Chapter 2 : Biological Classification

PRACTICE QUESTIONS

Biological Classification

Biological classification is defined as the process of grouping organisms according to certain similarities.

Linnaeus proposed the two kingdoms of classification, He classified organisms in the animal kingdom as Animalia and in the plant kingdom as Plantae. There were certain limitations related to biological classification. Classification of two kingdoms as it does not distinguish between eukaryotes and prokaryotes, unicellular and multicellular organisms, and photosynthetic and non-photosynthetic organisms. Also, the organisms that are aware and are conscious of their surroundings will be living organisms.

Five Kingdoms Rankings

RH Whittaker suggested the five rankings. The classification of these five kingdoms is as follows: Monera, Protista, Fungi, Plantae, and Animalia. The classification was based on the organization of the thallus, the cell structure, the diet, the phylogenetic relationship, and the reproduction.

Kingdom Monera

Kingdom Monera is considered as the most primitive group of organisms and monerans are most abundant of all. 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 features of Monerans are:

  • Monerans are present in both aerobic and anaerobic environment.
  • Some have rigid cell walls, while some do not.
  • The 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 also inside the body of plants and animals.
  • They can be autotrophic, i.e., they can synthesize food on their own while some others have a heterotrophic, saprophytic, parasitic, symbiotic, commensalistic and mutualistic modes 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.

Kingdom Protista

All unicellular eukaryotic 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 very first protist probably evolved 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 features of Kingdom Protista are as follows:

  • They are simple, unicellular, eukaryotic organisms.
  • Most of the protists 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.

Kingdom Fungi

Fungi are a group of organisms that are found everywhere from air, water, land to the soil. They are also found in plants and animals.

Some fungi are microscopic, and others are gargantuan – almost extending over a thousand acres. And even though fungi appear like plants, they are in fact 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 moulds

General features 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.
  • 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 complex sugar molecules called chitin. But unlike plants, they do not undergo 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.

Kingdom Plantae

  • Includes all eukaryotic, multicellular, and photosynthetic plants.
  • The characteristics of the members of Plantae are as follows:
  • Most of them are eukaryotic in nature.
  • The main pigment present is chlorophyll.
  • Its cell wall is made of cellulose.
  • Photosynthesis helps in the synthesis of food.
  • The process of reproduction can be both sexual and asexual.
  • They represent the phenomenon of alternation of generations, i.e., diploid sporophytes, and haploid gametophytes.

Kingdom Animalia

These types of organisms are heterotrophic, eukaryotic. Some of the characteristics of the members of Animalia are the following:

  • They are multicellular organisms of various sizes.
  • The organ systems are well developed such as the skeletal system, circulatory system, respiratory system, etc.
  • They are found to be bilaterally symmetrical.
  • They also have well-developed locomotor organs.
  • Breathing takes place through gills, book lungs, book gills, skin, lungs, etc.
  • Membrane-bound cell organelles with a nucleus bounded by a nuclear membrane.
  • The circulation takes place through the blood, the blood vessels, and the heart.
  • Reproduction takes place through the formation of haploid gametes. The fusion of the gametes creates a new diploid organism.
  • The kidneys are the most important respiratory organs.

Viruses

The viruses are acellular structures and therefore do not find a place in Whittaker’s five kingdom classification. They consist of nucleic acid (either DNA or RNA) that is surrounded by a protein coat. These viruses can grow and multiply only within a host cell. Viruses exist as crystals outside the host cell. They cause disease and severely damage the host. Examples of common viruses are the viruses that cause cold, flu, polio, AIDS, etc.

Viroids

They are the smallest known infectious structures and consist only of nucleic acid without a protein shell.

Lichens

They are known to be the symbiotic associations of algae and fungi. The pair of algae are autotrophic and synthesized and provide food. The mushroom pair offers protection and shelter.

Archaebacteria

Archaebacteria are one of the oldest living organisms (to be known) on Earth. They are classified as bacteria because many of their features resemble the bacteria when observed under a microscope. They belong to the kingdom Archaea and hence are named Archaebacteria. They share slightly common features with eukaryotes but are completely different from prokaryotes. They are known as extremophiles as they can easily survive under typically harsh conditions, for example, the bottom of the sea and the vents of a volcano.

Archaebacteria have made scientists reconsider the definition of species. Species are defined as a group with gene flow within its members whereas archaebacteria exhibit gene flow across its species.

Archaebacteria have the capability to produce methane, i.e., are methanogens. They do this by acting on the organic matter and hence decomposing it to release methane. Methane can hence for cooking and lighting purposes,

  • Archaebacteria cannot perform photosynthesis.
  • They do not produce spores, unlike bacteria.
  • 20% of all microbial cells living in the ocean are archaebacteria.
  • Archaea was discovered by Carl Woese in 1978.
  • Archaebacteria can only reproduce through the asexual mode.

Eubacteria

Eubacteria, also known as “true” bacteria, are single-celled prokaryotic microorganisms that have a variety of characteristics and can be found in a variety of environments around the world. Except for archaebacteria, this term encompasses all types of bacteria. Because eubacteria are so common, they belong to one of the three domains of life: Bacteria. Eubacterium treatment is accomplished through the use of medications.

Both heterotrophic and autotrophic organisms feed on Eubacteria. The most well-known type of nutrition in eubacteria is heterotrophic, which means they must consume food from other organic carbon sources, primarily plant or animal matter. Autotrophs, on the other hand, produce their own food through photosynthesis.

Types of Protozoan like Protists are there:

Protozoan like protists are heterotrophs in nature and can survive as predators and parasites. There are four main types of protozoans.

They are as follows:

  • Amoeboid protozoans have pseudopodia for swallowing food particles like Amoeba. These are found commonly in moist soil or seawater or freshwater.
  • Flagellated Protozoans are free-living or parasitic. They are responsible for the different parasitic diseases. For example, sleeping sickness is caused by the parasite Trypanosoma.
  • Ciliated Protozoans contain thousands of cilia. The movement of cilia helps the protozoans to move backwards or forward and also allows it to procure food from outside. An example of Ciliated Protozoans is Paramecium.
  • Sporozoans are parasitic and pathogenic. They form reproductive cells in them known as spores and thus derive their name. Their reproduction may be asexual or sexual. An example of Sporozoans is the Plasmodium species which causes Malaria.

Classes of kingdom fungi:

There are five main classes in the Fungi Kingdom.

They are as follows:

  • Phycomycetes or Lower Fungi: Asexual reproduction occurs in Phycomycetes. This takes place by motile spores known as Zoospores and non-motile spores known as Aplanospores. These spores are produced inside the sporangium. Examples of Phycomycetes are Mucor, Rhizopus, Albugo, etc.
  • Zygomycetes or Conjugation Fungi: Zygomycetes are a primitive group of fungi. Here, asexual reproduction occurs with the help of non-motile sporangiospores. An example of Zygomycetes is Rhizopus.
  • Deuteromycetes: These are also known as Imperfect Fungi as they do not engage in sexual reproduction at any stage. Only asexual reproduction occurs by conidia. The Mycelium gets separated and ranched due to this. Some examples of Deuteromycetes are Alternaria, Trichoderma, etc.
  • Basidiomycetes or Club Fungi: In Club Fungi, vegetative reproduction occurs through Fragmentation due to the absence of sexual reproductive organs. Plasmogamy between two vegetative or somatic cells leads to basidium growth, which then undergoes karyogamy and meiosis to produce four basidiospores. Some examples of Club Fungi are Agaricus, commonly known as mushrooms, Ustilago or Smut, etc.
  • Ascomycetes or Sac Fungi: Sac Fungi are saprophytic as well as parasitic in nature. Here, sexual reproduction occurs by ascospores, while asexual reproduction occurs by conidia. It is branched and separates mycelium in Ascomycetes. Some examples of Sac Fungi are Penicillium, Claviceps, Aspergillus, etc.

Characteristics of the members of Animalia:

The Animalia Kingdom is heterotrophic and eukaryotic.

Their characteristics are as follows:

  • They are multicellular and can be of various sizes.
  • They are bilaterally symmetrical.
  • Their respiratory organs are lungs, book lungs, gills, book gills, skin, etc.
  • They have membrane-bound cell organelles that have a nucleus bounded by a nuclear membrane.
  • Blood circulation happens through blood, blood vessels, and heart.
  • They possess well-developed organs and organ systems like the circulatory system, skeleton system, respiratory system, etc.