Some biologists did not agree with the classification of fungi like bread mold, yeast, and mushrooms. Fungi resemble with plants in many ways. But fungi are heterotrophs. They decompose the surrounding material and absorb food. This food is used for obtaining energy and for structural material. Their cell wall contains major structural component called chitin. So the fungi were given the status of a separate kingdom.The general characteristics of five kingdom classification system are as follows:
It includes prokaryotic organisms i.e. they are made of prokaryotic cells. Monerans are unicellular, although some types form chains, or colonies of cells. Prokaryotic cells are radically different from eukaryotic cells. Most are heterotrophic but some perform photosynthesis because they have chlorophyll in their cytoplasm. Within this kingdom, there are two different kinds of organisms i.e. bacteria and cyanobacteria.
It includes eukaryotic unicellular and simple multicellular organisms. There are three main types of protists.
- Algae are unicellular, colonial or simple multicellular. They resemble plant cells with cell walls and chlorophyll in chloroplasts. Simple multicellular means that they do not have multicellular sex organs and do not form embryos during their life cycles.
- Protozoans resemble animals whose cells lack chlorophyll and cell walls.
- Some protists are fungi-like.
It includes eukaryotic multicellular heterotrophs which are absorptive in their nutritional mode e.g. mushrooms. Most fungi are decomposers. They live on organic materials, secrete digestive enzymes and absorb small organic molecules formed by the digestion of enzymes.
It includes eukaryotic multicellular autotrophs. Plants are autotrophic in nutritional mode, making their own food by photosynthesis. They have multicellular sex organs and form embryos during their life cycles. Mosses, ferns and flowering plants are included in this kingdom.
Biologist believes that kingdom protista evolved from monera and then it gave rise to the other 3 eukaryotic kingdoms i.e. fungi, plantae and Animalia.
Status of Viruses
|Prions are composed of protein only and Viroids are composed of circular RNA only. Both of these particles causes infectious diseases in certain plants.|
Viruses are at the borderline of living and nonliving. Due to their crystalline nature, they are considered as non-living. They are acellular i.e. they do not have cellular organization yet show some characters of living organisms (e.g. they possess DNA). Viruses contain either RNA or DNA, normally encased in protein coat. They reproduce only in living cells, where they cause a number of diseases. They are not considered as organisms and thus are not included in five-kingdom classification system.Prion and Viroids are also acellular particles and are not included in the five-kingdom classification system.
The five kingdom system was proposed by Robert Whittekar in 1969. It is a recent system of classification. This system of classification is based on two principles:
- Three levels of organizations: cellular, tissue and organ level.
- Three principle mode of nutrition: photosynthesis, absorption, and ingestion.
There are following five kingdoms:
It contains all the prokaryotic unicellular organisms. It includes bacteria and blue green algae.
These are eukaryotes. All the organisms which do not come in the definition of plants, animals or fungi are included in this kingdom. Most of the protists are unicellular like Euglena and Amoeba. But this kingdom also contains relatively simple multicellular organisms. These simple multicellular organisms have been directly derived from the unicellular protists.
All the eukaryotic multicellular autotrophs are included in this kingdom. Plants are autotrophs. They synthesize their own food by photosynthesis. It includes mosses, ferns and flowering plants.
These are eukaryotic multicellular reducers. Fungi are heterotrophic organisms. They have absorptive mode of nutrition. Most of the fungi are decomposer. They live on organic material. They secrete (t/L) digestive enzymes. These enzymes break the organic matter into small organic molecules. Fungi absorb these organic molecules.
It includes all the eukaryotic multicellular consumers. Animals mostly ingest and digest foods in the specialized cavities. They lack cell wall and cellulose. They show movements. For example: birds, reptiles.
In this section, Five Kingdom System of Robert Whittaker is followed. In this system:
- The protozoans are placed in the kingdom Protoctista (Protista).
- All other animal are placed in the kingdom Animalia. These animals are multicellular, diploid, eukaryotic and ingestive heterotrophs. They develop from two dissimilar haploid gametes. The large gamete is called egg and the small gamete is called sperm.
All the biologists agree that the animals have evolved from protoctists (protists). But it is not known form which protists they have evolved.
- The relationship of different phyla.
- Development of complexity in Animals.
The animals in kingdom Animalia show advance multicellular structures. Their cells are joined by complex junctions. So the communication and flow of material take place between the cells. These animals have different size. Some are microscopic and their size is smaller than many protists. Some are have large size like whale. Whale is a sea mammal and it is the largest animal.
There are two are sub-kingdoms of kingdom Animalia:
(i) Sub-kingdom Parazoa (phylum porifera)
These animals lack tissues organized into organs. They have indeterminate (irregular) shape. So they are asymmetrical (without any shape)
All animals of other multicellular phyla are included in this group.
These animals have tissues organized into organs and organ system. This sub-kingdom is divided into two groups on the basis of their symmetry
(i) Radiata: The animals with radial symmetry are included in this group they have simple organization. The phylum Cnidaria belongs to this group.
(ii) Bilateria: They have bilateral symmetry. Most of the phyla belong to this group. These animals are divided into three groups on the basis of coelom or body cavity.
- Acoelomata: These animals are without body cavity e.g. phylum Platyhelminthes.
- Pseudocoelomata: These animals have false coelom. The false coelom is called pseudocoel e.g. Phylum Nematoda.
- Coelomata: These animals have true coelom e.g. Phylum Chordata.
- The animals of this group have radial symmetry. In this case body of an animal can be cut into two equal halves by more than one plane. The parts of the body are arranged around a central axis. Any plane (line) passing through this central axis divides the body into two equal halves. These halves are mirror image (similar) of each other.
- They are diploblastic (have two body layers).
- Examples: Cnidaria (coelentrata). The cylindrical body of a sea anemone can be cut in two equal halves in any plane.
- They have bilateral symmetry. In this case, an animal can be cut into two equal halves by only one plane (line). They have similar right side and left side of the body.
- They have prominent head at anterior end (head side) and a distinct posterior end (tail side).
- Grade Bilateria have distinct dorsal (upper side) and ventral (lower) sides.
- They are triploblastic (with three body layer).
- They may be acoelomate, pseudocoelomate or coelomate.
- Examples: Phylum Platyhelminthes, Phylum Nematoda, Phylum Annelida, Phylum Mollusca, Phylum Arthropoda, Phylum Echinodermata and Phylum Chordata.
The larvae of the Echinoderms have bilateral symmetry. However, the adult echinoderms have secondarily developed radial symmetry due to their special mode of life.
Modified Five Kingdom Classification System by L. Margulis and K. Schwartz
Lynn Margulis and Karlene Schwartz in 1988 gave modified five kingdom system of classification. They modified the basis of classification of Whittaker. There classification is based on:
- Cellular Organization.
- Mode of nutrition.
- Organelles of symbiotic origin like mitochondria and chloroplast.
These five kingdoms are:
(1) Kingdom Prokaryotae (Monera):
It includes unicellular prokaryotic organism e.g. Bacteria and Cyanobacteria.
(2) Kingdom Protoctista (Protista):
It includes all unicellular, eukaryotic organisms, which are no longer classified as animal, Plants or Fungi e.g. Euglena, Paramecium, Chlamydomonas Plasmodium.
(3) Kingdom Fungi:
It includes multicellular, eukaryotic non chlorophyllous organism e.g. Agaricus, Yeast.
(4) Kingdom Plantae:
It includes all multicellular, eukaryotic, chlorophylous, Photosynthetic autotrophs having cell wall made up of cellulose, Zygot retained to become embryo arid exhibiting heteromorphic alternation of generation e.g. Moss, Fern, Pinus, and Apple.
(5) Kingdom Animalia:
It includes all multicellular eukaryotic, non chlorophyllus ingestive heterophs e.g. Hydra, Earth worm and Human.