Biological Classification Class 11 Notes

Before Whittaker's five-kingdom system, biologists used simpler classification schemes. Understanding these earlier systems helps appreciate why the modern system is more comprehensive.

• Three-Kingdom system (Ernst Haeckel, 1866): Added Kingdom Protista to include unicellular eukaryotes that did not fit neatly into Plantae or Animalia.
• Four-Kingdom system (Copeland, 1938): Separated Monera (prokaryotes) from Protista, recognising the fundamental prokaryote–eukaryote divide.
• Five-Kingdom system (Whittaker, 1969): The most widely accepted system — Monera, Protista, Fungi, Plantae and Animalia — based on cell structure, body organisation, mode of nutrition, reproduction and evolutionary relationships.

Kingdom Monera includes all prokaryotic organisms — the simplest and most ancient forms of life. They are unicellular, lack a membrane-bound nucleus, and are found in almost every habitat on Earth.

• Cyanobacteria (blue-green algae) — photosynthetic prokaryotes that form colonies and have gelatinous sheaths. Examples: Nostoc, Anabaena. They are the earliest oxygen-evolving organisms.
• Chemosynthetic bacteria — obtain energy by oxidising inorganic substances (e.g. nitrifying bacteria, sulphur bacteria).
• Mycoplasma — the smallest known prokaryotes. They completely lack a cell wall and can survive without oxygen — they are obligate parasites.
• Bacteria reproduce primarily by binary fission. Under unfavourable conditions, some form endospores (highly resistant structures).

Kingdom Protista includes all eukaryotic, unicellular organisms. They are often referred to as 'the stepping stone between prokaryotes and multicellular eukaryotes'. Their cell structure is eukaryotic — with a well-defined nucleus and membrane-bound organelles.

• Chrysophytes (diatoms and golden algae) — diatoms have silica-containing cell walls that form two halves fitting together like a petri dish. Their cell wall deposits form diatomaceous earth.
• Dinoflagellates — mostly photosynthetic, with two flagella arranged in grooves. Some are red (Gonyaulax) causing red tide.
• Euglenoids — mixotrophic organisms with a protein-rich pellicle instead of a cell wall. They have a photosynthetic pigment (chlorophyll) and can also absorb nutrients.
• Slime moulds — saprophytic organisms that live on decaying plant matter. Under favourable conditions they form a plasmodium (multinucleate mass); under unfavourable conditions they form fruiting bodies (sporangia).
• Protozoans — the animal-like protists. They are heterotrophic and classified by their mode of locomotion.

Kingdom Fungi is a kingdom of eukaryotic, multicellular (except yeast) organisms with chitinous cell walls. They are heterotrophs with saprophytic nutrition — they secrete digestive enzymes on food and absorb the nutrients (absorptive heterotrophy).

• Fungal body consists of thread-like hyphae (singular: hypha) that collectively form a mycelium.
• Reproduction occurs by spores — asexual (conidia, oidia) and sexual (oospores, ascospores, basidiospores).
• Sexual reproduction involves three stages: plasmogamy (fusion of cytoplasm), karyogamy (fusion of nuclei) and meiosis.

Viruses, viroids and prions are non-cellular entities that challenge our definition of life. They are considered 'organisms at the edge of life' because they cannot reproduce independently.

Example: Why are viruses not classified in any of the five kingdoms?

Solution: Viruses are acellular — they lack a cellular structure, cannot reproduce independently, and do not exhibit metabolism outside a host cell. Since all five kingdoms are based on cellular life forms, viruses fall outside the system. They are often described as 'organisms at the edge of life'.

Example: A student observes a microorganism with a peptidoglycan cell wall that reproduces by binary fission and lives in hot springs. To which kingdom and domain does it belong?

Solution: The presence of a peptidoglycan cell wall and binary fission indicates it is a bacterium. However, hot-spring-dwelling organisms are typically Archaebacteria — but archaebacteria lack peptidoglycan. The correct answer is: Kingdom Monera (bacteria) and Domain Bacteria. The key discriminator is the peptidoglycan wall, which rules out Archaea.

Exercises (NCERT Q1–Q12)

• 1) Discuss how classification systems have undergone several changes over a period of time?
• 2) State two economically important uses of: (a) heterotrophic bacteria (b) archaebacteria
• 3) What is the nature of cell-walls in diatoms?
• 4) Find out what do the terms ‘algal bloom’ and ‘red-tides’ signify.
• 5) How are viroids different from viruses?
• 6) Describe briefly the four major groups of Protozoa.
• 7) Plants are autotrophic. Can you think of some plants that are partially heterotrophic?
• 8) What do the terms phycobiont and mycobiont signify?
• 9) Give a comparative account of the classes of Kingdom Fungi under the following: (i) mode of nutrition (ii) mode of reproduction
• 10) What are the characteristic features of Euglenoids?
• 11) Give a brief account of viruses with respect to their structure and nature of genetic material. Also name four common viral diseases.
• 12) Organise a discussion in your class on the topic – Are viruses living or non-living?

Answers at a glance

1. 1
   Q1 → Classification systems changed repeatedly as the available criteria improved. Linnaeus proposed a two-kingdom system of Plantae and Animalia, which failed because it treated all unicellular organisms as plants, an organism such as Paramoecium with its contractile vacuole and cilia being neither plant nor animal in the sense then meant, and because it did not separate fungi from plants. Haeckel added a third kingdom, Protista, in 1866 for the unicellular eukaryotes, which exposed the fungi as a group deserving its own place but did not give it one, the fungi being still carried with the plants. Copeland separated the prokaryotes as the kingdom Monera in 1956, creating a four-kingdom system and recognising for the first time the divide between cells with and without a membrane-bound nucleus. Whittaker proposed the five-kingdom system in 1957, adding the kingdom Fungi and dividing by mode of nutrition, so that Monera and Protista rest on cell type, Fungi and Plantae on nutrition, and Animalia on holozoic nutrition and organisation. Finally, Woese's 16S rRNA base-pair sequence work of 1990 separated the prokaryotes and gave the three-domain system of Bacteria, Archaea and Eukarya. The direction of change is from a single division of life, through increasing recognition of the prokaryote-eukaryote divide, to classification based on measured sequence similarity rather than on resemblance.
2. 2
   Q2 → (a) Heterotrophic bacteria are used in agriculture as biofertilisers, since Rhizobium in the root nodules of legumes, and free-living Azotobacter and the cyanobacteria, fix atmospheric nitrogen; in food preparation, to make curd and yoghurt with Lactobacillus and vinegar with Acetobacter aceti; and in industry and medicine, to produce antibiotics such as streptomycin, to treat sewage and tan leather, and, in the bacterium Escherichia coli into which the human insulin gene has been introduced, to make human insulin on a large scale. (b) Archaebacteria are used for the biogas produced by methanogens such as Methanobacterium from the dung of ruminants, which is a clean fuel, and for the high-fidelity DNA obtained from the halophiles, which tolerates the high salt and the repeated heating that damage the DNA of most other organisms and is therefore used in molecular biological experiments. Also used are the enzymes of the thermoacidophiles, such as the amylase and the pullulan, because they act at high temperature and low pH, where the enzymes of the mesophiles will not.
3. 3
   Q3 → The wall of a diatom is made of two thin shells that overlap, one fitting over the other as the two halves of a soap box fit together, the larger upper half sliding over the smaller lower one, and the two halves together forming the whole box, which is called the frustule. Into these shells silica is embedded, as hydrated silicon dioxide, and this is what makes the wall of the diatoms peculiar among the protists and what makes it indestructible; the golden-brown colour of the cells is a matter of the fucoxanthin in the chloroplast and not of the wall, which is organic elsewhere in the plankton. Since the wall cannot be destroyed, diatoms have left behind large amounts of wall deposit in their habitat, and the accumulation of this over billions of years forms the diatomaceous earth, which, being gritty, is used in polishing and in the filtration of oils and syrups. Since each daughter cell retains one of the two halves at division, the wall need never be made afresh. And since the form of the box is what is observed, the Pennate diatoms, which are bilaterally symmetrical with a feather-like outline, are distinguished from the centric ones, which are radially symmetrical, the class as a whole being called Bacillariophyceae for the rod or pencil shape many of them show.
4. 4
   Q4 → Both terms describe an explosive multiplication of plankton in water enriched with nutrients, especially the nitrates and phosphates carried in by agricultural runoff and industrial effluent. Algal bloom, also called a water bloom, is the resulting rapid multiplication of phytoplankton such as dinoflagellates, which appears as a scum on the surface and consumes so much dissolved oxygen that fish and other aquatic animals are killed. Red tides are caused by red dinoflagellates, chiefly Gonyaulax, multiplying so abundantly that the sea turns red, which is the literal meaning of the term. The danger of a red tide is toxicity rather than oxygen depletion, because the dinoflagellates produce neurotoxins such as saxitoxin which pass along the food chain and cause paralytic shellfish poisoning in humans who eat the contaminated shellfish. So an algal bloom is named for its effect on the water and kills by deoxygenation, while a red tide is named for its colour and kills by toxin.
5. 5
   Q5 → A virus is a nucleoprotein, consisting of a DNA or RNA genome enclosed in a protein coat built from capsid protein, and it replicates only by using a host cell's machinery. A viroid is a naked, circular, single-stranded RNA molecule with no protein coat, and it is much smaller than a virus. The key consequence is that a viroid carries no genes for protein at all, for it codes for nothing and not even a coat, and it is copied by an RNA polymerase already present in the host cell, so that it is infectious with nothing but its RNA, which is of low molecular weight. A further difference is host range: viroids are found only in plants, where they cause disease, and they do not infect animals, whereas viruses have a wide host range covering bacteria, plants and animals. The single missing protein coat is thus what produces the difference in size, in replication mechanism and in host range.
6. 6
   Q6 → The four major groups of Protozoa are separated by mode of nutrition and locomotion. The Amoeboids or Sarcodina move by pseudopodia formed by cytoplasmic streaming and feed by holozoic nutrition, engulfing food whole, as in Amoeba and Entamoeba, of which Entamoeba histolytica causes amoebic dysentery. The flagellated protozoans or Mastigophora are either free-living or parasitic and move by flagella, the parasitic forms being the familiar examples, Trypanosoma causing sleeping sickness and Giardia causing giardiasis, the photosynthetic flagellates being the dinoflagellates and the Euglenoids, which are placed elsewhere in the kingdom. The ciliates or Ciliophora move by cilia and feed holozoically, and are characterised by a differentiated ciliary apparatus used in locomotion and in feeding, the coordinated beating of its rows steering the water laden with food into the gullet, together with a large macronucleus for everyday function and one or more small micronuclei for reproduction, as in Paramoecium. The Sporozoans or Apicomplexa are all parasites whose life cycle includes an infectious spore-like stage, they move only by gliding and have no locomotory organ of the usual kind, and they are defined by the apical complex at the anterior end, the best known being Plasmodium, the malarial parasite, which causes malaria, along with the gregarines and the coccidia.
7. 7
   Q7 → Yes. Partially heterotrophic plants are of three kinds. Parasitic plants draw nourishment wholly or partly from another living plant, and although they possess chlorophyll their water and minerals come from the host, as in the mistletoes of the Loranthaceae and in Rafflesiaceae, of which Rafflesia is entirely non-green and lacks chlorophyll altogether. Saprophytes or saprotrophs obtain food from dead decaying organic matter rather than from a living host, as in the Indian pipe, Monotropa uniflora, which lacks chlorophyll and feeds on decaying matter with the help of a mycorrhizal fungus. Carnivorous or insectivorous plants photosynthesise for themselves but trap and digest insects to obtain nitrogen, phosphorus and sulphur, as in the pitcher plant, the Venus flytrap and Drosera. The distinction to hold on to is that the first two groups may have no chlorophyll at all, whereas a carnivorous plant is always green and eats only to supplement itself.
8. 8
   Q8 → Phycobiont and mycobiont are the two partners of a lichen, and the suffix -biont means an organism living in a particular way. The phycobiont is the algal partner, since phyco- refers to algae, and it is the photosynthetic half of the association, supplying the organic food produced by photosynthesis. The mycobiont is the fungal partner, since myco- refers to a fungus, and it supplies the water and the minerals, absorbs nutrients from the substrate, and gives protection and structural housing to the algal cells. The division of labour is therefore that the algal partner makes the food and the fungal partner supplies water, minerals and shelter. The algal partner is a green alga in most lichens but a cyanobacterium in many, so the phycobiont is any photosynthetic partner. The two are physiologically interdependent, with the algal cells living in a protected state inside the fungal tissue, so close is the association that a lichen seen in nature does not look like an organism with two members in it, and the whole association can photosynthesise while colonising bare rock and bark that neither partner could occupy alone.
9. 9
   Q9 → Mode of nutrition: Phycomycetes are saprophytic or parasitic, with an aseptate, coenocytic and multinucleate mycelium, as in Mucor, Rhizopus and Albugo; Ascomycetes are mostly saprophytic and some parasitic, mostly multicellular with a branched and septate mycelium, though the yeasts are unicellular, as in Aspergillus, Claviceps and Neurospora; Basidiomycetes are mostly saprophytic and rarely parasitic, with a branched and septate mycelium and a basidiocarp, as in Agaricus, Ustilago and Puccinia; Deuteromycetes are saprophytic or parasitic, many being decomposers of litter, with a septate and branched mycelium, as in Alternaria, Colletotrichum and Trichoderma. Mode of reproduction: Phycomycetes reproduce asexually by motile zoospores or non-motile aplanospores produced endogenously in a sporangium, and sexually by a zygospore formed by fusion of two gametes that are similar when isogamous and dissimilar when anisogamous or oogamous; Ascomycetes reproduce asexually by conidia produced exogenously on conidiophores, and sexually by ascospores formed endogenously in sac-like asci held in ascocarps such as apothecia, perithecia, cleistothecia or pseudothecia; Basidiomycetes have no sex organs and generally no asexual spores, vegetative reproduction being by fragmentation, while plasmogamy between two somatic cells gives a dikaryon that yields basidia, in which karyogamy and meiosis produce four basidiospores exogenously on a basidiocarp; Deuteromycetes reproduce only by asexual spores known as conidia, with no known sexual stage, which is why they are called the Fungi Imperfecti.
10. 10
    Q10 → Euglenoids are the flagellated protists, most of them living in stagnant fresh water, named for their type member Euglena. Their most characteristic feature is that they are both autotrophic and heterotrophic: their pigments are identical to those of higher plants, chlorophyll a, chlorophyll b and carotene, so they can photosynthesise in light, but when deprived of sunlight they behave like heterotrophs and predate on other smaller organisms. Instead of a cell wall they have a protein-rich pellicle, which makes the body flexible, and the food is stored as paramylon granules in the cytoplasm rather than as starch. The cell is elongated, spindle-shaped and tapering at both ends, with two flagella of unequal length and a single cup-shaped gullet or cytopharynx at the anterior end leading to a reservoir, which also serves for ingestion. A contractile vacuole handles osmoregulation, and an eyespot or stigma with a photoreceptor at the base of the longer flagellum makes the organism sensitive to light, so Euglenoids move towards it. The flexible pellicle also allows the body shape to change when squeezed, and the name euglena, meaning true eye, records the eyespot that the group is named for.
11. 11
    Q11 → Viruses are non-cellular, having no cell membrane, cell wall, cytoplasm, ribosomes or organelles, so they can multiply only inside a host cell. Structurally a virus is a nucleoprotein: a protein coat called a capsid, built from capsomeres arranged symmetrically, surrounds a nucleic acid core, and capsid plus core form the nucleocapsid, with some viruses also having an envelope of host-derived lipid bearing glycoprotein spikes. As to genetic material, a virus contains either DNA or RNA and never both, which is why the Baltimore classes are double-stranded DNA, single-stranded DNA, double-stranded RNA and single-stranded RNA; both kinds occur, with the pox virus of smallpox a DNA virus and the tobacco mosaic, influenza, polio and HIV viruses among the RNA viruses. Four common viral diseases are influenza, poliomyelitis, the common cold and AIDS, and dengue and COVID-19 are also routinely given. Viruses differ from bacteria in having hereditary material of only one type, in lacking the enzymes needed to replicate it, and in being obligate parasites, whereas a bacterium has both DNA and RNA, its own ribosomes, and can live independently.
12. 12
    Q12 → The evidence is balanced and the conclusion is that a virus is non-living by the cell definition but stands at the boundary of life. The living features are that a virus has a definite structure with a protein coat around a nucleic acid, contains a heredity material, can mutate, and once inside a host can grow and multiply by directing the synthesis of proteins and of new viral particles. The non-living features are that it has no cellular structure at all, with no membrane, cytoplasm or organelles, cannot grow or multiply on its own, lacks ribosomes and the enzymes for replicating its nucleic acid, does not respire, shows no independent metabolism, and is completely inert outside a host cell. The decisive consideration is location: outside a cell a virion performs no metabolism and cannot reproduce, while inside a host cell it hijacks the host's ribosomes to produce new particles. It is therefore not a degenerate bacterium but something categorically different, and it belongs to no kingdom, which is why it is placed outside all five. The class should conclude that by the criterion of independent metabolism and cellular organisation the virus is non-living, while its structure, heredity, mutability and capacity to multiply make it the connecting link between the living and the non-living, so both positions are defensible if the criterion is stated.

MCQ Questions

1 Mark Questions

1. 1
   Who proposed the two-kingdom classification? (Ans: Linnaeus)
2. 2
   Write the scientific name of the fungus used to make antibiotics from Penicillium. (Ans: Penicillium notatum)
3. 3
   What is a bacteriophage?
4. 4
   Name the cell wall material of fungi. (Ans: chitin)

2 Mark Questions

1. 1
   Mention any two given characters when archaebacteria are compared with true bacteria. (Ans: different cell wall, membrane lipids, live in extreme conditions; smaller ribosome)
2. 2
   Why are diatoms golden or brown in colour? (Ans: green-yellow pigment fucoxanthin in addition to chlorophyll; silica cell walls)
3. 3
   Why are viruses considered obligate parasites? (Ans: they cannot multiply or metabolise outside a living host cell)

3 Mark Questions

1. 1
   Describe the four classes of fungi with one example and the type of sexual reproduction found in each. (Ans: Phycomycetes − zygospores; Ascomycetes − ascospores; Basidiomycetes − basidiospores; Deuteromycetes − sexual stage absent)
2. 2
   Give the salient features of Kingdom Protista. (Ans: unicellular eukaryotes, membrane-bound organelles, some with cilia/flagella, photosynthetic (diatoms) or parasitic (slime moulds, protozoans))
3. 3
   Write the economic importance of bacteria (Ans: curd/cheese, antibiotics like streptomycin; nitrogen fixation; biogas and sewage treatment; harmful: disease, spoilage)

5 Mark Questions

1. 1
   Account for Whittaker's five-kingdom classification, stating the basis of classification and giving the main features and examples of each kingdom. (Ans: cell structure, thallus organisation, mode of nutrition, reproduction, phylogenetic relationships; Monera, Protista, Fungi, Plantae, Animalia)
2. 2
   Describe the structure, and the lytic cycle of multiplication of a bacteriophage, listing four viral diseases in humans. (Ans: capsid + DNA/RNA; adsorption → injection → synthesis → assembly → lysis; e.g. AIDS, influenza, polio, dengue)