Biological classification

Biological classification 

• Biological classification is defined as the process of grouping the organisms according to certain similarities Linnaeus proposed the two kingdoms of classification. 
• He classified organism into animal kingdom as Animalia and plant kingdom as Plantae. 
• There were certain limitations associated with two kingdom classification such as- it does not differentiate between eukaryotes and prokaryotes, unicellular and multicellular organisms, and photosynthetic and non-photosynthetic organisms. 

Five kingdom classifications 

• R.H. Whittaker proposed the five kingdoms of classification. 
• The five kingdoms classification is as follows- 

1. Monera, 
2. Protista, 
3. Fungi, 
4. Plantae, and 
5. Animalia. 

• The classification was based on thallus organization, cell structure, mode of nutrition, phylogenetic relationship, and reproduction.

          Fig.1. Five kingdom classifications 

Kingdom Monera 

• The most important members of Monera are Bacteria. 
• All organisms present in this kingdom are prokaryotes. 
• It includes bacteria as well as blue-green algae. 
• They can be present in extreme habitats such as hot springs, deserts, snow as well as deep oceans. 
• Certain organisms act as parasites also. 
• All monerans are prokaryotes. 
• They do not have the true nucleus, that is, nucleus not surrounded by nuclear membrane. 
• They can be autotrophs, heterotrophs as well as parasites. 
• They do have cell wall. 
• There respiration can be aerobic as well as anaerobic. 
• The movement occurs through flagella. 
• Exchange of nutrients or gases occurs by diffusion.

        Fig.2. Examples of Kingdom Monera

• They are divided into Archaebacteria and Eubacteria.

Archaebacteria

• Archaebacteria can be thermoacidophiles (hot springs), halophiles (survive in high salt concentration), and methanogens (marshy areas). 
• There structure of cell wall is different from other monerans. 
• Methanogens are found in the gut of some ruminants such as buffaloes and cows. 
• This helps in production of biogas from the dung of the animal.

Eubacteria

• They are referred as “True bacteria”. 
• The presence of motile flagellum and a rigid cell wall characterize them.
• It includes cyanobacteria (blue-green algae). They are like plants in having chlorophyll a. 
• So, known as photosynthetic autotrophs.

Cyanobacteria 

• They are unicellular, filamentous algae. 
• They form colonies which are surrounded by gelatinous sheath.
• Some have specialized structures known as heterocyst which helps in fixing nitrogen. For example, Nostoc.
• There are certain bacteria that can oxidize certain inorganic substances such as nitrites, nitrates, they are known as chemosynthetic autotrophs. 
• The energy released during oxidation is used in the production of the ATP. 

• Bacteria can reproduce asexually by fission. They can also reproduce through spores under unfavorable conditions. 

Monera also includes Mycoplasma. 

• They do not have cell wall and survive in anaerobic conditions. 
• Mycoplasma are the smallest living organisms known.

Kingdom Protista

• Single celled eukaryotes are kept under Protista. 
• They are mostly aquatic in nature. For example, 
• Algae serves as primary producers in aquatic ecosystem. 
• They are unicellular, eukaryotic organisms. They have true nucleus, bounded by nuclear membrane. 
• They can move by cilia as well as via flagella. 
• Respiration can be both aerobic as well as anaerobic. 
• Similarly, reproduction can be sexual as well as asexual.
• They are divided into Chrysophytes, Dinoflagellates, Euglenoids, Slime moulds and Protozoans.

1. Chrysophytes

• It consists of diatoms and golden algae. 
• They can live in freshwater water as well as marine water. 
• Mostly they are photosynthetic. 
• Their cell wall is impregnated with silica. 
• They are the major producers in the oceans. 
• Mostly they are unicellular flagellates. But some are amoeboid. 

Nostoc

Fig.4. Diatoms

2. Dinoflagellates

• Dinoflagellates are marine and photosynthetic organisms. 
• They can be of different colors such as yellow, brown, red, or blue in color. 
• The color is due to the presence of different colored pigments present in the cells of the dinoflagellates. 
• The cell wall is made up of cellulose. 
• They are biflagellate with one flagella placed longitudinally and other flagella placed transversely. For example, Gonyaulax catenella, Noctiluca scintillans.

3. Euglenoids

• Euglenoids are freshwater organisms that are found in stagnant water. 
• They are surrounded by a protein rich layer known as pellicle. 
• They have one short flagella and one long flagella. 
• They can behave as autotrophic as well as heterotrophic organisms. For example, Euglena.
• Most of the euglenoids have chloroplast so they can synthesize their own food. 
• The reserve food material found is euglenoids are Paramylon (carbohydrate).

4. Slime moulds

• They are saprophytic in nature. 
• Under favorable conditions, they form aggregates which are known as Plasmodium. 
• During unfavorable condition, they form fruiting bodies with spores. 
• They possess true cell walls.
• They are extremely resistant to adverse conditions.

5. Protozoan like protists

• They are heterotrophs. 
• They can live as predators as well as parasites. 
• There are four major types of protozoans found-

Amoeboid protozoans 

• They are found in fresh water, moist soil, or sea water. 
• They possess pseudopodia for engulfing the food particles, such as Amoeba.

Flagellated protozoans 

• They can be free-living or parasitic. 
• This group of organisms are responsible for different parasitic diseases. For example, Trypanosoma causes sleeping sickness.

Ciliated protozoans 

• They possess thousands of cilia. 
• The movement of cilia helps in propelling forward or backward. 
• It also helps in obtaining food from the outside. For example, Paramecium.

                 Fig.7. Paramecium

Sporozoans 

• They form reproductive cells which are known as spores. 
• They are parasitic and pathogenic in nature. For example, Plasmodium species causes Malaria. 
• Reproduction can be asexual or sexual.

Kingdom Fungi

• Fungi are heterotrophic in nature. 
• Some acts on dead organic matter and so known as saprophytes. 
• Some can feed on living organisms so known as parasites. 
• Some fungi can live in mutual relationship with other organisms such as algae, they are known as lichens. Such fungi are known as symbionts. 
• Lichens are symbiotic association between the fungi and the algae. 
• Mycorrhiza are the symbiotic association between the fungi and the roots of the higher plants. 
• Reproduction in fungi can occur by- budding, fragmentation, and fission. 
• Asexual reproduction occurs by means of spores known as conidia, zoospores or sporangiospores. 
• Sexual reproduction occurs by oospores, ascospores and basidiospores. 
• There are three steps of sexual cycle in fungi-the first step is the fusion of the protoplasm which is known as plasmogamy. 
• Then, the fusion of nuclei occurs which is known as karyogamy. 
• This results in the formation of the zygote. Then finally, the meiosis of zygote results in the formation of the spores.
• The vegetative part of the fungus is known as mycelium. 
• The network of hyphae is known as mycelium. 
• During sexual cycle of fungi, two haploid hyphae fuses to form diploid cells. 

Kingdom fungi is divided into different classes-

Phycomycetes (lower fungi)-

• Asexual reproduction occurs by motile spores known as zoospores and non-motile spores are known as aplanospores. 
• Spores are produced inside the sporangium. Fusion of gametes give rise to zygospore. 
• Mycelium is coenocytic (multiple nucleus) and multicellular. For example, such as Mucor, Rhizopus, Albugo.

•Ascomycetes (sac fungi)-

• They can be saprophytic as well as parasitic. Asexual reproduction occurs by conidia. 
• Sexual reproduction occurs through ascospores. 
• Branched and septate mycelium is found in Ascomycetes. For example, Penicillium, Aspergillus, Claviceps etc.

                

                 Fig.10. Ascomycetes

Basidiomycetes (club fungi)-

• Vegetative reproduction occurs through fragmentation. 
• Absence of sex organs. 
• Plasmogamy between two vegetative or somatic cells results in the formation of basidium. 
• Basidium undergo karyogamy and meiosis to form four basidiospores. For example, Agaricus (mushrooms), Ustilago (smut) etc.

Fig.11. Agaricus

Deuteromycetes 

• Deuteromycetes are commonly known as imperfect fungi as they do not have sexual reproduction at any stage of the life. 
• The asexual reproduction occurs by conidia. Mycelium is septate and branched. 
• For example, Alternaria, Trichoderma etc.

Zygomycetes (conjugation fungi) 

• Zygomycetesare primitive group of fungi. Asexual reproduction occurs by non-motile sporangiospores.
• For example, Rhizopus.

Kingdom Plantae

• It includes all photosynthetic, multicellular, eukaryotic plants. 

Characteristics of members of Plantae are as

follows-

• Mostly they are eukaryotic in nature.
• The main pigment present is chlorophyll.
• Their cell wall is made up of cellulose.
• Photosynthesis helps in the synthesis of the food.
• Reproduction can be sexual as well as asexual.
• They exhibit the phenomenon of alternation of generation, that is, diploid sporophyte and haploid gametophyte.

Kingdom Animalia

• They are heterotrophic, eukaryotic organisms. 

Some of the characteristics of members of Animalia are as follows-

• They are multicellular organisms with variable size.
• They have well developed organ system such as skeleton system, circulatory system, respiratory system etc
• They are bilaterally symmetrical.
• They have well developed locomotory organs.
• Respiration occurs by gills, book lungs, book gills, skin, lungs etc.
• Membrane bound cell organelles with nucleus bounded by a nuclear membrane.
• Circulation occurs via blood, blood vessels, and heart.
• Reproduction occurs by the formation of the haploid gametes. The fusion of gametes give rise to a new diploid organism.
• Kidneys are the main respiratory organs.

Viruses:

• They are acellular structures and therefore do not find a place in Whittaker’s five kingdom classification. 
• They consist of a nucleic acid (either DNA or RNA) that is surrounded by a protein coat. 
• They can grow and multiply only inside a host cell. 
• Outside the host cell the viruses exist as crystals. 
• They cause diseases and damage to the host. 
• Examples of common viruses are the viruses causing common cod, influenza, polio, AIDS, etc.

Viroids:

• They are the smallest structures known that are capable of infection. 
• They consist only the nucleic acid without the protein coat.

Lichens:

• They are symbiotic associations of algae and fungi. 
• The algal partner is autotrophic and synthesizes and provides food. 
• The fungal partner offers protection and shelter.