Over thousands and millions of years, a bewildering variety of species has evolved on the earth. Classification of these organisms was a serious challenge for the biologists. Many researchers had come forward with different categories to classify living things. Among them, Ernst Haeckel (1894), Robert Whittaker (1959) and Carl Woese (1977) are few whose contributions are notable.
Modern-day of taxonomy has accepted the five kingdom classification which was proposed by R. H. Whittaker. The basis of his classification is the cell structure, mode, and source of nutrition and the structure of the body.
The classification of living organisms according to Whittaker is divided into five kingdoms namely: Monera, Protista, Fungi, Plantae, and Animalia
Let’s learn about kingdoms Monera, Protista, and Fungi.
Kingdom Monera is considered as the most primitive group of organisms and monerans are most abundant of all organisms. It generally comprises unicellular organisms with a prokaryotic cell organization. They lack well-defined cell structures including the nucleus and other cell organelles. They consist of prokaryotes which include species like the Cyanobacteria, archaebacteria, mycoplasma, and bacteria are a few members of this kingdom.
The general characteristic features of Monerans are:
- Monerans are present in both living and non-living environment.
- Some have rigid cell walls, while some do not.
- Membrane bound nucleus is absent in monerans.
- Habitat – Monerans are found everywhere in hot or thermal springs, in the deep ocean floor, under ice, in deserts and on or inside the body of plants and animals.
- The mode of nutrition includes autotrophs, they can synthesize food on their own while some other have a heterotrophic, saprophytes, parasitic, symbiotic, commensalism and mutualism mode of nutrition.
- Locomotion is with the help of flagella.
- Circulation is through diffusion.
- Respiration in these organisms vary, few are obligate aerobes, while some are obligate anaerobes and facultative anaerobes
- Reproduction is mostly asexual and few also reproduce by sexual reproduction. Sexual reproduction is by conjugation, transformation, and transduction. Asexual reproduction is by binary fission.
All unicellular organisms are placed under the Kingdom Protista. The term Protista was first used by Ernst Haeckel in the year 1886. This kingdom forms a link between other kingdoms of fungi, plants, and animals. Kingdom Protista is an important phase in early evolution and the first protist evolved was probably 1.7 billion years ago.
Kingdom Protista is a very large group comprising of at least 16 phyla. Many species of this kingdom are the primary producers in the aquatic ecosystem and some are responsible for serious human diseases like malaria.
General characteristics of Kingdom Protista are as follows:
- They are simple, unicellular, eukaryotic organisms.
- Most of the protist live in water, some in moist soil or even the body of human and plants.
- These organisms have a membrane-bound nucleus, endomembrane systems, mitochondria for cellular respiration and some have chloroplasts for photosynthesis.
- Nuclei contain multiple DNA strands and the number of nucleotides is significantly less.
- Respiration – cellular respiration is the primarily aerobic process, but some living in the moist soil underneath ponds or in digestive tracts of animals are facultative anaerobes.
- Locomotion is often by flagella or cilia.
- Nutrition- include both heterotrophic and autotrophic.
- Reproduction – Some reproduce sexually and others asexually.
- Some protists are pathogens of both plants and animals. Example: Plasmodium falciparum causes malaria in humans.
The fungi are the independent group of plants and animals. They are presently found everywhere around including air, water, land, in the soil, and even in plants and animals. Some fungi are microscopic and others can be extended for more than a thousand acres. Fungi appear like plants but are closely related to animals.
Fungi have great economic importance and show a great diversity in morphology and habitat.
More than 70,000 species of fungi have been recognized and the organisms of kingdom fungi include mushrooms, smuts, yeasts, puffballs, rusts, smuts, truffles, morels, and molds.
General characteristics of fungi are as follows:
- Fungi are eukaryotic, non-vascular and non-motile organisms.
- The growth rate of fungi is slower than that of bacteria.
- The Kingdom Fungi grow best in an acidic environment.
- The Kingdom Fungi consist of both unicellular (e.g. Yeast, Molds) and multicellular (e.g. mushrooms) organisms.
- Like plant cells, fungi have cell walls made up of a complex sugar called chitin. But they do not perform photosynthesis.
- The cell wall is composed of chitin. The vegetative body of the fungi may be unicellular or composed of microscopic threads called hyphae.
- They have a heterotrophic mode of nutrition. Few species are saprophytes i.e., they feed on dead and decaying organic matters.
- Some fungi are parasitic while some are symbionts. They can live in a symbiotic relationship with algae like blue-green algae. These are called lichens.
- Reproduction in fungi is both by sexual and asexual means. Asexual reproduction takes place by means of spores and sexual reproduction takes place by means of gametic copulation, somatic copulation, and Spermatization.
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