Biological Classification | CBSE Class 11 Biology Notes

This note covers NCERT Class 11 Biology Chapter 2, Biological Classification: how classification systems changed, Whittaker's five kingdoms, and the kingdoms Monera, Protista and Fungi in detail, with Plantae and Animalia in outline, followed by viruses, viroids, prions and lichens. Questions on this chapter mostly ask which group has which feature, so the groups are compared in tables.

How did classification systems change over time?

Since the dawn of civilisation there have been many attempts to classify living organisms. At first this was done instinctively, not with scientific criteria, out of a need to use organisms for food, shelter and clothing.

Why the two kingdom system was found inadequate

Classifying organisms into plants and animals was easy to do and to understand, but a large number of organisms did not fall into either category. The two kingdom system did not distinguish between:

• eukaryotes and prokaryotes,
• unicellular and multicellular organisms, and
• photosynthetic organisms (green algae) and non-photosynthetic organisms (fungi).

Besides gross morphology, a need was felt to include other characteristics like cell structure, nature of the wall, mode of nutrition, habitat, methods of reproduction and evolutionary relationships. Plant and animal kingdoms have been a constant under all systems, but what is included in them, and the number and nature of the other kingdoms, have been understood differently over time.

How do the five kingdoms compare?

NCERT Table 2.1 compares the characteristics of the five kingdoms.

Note: Read the table down each column. Only Monera is prokaryotic and lacks a nuclear membrane. In Table 2.1 only Animalia is shown with the cell wall absent; in Protista it is present in some. The wall material separates the rest: chitin in fungi, cellulose in plants.

Why did the five kingdom system regroup organisms?

Earlier classification systems included bacteria, blue green algae, fungi, mosses, ferns, gymnosperms and angiosperms under "Plants". The character that unified this whole kingdom was that all the organisms had a cell wall. This placed together groups which differed widely in other characteristics.

Kingdom Protista brought together Chlamydomonas and Chlorella, which were earlier placed in algae within plants and have cell walls, with Paramoecium and Amoeba, which were earlier placed in the animal kingdom and lack a cell wall. This happened because the criteria for classification changed, and such changes will take place in future too. The aim is a classification that is phylogenetic, that is based on evolutionary relationships, as well as on morphological, physiological and reproductive similarities.

What are the features of Kingdom Monera?

Bacteria are the sole members of Kingdom Monera. They are the most abundant micro-organisms and occur almost everywhere. Hundreds of bacteria are present in a handful of soil. They also live in extreme habitats such as hot springs, deserts, snow and deep oceans, and many live in or on other organisms as parasites.

Four groups of bacteria are drawn in a row. The cocci are small round dots. The bacilli are short rods, one of them with a spore inside. The spirilla are long wavy threads, with a flagellum labelled. The vibrio are curved, comma-shaped cells.

See Fig. 2.1 in your NCERT textbook

Though the bacterial structure is very simple, bacteria are very complex in behaviour. As a group they show the most extensive metabolic diversity.

• Some are autotrophic: they synthesise their own food from inorganic substrates, either as photosynthetic autotrophs or as chemosynthetic autotrophs.
• The vast majority are heterotrophs: they depend on other organisms or on dead organic matter for food.

Archaebacteria

Archaebacteria are special because they live in some of the most harsh habitats. They differ from other bacteria in having a different cell wall structure, and this feature is responsible for their survival in extreme conditions.

Methanogens in the gut of ruminants are responsible for the production of methane (biogas) from the dung of these animals.

What are eubacteria and mycoplasma?

There are thousands of different eubacteria or "true bacteria". They are characterised by the presence of a rigid cell wall, and, if motile, a flagellum.

Nostoc is shown as a winding chain of oval cells, like a string of beads, surrounded by a thin mucilaginous sheath. Two cells with purple contents are the heterocysts, in which nitrogen is fixed; one is labelled.

See Fig. 2.2 in your NCERT textbook

Reproduction in bacteria

• Bacteria reproduce mainly by fission.
• Under unfavourable conditions they sometimes produce spores.
• They also reproduce by a sort of sexual reproduction, adopting a primitive type of DNA transfer from one bacterium to another.

Mycoplasma

The Mycoplasma are organisms that completely lack a cell wall. They are the smallest living cells known and can survive without oxygen. Many mycoplasma are pathogenic in animals and plants.

What is Kingdom Protista?

All single-celled eukaryotes are placed under Protista, but the boundaries of this kingdom are not well defined: what may be "a photosynthetic protistan" to one biologist may be "a plant" to another. NCERT includes five groups: chrysophytes, dinoflagellates, euglenoids, slime moulds and protozoans.

• Members of Protista are primarily aquatic.
• The kingdom forms a link with the others dealing with plants, animals and fungi.
• Being eukaryotes, protists have a well defined nucleus and other membrane-bound organelles. Some have flagella or cilia.
• Protists reproduce asexually, and sexually by a process involving cell fusion and zygote formation.

Diatomaceous earth and red tides

Because diatom walls are indestructible, diatoms have left behind large amounts of cell wall deposits in their habitat. This accumulation over billions of years is called diatomaceous earth. Being gritty, it is used in polishing and in the filtration of oils and syrups.

Red dinoflagellates such as Gonyaulax very often multiply so rapidly that they make the sea appear red. These are red tides. Toxins released by such large numbers may even kill other marine animals such as fishes.

Slime moulds under unfavourable conditions

1. During unfavourable conditions the plasmodium differentiates and forms fruiting bodies.
2. The fruiting bodies bear spores at their tips.
3. The spores possess true walls, are extremely resistant and survive for many years, even under adverse conditions.
4. The spores are dispersed by air currents.

Four protists are drawn one below the other. (a) A dinoflagellate, covered in stiff plates, with a groove running round the upper part of the cell and a flagellum trailing from it. (b) Euglena, a spindle-shaped cell with a long flagellum. (c) A slime mould, a spreading irregular mass. (d) Paramoecium, a slipper-shaped cell covered in fine cilia.

See Fig. 2.4 in your NCERT textbook

What are the four groups of protozoans?

Note: Match each protozoan group to how it moves: pseudopodia for amoeboid, flagella for flagellated, cilia for ciliated. Sporozoans are defined not by movement but by the infectious spore-like stage.

What are the features of Kingdom Fungi?

The fungi constitute a unique kingdom of heterotrophic organisms, with great diversity in morphology and habitat. They are cosmopolitan and occur in air, water and soil and on animals and plants. They prefer to grow in warm and humid places, which is why food kept in a refrigerator does not go bad so quickly.

Structure and nutrition

• With the exception of yeasts, which are unicellular, fungi are filamentous. Their bodies consist of long, slender thread-like structures called hyphae, and the network of hyphae is known as the mycelium.
• Some hyphae are continuous tubes filled with multinucleated cytoplasm; these are coenocytic hyphae. Others have septae or cross walls.
• The cell walls of fungi are composed of chitin and polysaccharides.
• Most fungi absorb soluble organic matter from dead substrates and are called saprophytes. Those that depend on living plants and animals are parasites.
• They can also live as symbionts: with algae as lichens, and with the roots of higher plants as mycorrhiza.

Reproduction

The sexual cycle involves three steps.

1. Plasmogamy: fusion of protoplasms between two motile or non-motile gametes.
2. Karyogamy: fusion of two nuclei.
3. Meiosis in the zygote, resulting in haploid spores.

In some fungi the fusion of two haploid cells immediately results in diploid cells (2n). In the ascomycetes and basidiomycetes the sequence has an extra stage:

1. Two haploid hyphae of compatible mating types come together and fuse.
2. An intervening dikaryotic stage (n + n, two nuclei per cell) follows. This condition is called a dikaryon, and the phase is the dikaryophase.
3. Later the parental nuclei fuse and the cells become diploid.
4. The fungi form fruiting bodies in which reduction division occurs, leading to the formation of haploid spores.

The morphology of the mycelium, the mode of spore formation and the fruiting bodies form the basis for dividing the kingdom into classes.

How do the four classes of fungi differ?

• Neurospora is used extensively in biochemical and genetic work, and many ascomycetes like morels and truffles are edible delicacies.
• In basidiomycetes the dikaryotic structure formed by plasmogamy gives rise to the basidium, where karyogamy and meiosis take place.
• Deuteromycetes are called imperfect fungi because only their asexual or vegetative phases are known. When the perfect (sexual) stages of a member are discovered, it is moved to its rightful class, often ascomycetes or basidiomycetes.

Three fungi are shown. (a) Mucor, in a photograph: a cottony web of fine hyphae dotted with small round sporangia. (b) Aspergillus, with a hypha ending in a swollen head that bears chains of conidia. (c) Agaricus, the familiar umbrella-shaped mushroom with a cap and a stalk.

See Fig. 2.5 in your NCERT textbook

What are Kingdoms Plantae and Animalia?

What are viruses, viroids, prions and lichens?

Whittaker's five kingdom classification makes no mention of lichens or of acellular organisms like viruses, viroids and prions.

Viruses

How viruses and viroids were discovered

1. 1892: Dmitri Ivanowsky recognised certain microbes, smaller than bacteria, as the cause of the mosaic disease of tobacco.
2. 1898: M.W. Beijerinck showed that the extract of infected tobacco plants could infect healthy plants and named the pathogen "virus".
3. 1935: W.M. Stanley showed that viruses could be crystallised and that the crystals consist largely of proteins.
4. 1971: T.O. Diener discovered viroids, infectious agents smaller than viruses.

Viruses did not find a place in classification because they are not considered truly "living", if living means having a cell structure. They are non-cellular organisms with an inert crystalline structure outside the living cell. Once they infect a cell, they take over the machinery of the host cell to replicate themselves, killing the host. Viruses are obligate parasites. The word virus means venom or poisonous fluid.

• Besides proteins, viruses contain genetic material, either RNA or DNA. No virus contains both. A virus is a nucleoprotein, and the genetic material is infectious.
• Plant viruses generally have single-stranded RNA. Animal viruses have either single or double stranded RNA or double stranded DNA. Bacteriophages are usually double stranded DNA viruses.
• The protein coat, the capsid, is made of small subunits called capsomeres, arranged in helical or polyhedral geometric forms. It protects the nucleic acid.
• Viruses cause mumps, small pox, herpes and influenza, and AIDS in humans. In plants the symptoms can be mosaic formation, leaf rolling and curling, yellowing and vein clearing, dwarfing and stunted growth.

Part (a) shows the tobacco mosaic virus as a rod built of many capsomeres wound in a helix, with a strand of RNA running inside; the labels are RNA and capsid. Part (b) shows a bacteriophage: a many-sided head on top of a collar, a sheath below it and thin tail fibres spreading out at the bottom.

See Fig. 2.6 in your NCERT textbook

Lichens

Lichens are symbiotic, that is mutually useful, associations between algae and fungi.

The association is so close that a lichen in nature would never seem to be two different organisms. Lichens are very good pollution indicators: they do not grow in polluted areas.

Glossary

• Five Kingdom Classification — Whittaker's 1969 system of Monera, Protista, Fungi, Plantae and Animalia, based on cell structure, body organisation, nutrition, reproduction and phylogeny.
• Phylogenetic — Based on evolutionary relationships, as a modern classification system aims to be.
• Archaebacteria — Bacteria with a different cell wall structure that live in harsh habitats: halophiles, thermoacidophiles and methanogens.
• Heterocysts — Specialised cells of some cyanobacteria, such as Nostoc and Anabaena, in which atmospheric nitrogen is fixed.
• Mycoplasma — Organisms that completely lack a cell wall; the smallest living cells known, able to survive without oxygen.
• Diatomaceous earth — Gritty deposits of indestructible diatom cell walls accumulated over billions of years, used in polishing and filtration.
• Pellicle — The protein-rich layer that replaces a cell wall in euglenoids and makes their body flexible.
• Plasmodium (of slime moulds) — The aggregation formed by slime moulds under suitable conditions, which may spread over several feet.
• Mycelium — The network of hyphae that forms the body of a filamentous fungus.
• Coenocytic hyphae — Fungal hyphae that are continuous tubes filled with multinucleated cytoplasm, without cross walls.
• Dikaryon — The condition of two nuclei per cell (n + n) in ascomycetes and basidiomycetes before the nuclei fuse.
• Capsid — The protein coat of a virus, made of capsomeres, which protects the nucleic acid.
• Viroid — An infectious agent smaller than a virus, made of free RNA without a protein coat.
• Phycobiont — The algal, autotrophic partner of a lichen, which prepares food for the fungus.

Common errors and misconceptions

• Misconception: Whittaker's classification was proposed by Linnaeus. Correct: Linnaeus' time saw the two kingdom system. R.H. Whittaker proposed the five kingdom classification in 1969.
• Misconception: Blue-green algae are algae in Kingdom Plantae. Correct: Blue-green algae are cyanobacteria, prokaryotes placed in Kingdom Monera.
• Misconception: Fungal cell walls are made of cellulose. Correct: Fungal cell walls are made of chitin and polysaccharides. Cellulose walls belong to plants.
• Misconception: Euglenoids have a cell wall like plants. Correct: Instead of a cell wall they have a flexible protein-rich pellicle.
• Misconception: Plasmodium is the name of the slime mould body. Correct: The slime mould aggregation is a plasmodium, but Plasmodium in italics is the sporozoan malarial parasite.
• Misconception: Deuteromycetes reproduce sexually by ascospores. Correct: Deuteromycetes reproduce only by asexual spores called conidia, which is why they are called imperfect fungi.
• Misconception: Viroids are small viruses with a protein coat. Correct: Viroids are free RNA that lacks the protein coat found in viruses.
• Misconception: Some viruses contain both DNA and RNA. Correct: A virus contains either RNA or DNA. No virus contains both.

Exam-style questions with model answers

Q1. What is the nature of the cell walls in diatoms? [1 mark]

1. In diatoms the cell walls form two thin overlapping shells that fit together as in a soap box; they are embedded with silica and are therefore indestructible.

Q2. How are viroids different from viruses? [2 marks]

1. Viroids, discovered by T.O. Diener in 1971, are smaller than viruses and consist of free RNA of low molecular weight.
2. They lack the protein coat that is found in viruses. A viroid causes potato spindle tuber disease.

Q3. What do the terms phycobiont and mycobiont signify? [2 marks]

1. In a lichen the algal component is the phycobiont; it is autotrophic and prepares food for the fungus.
2. The fungal component is the mycobiont; it is heterotrophic and provides shelter and absorbs mineral nutrients and water for its partner.

Q4. State two economically important uses each of heterotrophic bacteria and archaebacteria. [3 marks]

1. Heterotrophic bacteria are helpful in making curd from milk and in the production of antibiotics; they also fix nitrogen in legume roots.
2. The majority of heterotrophic bacteria are important decomposers.
3. Among archaebacteria, methanogens in the gut of ruminants such as cows and buffaloes are responsible for the production of methane (biogas) from the dung of these animals, and this biogas is used as a fuel. Methanogens also produce biogas from sewage sludge in anaerobic digesters.

Q5. What are the characteristic features of euglenoids? [3 marks]

1. The majority are fresh water organisms found in stagnant water. Instead of a cell wall they have a protein-rich pellicle that makes the body flexible.
2. They have two flagella, a short and a long one.
3. They are photosynthetic in sunlight but behave like heterotrophs when deprived of sunlight, predating on smaller organisms. Their pigments are identical to those of higher plants. Example: Euglena.

Q6. Describe briefly the four major groups of protozoans. [3 marks]

1. Amoeboid protozoans move and capture prey by pseudopodia, as in Amoeba; marine forms have silica shells and some, such as Entamoeba, are parasites. Flagellated protozoans have flagella; parasitic forms like Trypanosoma cause sleeping sickness.
2. Ciliated protozoans such as Paramoecium move by thousands of cilia, which steer water laden with food into a gullet.
3. Sporozoans have an infectious spore-like stage in their life cycle; Plasmodium causes malaria.

Q7. Give a comparative account of the classes of Kingdom Fungi under mode of nutrition and mode of reproduction. [5 marks]

1. Phycomycetes: found in aquatic habitats, on decaying wood (saprophytic) or as obligate parasites on plants. Asexual reproduction by zoospores or aplanospores produced endogenously in sporangium; sexual reproduction by zygospore formed by fusion of gametes.
2. Ascomycetes: saprophytic, decomposers, parasitic or coprophilous. Asexual spores are conidia produced exogenously on conidiophores; sexual spores are ascospores produced endogenously in asci within ascocarps.
3. Basidiomycetes: grow in soil and on logs and tree stumps (saprophytic) and in living plants as parasites such as rusts and smuts. Asexual spores are generally absent; vegetative reproduction by fragmentation is common. Plasmogamy is by fusion of somatic cells, and four basidiospores are produced exogenously on the basidium.
4. Deuteromycetes: saprophytes, parasites or decomposers of litter. They reproduce only by asexual conidia; no sexual stage is known.
5. Examples in order: Mucor and Rhizopus; Penicillium and Aspergillus; Agaricus and Puccinia; Alternaria and Trichoderma.

Q8. Give a brief account of viruses with respect to their structure and genetic material, and name four viral diseases. [5 marks]

1. Viruses are non-cellular obligate parasites, with an inert crystalline structure outside the living cell. Inside a host cell they take over its machinery to replicate themselves.
2. A virus is a nucleoprotein. Its protein coat, the capsid, is made of subunits called capsomeres arranged in helical or polyhedral forms, and it protects the nucleic acid.
3. The genetic material is either RNA or DNA, never both, and it is infectious. Plant viruses generally have single stranded RNA.
4. Animal viruses have single or double stranded RNA or double stranded DNA, and bacteriophages usually have double stranded DNA.
5. Viral diseases include mumps, small pox, herpes and influenza; AIDS in humans is also caused by a virus.

Key takeaways

• Aristotle classified by simple morphology, the two kingdom system followed in Linnaeus' time, and Whittaker proposed five kingdoms in 1969.
• Whittaker's criteria were cell structure, body organisation, mode of nutrition, reproduction and phylogenetic relationships.
• Monera is the only prokaryotic kingdom; bacteria show the most extensive metabolic diversity, and archaebacteria survive extremes because of their different cell wall.
• Cyanobacteria such as Nostoc and Anabaena fix nitrogen in heterocysts; mycoplasma lack a cell wall and are the smallest living cells.
• Protista includes chrysophytes, dinoflagellates, euglenoids, slime moulds and protozoans, all single-celled eukaryotes, mostly aquatic.
• Fungi have chitin walls and a mycelium of hyphae; their sexual cycle is plasmogamy, karyogamy and meiosis, with a dikaryotic stage in some classes.
• The four fungal classes differ in mycelium, asexual spores and sexual spores; deuteromycetes are imperfect fungi with no known sexual stage.
• Viruses contain either RNA or DNA within a capsid, viroids are free RNA, prions are abnormally folded proteins, and lichens pair an alga with a fungus.

Test yourself

Who proposed the five kingdom classification, and in which year?

R.H. Whittaker proposed the five kingdom classification in 1969, with the kingdoms Monera, Protista, Fungi, Plantae and Animalia.

What are the three groups of archaebacteria and their habitats?

Halophiles live in extreme salty areas, thermoacidophiles in hot springs, and methanogens in marshy areas and the gut of ruminants.

What is diatomaceous earth used for?

Diatomaceous earth, being gritty, is used in polishing and in the filtration of oils and syrups.

Which organism causes red tides?

Red dinoflagellates such as Gonyaulax multiply so rapidly that the sea appears red, and their toxins may kill fishes.

Name three plants that are partially heterotrophic.

Bladderwort and Venus fly trap are insectivorous plants, and Cuscuta is a parasite; all three are partially heterotrophic.

What are the three steps of the sexual cycle in fungi?

The steps are plasmogamy, the fusion of protoplasms; karyogamy, the fusion of two nuclei; and meiosis in the zygote, giving haploid spores.

What did W.M. Stanley show in 1935?

W.M. Stanley showed that viruses could be crystallised, and that the crystals consist largely of proteins.

Which diseases are caused by prions?

Prions cause bovine spongiform encephalopathy (mad cow disease) in cattle and its analogous variant, Creutzfeldt-Jacob disease, in humans.