Biological Classification

Notes for NCERT Class 11 Chapter 2 Biological Classification: Biological classification is the process by which biologists group living organisms which, are classified on the basis of their similarity. Classification is essential for the convenient study of living organisms. It is required to identify different varieties of organisms. It helps in the correct identification of many organisms. It leads to the evolution of organisms. It also establishes phylogenetic relationships among organisms. Carolus Linneuas was one of the scientists to classify organisms.

NCERT Class 11 Biology Chapter 02 Biological Classification: The practice of classifying organisms based on shared characteristics is known as biological classification. Linnaeus proposed two areas of classification. He divided organisms into two kingdoms: the animal kingdom (Animalia) and the plant kingdom (Plantae). The classification of the two kingdoms had some disadvantages, such as the impossibility of distinguishing between eukaryotes and prokaryotes, unicellular and multicellular species, and photosynthetic and non-photosynthetic organisms. As a result, the field continued to grow and served as a primary example of R.H. Whittaker's classification of the five domains or kingdoms.

Biological Classification

Two Kingdom Classification

Two kingdom classification was given by a biologist, Carolus Linnaeus. He classified organisms into two kingdoms, i.e. Plantae (included all plants) and Animalia (included all animals). 

Disadvantages of Two Kingdom Classification

This system didn't distinguish between the following types of organisms-

1. Eukaryotes and prokaryotes
2. Unicellular and multicellular organisms
3. Photosynthetic (green algae) and non-photosynthetic (fungi) organisms

Five Kingdom Classification

In 1969, R.H. Whittaker proposed the five-kingdom classification. He classified those five kingdoms as Monera, Protista, Fungi, Plantae, and Animalia. He primarily used the following criteria for classification:

1. Cell structure
2. Body organisation
3. Mode of nourishment
4. Reproduction
5. Phylogenetic linkages or relationships

Kingdom Monera

Bacteria are the main members of this kingdom. Kingdom Monera is further divided into:

1. Archaebacteria 
2. Eubacteria or true bacteria

Archaebacteria

They are special bacteria as they can withstand extreme environmental conditions because of their different cell wall structure. They can be:

• Thermoacidophiles: They are found in the hot springs 
• Halophiles: They are found in the salty areas
• Methanogens: They are found in the marshy areas/ gut of ruminant animals (production of biogas)

Eubacteria or True Bacteria

They have rigid cell walls and flagellum (locomotion), if motile. They can be photosynthetic autotrophs, chemosynthetic autotrophs and heterotrophs.

• Photosynthetic Autotrophs: Cyanobacteria (blue-green algae, have chlorophyll a), Nostoc and Anabaena are their common examples. They are surrounded by a gelatinous sheath or mucilaginous covering, which protects them from wetting. They fix atmospheric nitrogen in specialised cells called heterocysts (significance)
• Chemosynthetic Autotrophs: These are the bacteria which oxidise inorganic substances, e.g. nitrates, nitrites and ammonia and use the released energy for ATP production. They recycle nutrients, e.g. nitrogen, phosphorous, iron and sulphur (significance).
• Heterotrophs: They are decomposers. Some of them are pathogens and some are beneficial as they are helpful in making curd from milk, producing antibiotics, and fixing atmospheric nitrogen in leguminous plants (significance).

Reproduction in Bacteria 

They reproduce by asexual mode- binary fission, sexual mode- transfer of DNA and spore formation in unfavourable conditions.

Mycoplasma

They are the smallest organisms which lack cell walls. They can survive in the absence of oxygen (anaerobic). They cause diseases (pathogens).

Kingdom Protista

They are single-celled eukaryotes. They include:

1. Chrysophytes
2. Dinoflagellates
3. Euglenoids
4. Slime moulds
5. Protozoans

Types of Protozoans and their Features/ Diseases Caused

Kingdom Fungi

Fungi are multicellular and the how heterotrophic mode of nutrition (saprophytes/parasites/symbiotic- mycorrhiza). Some fungi are unicellular, e.g. yeast.

Some Useful Fungi

Mushrooms and yeast are the most valuable fungi. Mushrooms are edible and are a good source of proteins. Yeast is used to make bread and cheese. Penicillium fungi are used to produce antibiotics.

Some Harmful Fungi

Some fungi cause diseases in both plants and animals, e.g. wheat rust disease is caused by Puccinia fungus.

Reproduction in Fungi

There are three modes of reproduction in fungi, i.e. vegetative, asexual and sexual.

• Vegetative Propagation: It takes place by fragmentation, fission and budding.
• Asexual Reproduction: It takes place by conidia or sporangiospores or zoospores.
• Sexual Reproduction: It takes place by oospores, ascospores and basidiospores.

Stages of Sexual Reproduction in Fungi

1. Plasmogay- It is the fusion of protoplasms between two motile or non-motile gametes.
2. Karyogamy- It is the fusion of two nuclei.
3. Meiosis in zygote, gives rise to haploid spores.

In ascomycetes and basidiomycetes, the dikaryotic stage (n + n, i.e., two nuclei per cell) is formed, known as dikaryon and the phase is dikaryophase.

Four Major Groups of Fungi

Kingdom Plantae

All eukaryotic organisms that contain chlorophyll, usually known as plants, are classified as Plantae. A few species, like parasites and plants that feed on insects, are partially heterotrophic. Insectivorous plants include bladderwort and Venus fly traps, and parasites like Cuscuta feed on them. The eukaryotic structure of plant cells has large chloroplasts and a cell wall comprised primarily of cellulose. Algae, bryophytes, pteridophytes, gymnosperms, and angiosperms are all part of the plant kingdom.

Alternation of Generation

The haploid gametophytic and the diploid sporophytic phases of a plant's life cycle alternate with one another. Various plant families have different haploid and diploid phase lengths and whether they are independent or reliant on others.

Kingdom Animalia

They are multicellular and heterotrophic (show the holozoic mode of nutrition) eukaryotes. They lack cell walls. Almost, all the animals show locomotion. Sexual reproduction occurs by the fusion of male and female gametes which give rise to an embryo followed by repeated cell divisions.

Viruses, Viroids, Prions And Lichens

The differences between viruses, viroids and prions are given below:

Bacteriophage

Bacteriophages are also known as phages. These are the viruses which infect and replicate in the bacterial cells. 

Tobacco Mosaic Virus

The tobacco mosaic virus (TMV) consists of single-stranded RNA. It infects tobacco plants and members of the family Solanaceae. The infection can cause some patterns like a mosaic, which shows mottling and discolouration on the surface of the leaves.

Lichens

The close association of fungus and algae form lichens. They are found in a pollution-free environment. Lichens are used in deodorant, pH papers, insense-sticks, toothpaste and perfumes. The fungal component is known as mycobiont and the algal component is known as phycobiont.