Three domain system
This system was proposed in 1978 by Carl Woese. Molecular biology and biochemistry provide the basis of this newly proposed system. Based on distinct differences in the ribosomal RNA sequences two distinct taxons were discovered among the kingdom Monera.

The three domains are:

  1. Eubacteria are the true bacteria.
  2. Archaea are odd bacteria that live in extreme environments, high salt, heat, etc.
  3. Eukarya have a nucleus & organelles for example humans, animals, plants.

The kingdoms- Animalia, Plantae, Fungi and Protista are in the domain Eukarya.

Five kingdom system:
This system was proposed in 1969 by Robert Whittaker. This is the system most of us are familiar with. The kingdoms include

  1. Monera (Prokaryote), the bacteria
  2. Protista, the unicellular Eukaryotes
  3. Fungi
  4. Plantae
  5. Animalia

Bacteria is a unicellular organism with a body size of 3-5um – Micro Meter. Their cells contain cytoplasm, but the nucleus membrane is not available in the cells. A bacterium has a cell wall formed with protein – Mucopeptide. Some bacteria can be harmful as they are disease causing ones.

There are 5 another types of bacteria according to their nutritional habits.

  1. Saprophytic Bacteria
    This type of bacteria can be found on decomposing tree trunks.
  2. Parasitic Bacteria
    These are pathogenic type and live on living organisms in their tissues. They can harm the host.
  3. Autotrophic Bacteria
    These are purple color and they are photosynthetic. So they can produce their own food.
  4. Symbiotic Bacteria
    This type can be found on leguminous plants in the root nodules. They can perform the nitrogen fixation which means changing the atmospheric nitrogen to an absorb able form.
  5. Chemosynthetic Bacteria
    These bacteria too can produce their own food by doing chemical reactions. A chemosynthetic bacterium is the beginning of life on earth according to one theory. Fission of these bacteria occurs every 18-20 minutes and if a cholera bacteria reproduces at this rate, then 4.7 x 1022 amount of bacteria would be reproduced within 24hrs.

Algae
Algae also cannot be seen with the naked eyes. There are unicellular algae as well as muticellular filamentous algae. They produce their own food by photosynthesis, hence they can be known as Autotrophic.

Protozoa
Protozoa is also a unicellular organism with large number of aquatic species of around 40,000. Usually they move onto other living organisms and feed on them. Protozoa has a body size of 10-52um.

Fungi
Fungi are none green plants which are not photosynthetic. They are heterotrophic nutrition type because they are parasitic or saprophytic. They show a symbiotic relationship with Algae to form lichens. Some fungi can be seen with naked eyes.
There are fungi as mushrooms and molds with a multicellular structure known as mycelium.

Virus
Viruses have a very simple structure and the smallest microorganism discovered so far by the man. They can reproduce and don’t breathe at all. Viruses can capture living cells and reproduce on them. They have the crystallization ability.

The eukaryota domain consists of all eukaryotic groups:

  1. Protista,
  2. Fungi,
  3. Plantae
  4. Animalia.

Monera is the only kingdom consisting of prokaryotic organisms.

The archaebacteria is the most ancient of this kingdom and is very different. The other groups of monera include cyanobacteria and eubacteria. Protista is most ancient eukaryotic kingdom. Fungi are eukaryotic, heterotrophic, multicellular group. Plantae are multicellular eukaryotes, whereas animalia are heterotrophic eukaryotes.
Mollusks, large phylum of invertebrate animals, can be segregated into seven classes: Aplacophora, Monoplacophora, Polyplacophora, Bivalvia, Gastropoda, Cephalopoda and Scaphopoda.
• Aplacophora includes worm-like animals with no shell and a rudimentary body structure. These animals lack a calcareous shell, but possess aragonite spicules on their epidermis. They have a rudimentary mantle cavity and lack eyes, tentacles, and nephridia (excretory organs).
• Members of class Monoplacophora have a single shell that encloses the body. A looped digestive system, multiple pairs of excretory organs, many gills, and a pair of gonads are present in these animals.
• Members of class Polyplacophora are better known as “chitons”; these molluscs have a large foot on the ventral side and a shell composed of eight hard plates on the dorsal side. They have oval bodies with overlapping calcareous plates; underneath the plates, the body is not segmented; they creep along using a broad, flat foot surrounded but a groove or mantle cavity in which the gills are arranged.
• Class Bivalvia consists of mollusks with two shells held together by a muscle; these include oysters, clams, and mussels. They do not have distinct head or radula; they disperse from place to place largely as larvae.
• Members of class Gastropoda have an asymmetrical body plan and usually have a shell, which can be planospiral or conispiral. Their key characteristic is the torsion around the perpendicular axis on the center of the foot that is modified for crawling.
• Class Cephalopoda (“head foot” animals) includes octopi, squids, cuttlefish, and nautilus. They have highly-developed nervous system along with eyes, as well as a closed circulatory system. Class Scaphopoda consists of mollusks with a single conical shell through which the head protrudes, and a foot modified into tentacles known as captaculae that are used to catch and manipulate prey.