Chapter 2 Biological Classification

The classification systems have undergone several changes with time. The first attempt of classification was made by Aristotle. He classified plants as herbs, shrubs, and trees. Animals, on the other hand, were classified on the basis of presence or absence of red blood cells. This system of classification failed to classify all the known organisms.

Therefore, Linnaeus gave a two kingdom system of classification. It consists of kingdom Plantae and kingdom Animalia. However, this system did not differentiate between unicellular and multicellular organisms and between eukaryotes and prokaryotes. Therefore, there were large numbers of organisms that could not be classified under the two kingdoms.

So Ernest Haeckel then separated unicellular eukaryotic organisms into separate kingdom named Protista and hence gave three kingdom classification.

After that Copeland,separated all prokaryotic organisms into a separate kingdom named Monera and hence Four-Kingdom systems of classification came into existence.

And lastly, a five kingdom system of classification was proposed by R.H. Whittakaer in 1969 to separate achlorophyllous,decomposer organism into a separate kingdom named Fungi. So five kingdoms are now

1) Monera

2) Protista

3) Fungi

4) Plantae

5) Animalia

(a) Heterotrophic bacteria

(1) Many bacteria like Lactobacillus helps in the production of curd from milk.

(2) They act as decomposers and help in the formation of humus e.g. Pseudomonas.

(3) Many antibiotics are obtained from some species of bacteria like streptomyces, Bacillus etc.

(4) Many soil bacteria help in fixation of atmospheric nitrogen like Rhizobium (Symbiotic), Azotobacter (Free living).

 

(b) Archaebacteria

(1) Methane gas is produced from the dung of ruminants by the methanogens.

(2) Methanogens are also involved in the formation of biogas and sewage treatment.

The cell walls construction of diatoms is known as Frustule which consists of two thin overlapping shells fitting into each other just as a soap-box. Their walls are made up of silica. When the diatoms die, the silica in their cell walls gets deposited in the form of diatomaceous earth. This diatomaceous earth is very soft and quite inert. It is used in filtration of oils, sugars, and for other industrial purposes.

Algal bloom refers to vigorous growth of algae or blue-green algae in water, due to enrichment of water body with minerals nutrients coming from fields, resulting in discoloration of the water body. This causes an increase in the biological oxygen demand (BOD), resulting in the death of fishes and other aquatic animals, due to suffocation.

Viroids are free RNA molecules of low molecular weight without any protein coat while viruses can have either RNA or DNA molecules encapsulated in a protein coat. Viroids are smaller in size than viruses.

Protozoans are microscopic unicellular eukaryotic organisms with heterotrophic mode of nutrition. Their nutrition may be holozoic, saprobic, or parasitic. These are divided into four major groups.

(1) Amoeboid protozoans or sarcodines

They are unicellular, jelly-like protozoa found in fresh or sea water and in moist soil. Their body lacks a periplast. Therefore, they may be naked or covered by a calcareous shell. They usually lack flagella and have temporary protoplasmic outgrowths called pseudopodia for locomotion. These pseudopodia or false feet help in movement and capturing prey. They include free living forms such as Amoeba or parasitic forms such as Entamoeba.

(2) Flagellated protozoans or zooflagellates

They are free living, non-photosynthetic flagellates without a cell wall. They possess flagella for locomotion and capturing prey. They include parasitic forms such as Trypanosoma, which causes sleeping sickness in human beings and free living forms such as Noctiluca.

(3) Ciliated protozoans or ciliates

They are aquatic individuals that form a large group of protozoa. Their characteristic features are the presence of numerous cilia on the entire body surface and the presence of two types of nuclei. All the cilia beat in the same direction to move the water laden food inside a cavity called gullet. They include organisms such as Paramaecium, Vorticella etc.

(4) Sporozoans

They include disease causing endoparasites and other pathogens. They are uninucleate and their body is covered by a pellicle. They do not possess cilia or flagella. They include the malaria causing parasite Plasmodium.

Plants have autotrophic mode of nutrition as they have the ability to synthesize their own organic food by process of photosynthesis due to presence of green pigments chlerophylum. However, some insectivorous plants are partially heterotrophic. They have various means of capturing insects so as to supplement their diet with Nitrogen derived from insects, as they live in nitrogen deficient soil. The examples include pitcher plant (Nepenthes), Venus fly trap, Bladderwort and sundew plant (Drosera).

Phycobiont refers to the algal component of the lichens and mycobiont refers to the fungal component. Both of these are present in symbiotic relationship in which Algae prepare food for Fungi due to presence of chlorophyll whereas the fungus provides shelter to algae and absorbs water and nutrients from the soil.

(A) Phycomycetes- This group of fungi includes members such as Rhizopus, Albugo, etc.

(i) Mode of nutrition

They are obligate parasites on plants or are found on decaying matter such as wood.

(ii) Mode of reproduction

Asexual reproduction takes place through motile zoospores or non-motile aplanospores produced endogenously in sporangium.

Sexual reproduction may be of isogamous, anisogamous, or oogamous type resulting in the formation of thick-walled zygospore.

(B) Ascomycetes- This group of fungi includes members such as Penicillium, Aspergillus, Claviceps, and Neurospora.

(i) Mode of nutrition

They are sporophytic, decomposers, parasitic or coprophilous (growing on dung).

(ii) Mode of reproduction

Asexual reproduction occurs through asexual spores produced exogenously, such as conidia produced on conidiophores.

Sexual reproduction takes place through ascospores produced endogenously in sac-like asci and arranged inside ascocarps.

(C) Basidiomycetes- This group of fungi includes members such as Ustilago, Agaricus and Puccinia.

(i) Mode of nutrition

They grow as decomposers in soil or on logs and tree stumps. They also occur as parasites in plants causing diseases such as rusts and smuts.

(ii) Mode of reproduction

Asexual reproduction takes place commonly through fragmentation. Asexual spores are absent.

Sex organs are absent but sexual reproduction takes place through plasmogamy. It involves fusion of two different strains of hyphae. The resulting dikaryon gives rise to a basidium. Four basidiospores are produced inside a basidium.

(D) Deuteromycetes – This group of fungi includes members such as Alternaria, Trichoderma, and Colletotrichum.

(i) Mode of nutrition

Some members are saprophytes while others are parasites. However, a large number act as decomposers of leaf litter.

(ii) Mode of reproduction

Asexual reproduction is the only way of reproduction in deuteromycetes. Which occurs through asexual spores called conidia.

Sexual reproduction is absent in deuteromycetes.

Some characteristic features of Euglenoids are as follows.

• Euglenoids (such as Euglena) are unicellular protists commonly found in fresh water.

• Instead of cell wall, a protein-rich cell membrane known as pellicle is present.

• They bear two flagella on the anterior end of the body.

• A small light sensitive eye spot is present.

• They contain photosynthetic pigments such as chlorophyll and can thus prepare their own food. However, in absence of light, they behave similar to heterotrophs by capturing other small aquatic organisms.

• They have both plant and animal-like features, which makes them difficult to classify and hence they are called as connecting link between plants and animals.

Viruses are sub-microscopic, infectious, nucleoprotein particles that can infect all living organisms. A virus consists of genetic material either in the form of RNA or DNA surrounded by a protein coat.

Most of the viruses, infecting plants, have single stranded RNA as genetic material. On the other hand, the viruses infecting animals have single or double stranded RNA or double stranded DNA.

Bacteriophages or viruses infecting bacteria mostly have double stranded DNA. Their protein coat called capsid is made up of capsomere subunits. These capsomeres are arranged in helical or polyhedral geometric forms.

A.I.D.S, small pox, mumps, and influenza are some common examples of viral diseases.

Viruses are microscopic organisms that have characteristics of both living and non-living. A virus consists of a strand of DNA or RNA covered by a protein coat. This presence of nucleic acid (DNA or RNA) and Protein suggests that viruses are alive. In addition, they can also respond to their environment in a limited manner inside the host cell.

However, some other characters, such as their inability to reproduce without using the host cell machinery and their acellular nature, indicate that viruses are non-living. Therefore, classifying viruses has remained a mystery for modern systematics.