Kingdom Monera: This kingdom includes prokaryotic and unicellular organisms. It includes bacteria, blue-green algae or cyanobacteria, and mycoplasma.

This kingdom is classified into the following sub-kingdoms:

1. Archaebacteria: These are autotrophs, which obtain energy from the oxidation of chemical energy sources for their metabolic activities such as reduced gases like ammonia, methane or hydrogen sulphide. They produce their own amino acids and proteins.

They are divided into three groups:

(a) Methanogens: They produce methane by their metabolic activities. They cannot survive in the presence of oxygen.

(b) Thermoacidophiles: These archaea favour extremely hot and acidic environments. They use hydrogen sulphide as their energy source.

(c) Halophiles: They inhabit extremely salty and alkaline environments.

2. Eubacteria: These are prokaryotic unicellular organisms which are enclosed in a porous, rigid cell wall. The plasma membrane surrounds a non-compartmentalised cytoplasm. The eubacteria lack membrane-bound cell organelles such as nucleus, mitochondria, Golgi apparatus etc. They possess a single, circular DNA which is coiled into one region of the cell, referred to as the nucleoid. They are divided into different categories on the basis of their body shape.

These are:

(a) Bacilli (rod-shaped)

(b) Cocci (spherical)

(c) Spirilla (corkscrew)

Electron transport and photosynthesis take place on the plasma membrane, which sometimes folds into the interior of the cell. The cell wall of eubacteria consists of a unique material called peptidoglycam which is composed of chains of sugars cross-linked by peptides. The eubacteria can be differentiated into two distinct types based on their cell wall composition - gram-positive bacteria and gram-negative bacteria. Many bacteria are also motile by means of appendages called flagella. Another set of appendages called pilli help in sexual reproduction and attachment to the substrate.