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).

Heterocyst

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

Classification of Protista	Characteristic Features	Examples
Chrysophytes (chief producers in oceans)	Their cell walls form two overlapping shells, which are fit together and embedded with silica which, makes them indestructible. So, cell wall deposits and their accumulation leads to ‘diatomaceous earth’. This soil can be used to polish things, and filter oils and syrups.	Diatoms and golden algae (desmids)
Dinoflagellates	They show rapid multiplication and make the appearance of sea red (bioluminescence). Toxins released by them can kill other aquatic animals.	Red dinoflagellates (Example: Gonyaulax)
Euglenoids	They have a pellicle protein-rich layer) which keeps them flexible	Euglena
Slime Moulds	During suitable conditions, they form plasmodium and during unfavourable conditions, plasmodium differentiates and forms fruiting bodies (spores inside)	Acrasia, Plasmodiophorina
Protozoans	They are heterotrophs and live as predators or parasites. They are classified into four types	Plasmodium

Types of Protozoans and their Features/ Diseases Caused

Protozoans	Features/Diseases Caused	Examples
Amoeboid Protozoans	They form pseudopodia to capture their prey. Some of them are parasites	Amoeba, Entamoeba
Flagellated Protozoans	Sleeping sickness, a disease caused by the parasitic forms	Trypanosoma
Ciliated Protozoans	Cilia (locomotion) and gullet, a cavity is present on the body	Paramoecium
Sporozoans	Some species cause malaria	Plasmodium

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

Classification of Fungi	Characteristic Features	Examples
Phycomycetes	They grow on decaying wood in moist sites and obligate parasites on plant bodies	Mucor, Rhizopus (bread mould fungi) and Albugo (parasitic fungi on mustard)
Ascomycetes (sac-fungi)	Neurospora is used in biochemical and genetic work. Some are edible, e.g. morels and truffles	Penicillium, yeast, Aspergillus, Claviceps and Neurospora
Basidiomycetes	Some are edible, e.g. mushrooms. Mushrooms are rich in protein	Agaricus (mushroom), Ustilago (smut) and Puccinia (rust fungus), Mushrooms, bracket fungi, puffballs
Deuteromycetes	They are known as ‘Imperfect Fungi’ because in this group, only asexual or vegetative phases are seen. Some fungi of this group are saprophytes or parasites while the majority are decomposers of litter, which aid in mineral cycling	Alternaria, Colletotrichum and Trichoderma

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:

Viruses	Viroids	Prions
They are oblique intracellular agents	They are oblique intracellular agents	They are the abnormal form of a cellular protein
They have either DNA or RNA which is surrounded by a protein coat	They consist of only RNA. The protein coat is absent	They don’t possess DNA or RNA. Only protein coat is present

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. 

FAQs on Biological Classification

Q1: What are the Commercial Applications for Heterotrophic Bacteria and Archaea?

Answer:

Heterotrophic Bacteria: They help with nitrogen fixation, ammonification and nitrification. In addition, Rhizobium bacteria, they maintain soil fertility. Other members produce dairy products such as cheese and cottage cheese. Archaebacteria: Methanogens in animal feces produce biogas.

Q2: Write Some Plant like and Animal-like Features of Euglena.

Answer:

Plant-like features are:

• Euglena has plastids which help in photosynthesis
• Some of the species of euglena have carotenoid pigments, which give it red colour

Animal-like features are:

• Euglena doesn’t have a cell wall
• Flagella are present for locomotion

Q3: What Function Do Fungi Play in Our Daily Lives?

Answer:

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