule located in the nucleoid region. Many species also possess small additional rings of DNA called plasmids, which carry antibiotic resistance genes or the sex factor (F-factor).

Cell Organelles: Membrane-bound organelles (endoplasmic reticulum, mitochondria, chloroplasts, Golgi complex) are absent. Only 70S ribosomes and mesosomes are present.

Monera - General Body Functions

A. Nutrition

Bacteria exhibit four primary nutritional categories:

Autotrophs: Synthesize their own organic food.

Saprotrophs: Feed on dead organic matter.

Symbionts: Associate with other living organisms for mutual benefit.

Parasites: Absorb food from living hosts while causing harm.

B. Respiration

Respiration can be aerobic (using oxygen) or anaerobic (without oxygen). Cellular respiration and energy release occur in the mesosomes (inner extensions of the plasma membrane).

C. Reproduction

(i) Asexual Reproduction

Bacteria reproduce asexually primarily through binary fission. Under favorable conditions, a single bacterial cell can divide into two in about 20 minutes.

(ii) Sexual Recombination (Genetic Recombination)

Some bacteria exhibit a primitive mode of sexual reproduction via conjugation:

• Two conjugating bacteria lie close together and are held by pili.
• A segment of DNA or the F-factor (fertility factor) is transferred from a male donor cell to a female recipient cell.

Harmful and Beneficial Bacteria

Bacterial Pathogens

Many bacteria cause serious infectious diseases in humans:

Name of Bacterium	Disease Caused
Vibrio cholerae	Cholera
Salmonella typhi	Typhoid
Clostridium tetani	Tetanus
Corynebacterium diphtheriae	Diphtheria
Mycobacterium tuberculosis	Tuberculosis

Cyanobacteria (Blue-Green Algae)

Cyanobacteria were earlier called blue-green algae. As a successful group on primitive Earth, their photosynthetic activity released oxygen into the atmosphere, gradually driving up global oxygen levels.

Differences Between Bacteria and Cyanobacteria

• Photosynthesis:
  Cyanobacteria perform oxygenic photosynthesis using chlorophyll a and thylakoids. General bacteria are primarily heterotrophic and do not perform oxygenic photosynthesis.
  
  
• Pigmentation:
  General bacteria lack chlorophyll.
  Cyanobacteria contain chlorophyll a and accessory pigments like phycocyanin and phycoerythrin.
  
  
• Cellular Organization:
  General bacteria are predominantly unicellular.
  Cyanobacteria can be unicellular or colonial, often forming filaments with specialized cells.
  
  
• Nitrogen Fixation:
  Some general bacteria fix nitrogen, often in specialized cells or root nodules.
  Many cyanobacteria fix nitrogen in specialized cells called heterocysts.
  
  
• Internal Membranes:
  General bacteria lack complex internal membrane systems.
  Cyanobacteria possess complex internal thylakoid membranes for photosynthesis.
  
  
• Habitat:
  General bacteria are ubiquitous in diverse environments, including extreme ones.
  Cyanobacteria are mainly aquatic, found in freshwater, marine, and moist soil environments.

Major Groups of Kingdom Monera

Kingdom Monera is categorized into three primary groups:

1. Archaebacteria: Inhabit unusual and extreme environments with low oxygen levels:
   • Methanogenic bacteria: Found in sewage and animal digestive tracts.
   • Thermoacidophilic bacteria: Thrive in hot springs.
   • Halophilic bacteria: Adapt to high-salinity environments (concentrated sea water).
     
     
2. Eubacteria: Includes all true bacteria (excluding cyanobacteria).
3. Cyanobacteria: Encompasses all oxygenic photoautotrophic bacteria.