The five-kingdom concept of classification

The five-kingdom strategy of classification

Types in which organisms obtain nutrition from their food are the basis of a five-kingdom system of classification proposed in 1969 by Robert H. Whittaker. He expanded Haeckel’s system of classification and concluded that three levels of cellular organization have evolved to accommodate three principal modes of nutrition: (1) photosynthesis, the process in which light supplies energy to convert carbon dioxide and water to sugars; (2) absorption, the intake of chemical nutrients dissolved in water, and (3) ingestion, the intake of undissolved particles of food.

In the system , prokaryotes from the kingdom Monera, which until recently was considered the most primitive kingdom and beleived to be the ancestors of the eukaryotes. Procaryotes usually obtain nutrients only by absorption, and could not ingest or photosynthesize food. The kingdom Protista harbours the unicellular eukaryotic microorganisms, which represent all the three nutritional types: algae are photosynthetic, protozoa can ingest their food, and slime molds (the lower fungi) only absorbs nutrients. Higher eukaryotic organisms are placed in the kingdom called Plantae (photosynthetic green plants and higher algae), Animalia (animals, which ingest food), and fungi, the organism that posses cell walls but lacks the photosynthetic pigment chlorophyll found in other plants and thus absorb their energy as food.

Terefore, microorganisms were placed in three of the five kingdoms: Monera (bacteria), Protista (protozoa and microscopic algae), and fungi (the microscopic fungi called yeasts and molds). Whittaker’s system puts all bacteria in the kingdom Monera and tells a common ancestry for all members of this kingdom. However, results of extensive research during recent decades suggest a different ancestral pattern among microorganisms, as described in the following section.

Archaeobacteria, eubacteria, and eukaryotes

Till 1977 scientists thought prokaryotes were the most primitive of all organisms. The PREFIX PRO-, MEANING “earlier than”, implied for these organisms, because of those simple structure, were the ancestors of the more complex eukaryotes. Then Carl whose and his co-investigators at the University of Illinois found that neither group had developed from the other. They found intrestingly that procaryotes and eucaryotes apparently had evolved by completely different pathways from a common ancestral form.

Proofs to support this idea came from studies of ribosomal ribonucleic acid, or rRNA is composed many smaller units called ribonucleotides. There are four different kinds of ribonucleotides, arranged in various combination to form a single, long chain of several hundred units. The rRNA from any particular organism has a distinctive sequential management of ribonucleotides or a specific nucleotide sequence.

The genes that guide the nucleotide sequence of rRNA slowly change during millions of years of evolution. As one can compare those changes in different organisms, rRNA can be as an indicator of how nearly organisms are related. Some of the portion of the rRNA molecules of all living beings has remained almost the same, despite 3.5 to 4 billion years of evolution. This constancy supports the fact that all organisms have developed from a common ancestral form.

At the same time, the quantity of difference among the other portions of rRNA can be used to measure the scope of relatedness between organisms. For example, if the ribonucleotide sequences of two types of organisms differ greatly, they are only distantly or partially related; that is, the organisms diverged a long period ago from a common ancestor. However, if sequences show much more similarity, organisms are closely related to each otherc and have a rather recent common ancestor.

Using these techniques, Woese found that rRNA molecules in types of organisms differ in the arrangement, or sequence, of their nucleotides. Eukaryotes usually possess one general type of sequence and procaryotes the second type, but he also discovered that some prokaryotes have a third kind of rRNA. This rRNA arrangement differs each other from that of other prokaryotes as it does from that of eukaryotes. In other words, there are two major types of bacteria. It is now clearly understood that these two kinds of bacteria, designated archaebacteria and eubacteria, are as different from each other as they are from the eukaryotic organisms..

The most convincing explanation is that archaebacteria, eubacteria, and eukaryotes evolved through different pathways from a common ancestor. Within the eubacterial branch, there are at least 10 different paths of evolutionary descent; within the archaeobacteria branch, at least three. Woese formulated that archaebacteria, eubacteria, and eukaryotes represent the three primary kingdoms of life, a concept that is gaining support among scientists.

There also is considerable findings that bacteria may have possesed an unexpected role in the evolution of eukaryotic cells. Present-day eukaryotic cells- usually contain self-replicating organelles their ancestors did not posses,organelles are structures within cells that perform specific functions. The organelles called chloroplasts and mitochondria have their own genes and ribosomes. Moreover, in the process of comparative studies of the structural and biochemical properties of these organelles and eubacteria, mitochondria and chloroplasts appear to have been derived from eubacteria. Particularly strong evidence for this idea comes from rRNA nucleotide sequence analyses done since 1980. It is believed that at some stage evolution bacteria invaded a primitive eukaryotic cell. Instead of creating problem , the bacteria provided respiratory and photosynthetic abilities previously lacking in the cell. Both benefited from this association, and each gradually became dependent on the other. The bacteria eventually evolved to become mitochondria and chloroplasts, which are responsible for respiration and photosynthesis, respectively. The idea of a prokaryotic origin for eukaryotic organelles is also called as the endosymbiotic theory.

References

Arvind, Keshari K. and Kamal K Adhikari. A Textbook of Biology. Vidyarthi Pustak Bhander.

Michael J.Pleczar JR, Chan E.C.S. and Noel R. Krieg. Microbiology. Tata Mc GrawHill, 1993.

Powar. and Daginawala. General Microbiology.

Rangaswami and Bagyaraj D.J. Agricultural Microbiology.