From two kingdoms to the five-kingdom classification
Sorting your wardrobe into only 'summer' and 'winter' clothes works at first, but then where do raincoats go? Early biologists faced the same trouble with only 'plants' and 'animals'.
Aristotle first used simple structure to group organisms, and Linnaeus later put them in two kingdoms, Plantae and Animalia. R. H. Whittaker (1969) proposed the five-kingdom classification: Monera, Protista, Fungi, Plantae and Animalia.
In the figure : Whittaker's five kingdoms.
Rule 1: Why the two-kingdom system failed — It did not separate prokaryotes from eukaryotes, unicellular from multicellular organisms, or photosynthetic from non-photosynthetic ones. Fungi, with chitin in their walls, were also grouped with plants.
- Easy : A bacterium has no nucleus, but a mango tree has one; the two-kingdom system put both with plants.
- Medium : Fungi do not make their own food but were placed with plants.
- Exam-type : Q: Name one reason why the two-kingdom system was replaced. A: It placed prokaryotes and eukaryotes, and autotrophs and heterotrophs, together.
Rule 2: Criteria of Whittaker — He used cell structure, body organisation, mode of nutrition, reproduction and phylogenetic (evolutionary) relationships.
- Easy : Cell structure separates prokaryotes (Monera) from eukaryotes.
- Medium : Mode of nutrition separates green plants from fungi and animals.
- Exam-type : Q: Which kingdom contains all prokaryotes? A: Monera.
Five kingdoms: Monera (prokaryotes) and four eukaryotic kingdoms: Protista, Fungi, Plantae and Animalia.
Putting viruses and lichens into Whittaker's five kingdoms. They are not included in it.
Memory clue: MPF-PA — Monera, Protista, Fungi, Plantae, Animalia.
Spot the difference
| Point | Two-kingdom system | Five-kingdom system |
|---|---|---|
| Proposed by | Linnaeus | R. H. Whittaker (1969) |
| Kingdoms | Plantae, Animalia | Monera, Protista, Fungi, Plantae, Animalia |
| Fungi | Kept with plants | Separate kingdom |
Practice questions
- 1Who proposed the five-kingdom classification and when?
- 2Name the five kingdoms.
- 3Who divided organisms into two kingdoms?
- 4Name any two criteria of Whittaker's classification.
- 5Which kingdom is made up of prokaryotes?
- 6Why were fungi moved out of plants?
- 7State two drawbacks of the two-kingdom system.
- 8An organism has no nucleus and a rigid wall. Which of the five kingdoms does it fit?
Show answers (try them yourself first)
- 1R. H. Whittaker, in 1969.
- 2Monera, Protista, Fungi, Plantae and Animalia.
- 3Linnaeus (Plantae and Animalia).
- 4Cell structure and mode of nutrition (others: body organisation, reproduction, phylogenetic relationships).
- 5Monera.
- 6Fungi have chitin in their cell walls and are heterotrophs, unlike plants.
- 7It did not distinguish prokaryotes from eukaryotes and unicellular from multicellular organisms, and it grouped photosynthetic and non-photosynthetic organisms together.
- 8Monera, as it is a prokaryote.
Kingdom Monera: bacteria
A spoonful of soil holds more bacteria than there are people in a city. Curd, root nodules of pulses and a sore throat all involve these tiny cells.
Monera includes all prokaryotes, organisms without a membrane-bound nucleus. The main members are bacteria.
In the figure : Monera has archaebacteria, eubacteria and the cell-wall-less mycoplasma.
Rule 1: Archaebacteria — They live in harsh conditions and have a different cell-wall structure. Halophiles live in salty areas, thermoacidophiles in hot acidic springs and methanogens in marshes and in the gut of ruminants, where they produce methane (biogas).
- Easy : Methanogens are present in the dung of cattle and make biogas.
- Medium : A bacterium found in a hot spring is likely to be a thermoacidophile.
- Exam-type : Q: Which bacteria produce methane? A: Methanogens (archaebacteria).
Rule 2: Eubacteria — They have a rigid cell wall and may be motile with flagella. Cyanobacteria (blue-green algae) such as Nostoc and Anabaena are photosynthetic autotrophs, and some fix atmospheric nitrogen in heterocysts. Heterotrophic bacteria are decomposers or cause diseases. Shapes: coccus (spherical), bacillus (rod), vibrio (comma) and spirillum (spiral).
- Easy : Curd is made by the action of bacteria on milk.
- Medium : A rod-shaped bacterium is a bacillus, and a comma-shaped one is a vibrio.
- Exam-type : Q: Which Monera can fix nitrogen and photosynthesise? A: Cyanobacteria such as Nostoc and Anabaena.
Rule 3: Mycoplasma — They are the smallest living cells known. They have no cell wall and can survive without oxygen. Many cause diseases in plants and animals.
- Easy : Mycoplasma are smaller than ordinary bacteria.
- Medium : They are unaffected by antibiotics that act on the cell wall, because they have none.
- Exam-type : Q: Name the smallest living cells that lack a cell wall. A: Mycoplasma.
Bacteria reproduce mainly by binary fission. Under harsh conditions they form spores, and some exchange DNA by conjugation, transduction or transformation.
Calling cyanobacteria algae in the strict sense. They are prokaryotes and belong to Monera.
Memory clue: A-E-M — Archaebacteria harsh, Eubacteria true, Mycoplasma wall-less.
Spot the difference
| Group | Habitat or feature | Example |
|---|---|---|
| Archaebacteria | Extreme habitats; unusual cell wall | Methanogens, halophiles |
| Eubacteria | Rigid cell wall; many types | Nostoc, Anabaena |
| Mycoplasma | No cell wall; smallest cells | Mycoplasma |
Practice questions
- 1What type of cell do Monera have?
- 2Name the shapes of bacteria.
- 3What are halophiles?
- 4Name two cyanobacteria.
- 5Which organisms lack a cell wall in Monera?
- 6How do methanogens help humans?
- 7Why are archaebacteria able to survive in extreme conditions?
- 8Cyanobacteria can make their own food and also enrich a rice field. Explain.
Show answers (try them yourself first)
- 1Prokaryotic cells with no membrane-bound nucleus.
- 2Coccus, bacillus, vibrio and spirillum.
- 3Archaebacteria that live in very salty places.
- 4Nostoc and Anabaena.
- 5Mycoplasma.
- 6They produce methane in biogas plants from the dung of cattle.
- 7Their cell walls have a special structure that suits harsh habitats such as salty places, hot springs and marshes.
- 8They photosynthesise, so they are autotrophs, and some fix atmospheric nitrogen into usable form.
Kingdom Protista: the unicellular eukaryotes
A drop of pond water seen under a microscope looks like a crowded city: tiny round, oval and slipper-shaped cells swim or glide about. Most of them are protists.
Protista includes single-celled eukaryotes. Their cells have a well-defined nucleus and organelles. They link the other eukaryotic kingdoms and include photosynthetic as well as heterotrophic forms.
In the figure : Five groups of Protista.
Rule 1: Chrysophytes, dinoflagellates and euglenoids — Chrysophytes include diatoms and golden algae; diatom walls are made of silica, and their deposits form gritty diatomaceous earth. Dinoflagellates are mostly marine and photosynthetic; some, such as Gonyaulax, multiply so fast that the sea turns red (red tide). Euglenoids such as Euglena have a flexible protein layer, the pellicle, and make food in sunlight but behave as heterotrophs without light.
- Easy : Diatomaceous earth is used for filtering in industry.
- Medium : Euglena has both chlorophyll and the ability to eat small organisms when light is short.
- Exam-type : Q: What causes red tides? A: Rapid multiplication of dinoflagellates such as Gonyaulax.
Rule 2: Slime moulds and protozoans — Slime moulds are saprophytes that move over decaying twigs and leaves as a plasmodium. Protozoans are heterotrophs and include amoeboid forms (Amoeba), flagellated forms (Trypanosoma), ciliated forms (Paramoecium) and sporozoans (Plasmodium, the malaria parasite).
- Easy : Amoeba moves by pseudopodia.
- Medium : Paramoecium moves by cilia, and Trypanosoma by flagella.
- Exam-type : Q: Which sporozoan causes malaria? A: Plasmodium.
Protists are eukaryotic and mostly aquatic. Their locomotion is by flagella, cilia or pseudopodia.
Thinking all protists make their own food. Protozoans and slime moulds are heterotrophs.
Memory clue: C-D-E-S-P — Chrysophytes, Dinoflagellates, Euglenoids, Slime moulds, Protozoans.
Spot the difference
| Group | Key feature | Example |
|---|---|---|
| Chrysophytes | Silica walls | Diatoms |
| Dinoflagellates | Red tides | Gonyaulax |
| Euglenoids | Pellicle; autotroph and heterotroph | Euglena |
| Protozoans | Heterotrophs | Amoeba, Plasmodium |
Practice questions
- 1What is the nature of a protistan cell?
- 2Name the protist that causes malaria.
- 3What is diatomaceous earth?
- 4Which organelle-like protein layer is found in Euglena?
- 5What are red tides?
- 6How do Paramoecium and Amoeba move?
- 7Why is Euglena called a link between plants and animals?
- 8Why can red tides kill fish?
Show answers (try them yourself first)
- 1It is eukaryotic: it has a nucleus and membrane-bound organelles.
- 2Plasmodium, a sporozoan.
- 3Gritty silica deposits of the cell walls of diatoms.
- 4The pellicle.
- 5Reddening of sea water due to rapid multiplication of dinoflagellates such as Gonyaulax.
- 6Paramoecium by cilia, Amoeba by pseudopodia.
- 7It is photosynthetic in light like plants but takes in food heterotrophically in the absence of sunlight.
- 8The rapidly multiplying dinoflagellates release toxins that can poison marine animals.
Kingdom Fungi
Bread kept in a damp box turns fuzzy after a few days. The fuzz is a fungus that has used the bread as food.
Fungi are heterotrophic eukaryotes whose cell walls contain chitin and polysaccharides. Their body is a network of thread-like hyphae, which form a mycelium. They feed as saprophytes, parasites or symbionts.
In the figure : Four classes of fungi.
Rule 1: Phycomycetes and Ascomycetes — Phycomycetes live on decaying wood, in damp places or as obligate parasites on plants (examples: Mucor, Rhizopus, Albugo); their hyphae are aseptate and coenocytic. Ascomycetes (sac fungi) include Aspergillus, Claviceps and Neurospora, and the unicellular yeast and Penicillium; they have septate hyphae and make sexual spores, ascospores, inside asci.
- Easy : Rhizopus grows on stale bread.
- Medium : Yeast is a single-celled fungus used in baking and brewing.
- Exam-type : Q: In which fungi are ascospores formed in asci? A: Ascomycetes.
Rule 2: Basidiomycetes and Deuteromycetes — Basidiomycetes include mushrooms, bracket fungi and puffballs (examples: Agaricus, Ustilago and Puccinia). Spores form on a club-shaped basidium. Deuteromycetes are 'imperfect fungi' because only their asexual (conidial) stage is known; examples: Alternaria, Colletotrichum and Trichoderma.
- Easy : A button mushroom is a basidiomycete.
- Medium : A fungus that is seen only in its conidial stage is placed in deuteromycetes.
- Exam-type : Q: Why are deuteromycetes called imperfect fungi? A: Their sexual phase is not known.
Fungal wall = chitin; plant wall = cellulose. Nutrition is heterotrophic and absorptive.
Calling mushrooms plants because they grow from the soil. Fungi are not photosynthetic.
Memory clue: P-A-B-D — Phyco (algae-like), Asco (sac), Basidio (club), Deutero (imperfect).
Spot the difference
| Class | Hyphae | Example |
|---|---|---|
| Phycomycetes | Aseptate, coenocytic | Mucor, Rhizopus |
| Ascomycetes | Septate; ascospores in asci | Yeast, Penicillium |
| Basidiomycetes | Septate; basidiospores | Mushroom, Puccinia |
| Deuteromycetes | Septate; only asexual stage known | Alternaria, Trichoderma |
Practice questions
- 1What is the cell wall of fungi made of?
- 2What is a mycelium?
- 3Name two phycomycetes.
- 4Which fungal class is called the sac fungi?
- 5Give two examples of basidiomycetes.
- 6What is meant by coenocytic hyphae?
- 7Why are deuteromycetes called imperfect fungi?
- 8How does a fungus differ from a green plant in the way it gets food?
Show answers (try them yourself first)
- 1Chitin and polysaccharides.
- 2The network formed by the hyphae of a fungus.
- 3Mucor and Rhizopus.
- 4Ascomycetes.
- 5Mushroom (Agaricus) and puffball, bracket fungus, Ustilago or Puccinia.
- 6Hyphae without cross walls, containing many nuclei in one continuous cytoplasm.
- 7Only their asexual stage is known; when the sexual stage is found they are moved to another class.
- 8A fungus is heterotrophic and absorbs food from its source, whereas a green plant is photosynthetic.
Kingdoms Plantae and Animalia
A banyan tree makes its own food in the sun, and a cow has to graze to get food. This one difference in feeding separates the last two kingdoms.
Plantae are eukaryotic, chlorophyll-containing organisms with cellulose cell walls, which make their food by photosynthesis. Animalia are heterotrophic eukaryotes without cell walls that take food by ingestion (holozoic nutrition).
In the figure : Plantae and Animalia differ mainly in their mode of nutrition.
Rule 1: Plantae — Algae, bryophytes, pteridophytes, gymnosperms and angiosperms are in this kingdom. Plants show alternation of generations between a haploid gametophyte and a diploid sporophyte. A few plants, such as Cuscuta (a parasite), are partly or fully heterotrophic, and some, such as the pitcher plant and Venus fly trap, are insectivorous.
- Easy : A mango tree is an autotroph.
- Medium : Cuscuta is a parasite on other plants although it is placed with plants.
- Exam-type : Q: Name an insectivorous plant. A: Pitcher plant or Venus fly trap.
Rule 2: Animalia — Animals have no cell walls and digest food inside the body. They show different levels of body organisation and symmetry, and most reproduce sexually with embryonic development.
- Easy : A frog eats insects, so it is a heterotroph.
- Medium : Animals have no cell walls, so their cells can change shape and move.
- Exam-type : Q: What type of nutrition do animals show? A: Holozoic, meaning they ingest the food and digest it internally.
Plants are autotrophic with cellulose walls; animals are heterotrophic with no cell wall. A few plants are exceptions.
Saying that all plants are autotrophs. Cuscuta is a parasite, and insectivorous plants also catch insects.
Memory clue: PAC / AHN — Plants: Autotroph, Cellulose; Animals: Heterotroph, No wall.
Spot the difference
| Point | Plantae | Animalia |
|---|---|---|
| Cell wall | Cellulose | Absent |
| Nutrition | Autotrophic (mostly) | Heterotrophic, holozoic |
| Example | Mango, Cuscuta | Frog, human |
Practice questions
- 1What is the cell wall of plants made of?
- 2What type of nutrition do animals show?
- 3Name one parasitic plant.
- 4Name two insectivorous plants.
- 5Which pigment makes plants green?
- 6What is alternation of generations?
- 7Why are insectivorous plants still placed in Plantae?
- 8Name two features that separate Animalia from Plantae.
Show answers (try them yourself first)
- 1Cellulose.
- 2Holozoic: ingestion and internal digestion.
- 3Cuscuta (dodder).
- 4Pitcher plant and Venus fly trap.
- 5Chlorophyll.
- 6A life cycle in which a haploid gametophyte and a diploid sporophyte stage follow each other.
- 7They have chlorophyll and photosynthesise, and catch insects mainly to get extra nitrogen.
- 8Animals have no cell wall and are heterotrophic, whereas plants have a cellulose wall and photosynthesise.
Viruses, viroids, prions and lichens
A cold spreads from one student to the whole class within days, though no one can see the cause. Viruses are the cause of many such illnesses and sit on the border of the living and the non-living.
Viruses are acellular particles with a nucleic acid core (DNA or RNA) in a protein coat called the capsid. Viroids are free RNA without a protein coat; prions are infectious proteins; lichens are partnerships of algae and fungi.
In the figure : Groups not placed in Whittaker's five kingdoms.
Rule 1: Viruses — Ivanowsky (1892) found that the tobacco mosaic disease passes through bacteria-proof filters. Beijerinck (1898) called the fluid 'contagium vivum fluidum'. W. M. Stanley (1935) crystallised viruses and showed they are mainly proteins. A virus is inactive outside a host and becomes active only inside a living host cell. Bacteriophages are viruses that infect bacteria.
- Easy : A virus cannot make copies of itself on a table or in water.
- Medium : Mumps, influenza and AIDS are viral diseases.
- Exam-type : Q: Who crystallised viruses? A: W. M. Stanley, in 1935.
Rule 2: Viroids, prions and lichens — T. O. Diener (1971) discovered viroids: small RNA molecules without a protein coat that cause potato spindle tuber disease. Prions are abnormally folded proteins that cause diseases such as mad cow disease (BSE) in cattle and Creutzfeldt-Jakob disease in humans. Lichens are a symbiotic association between an alga (phycobiont) and a fungus (mycobiont); the alga makes food and the fungus gives shelter and absorbs water. Lichens are sensitive to air pollution, so they act as pollution indicators.
- Easy : Lichen on a rock is two organisms living together.
- Medium : A lichen disappearing from a city tree may point to air pollution.
- Exam-type : Q: Which part of a lichen makes the food? A: The algal partner (phycobiont).
Virus = nucleic acid + protein coat. Viroid = RNA only. Prion = protein only. Lichen = alga + fungus.
Thinking viroids have a protein coat. They have free RNA only.
Memory clue: VVPL — Virus (coat), Viroid (RNA), Prion (protein), Lichen (partners).
Spot the difference
| Item | Made of | Example |
|---|---|---|
| Virus | DNA or RNA + protein coat (capsid) | TMV, bacteriophage |
| Viroid | Free RNA only | Potato spindle tuber |
| Prion | Abnormal protein | Mad cow disease |
| Lichen | Alga + fungus | Rock and tree crusts |
Practice questions
- 1Who discovered viroids and in which year?
- 2What is the protein coat of a virus called?
- 3Name the partners of a lichen.
- 4Which disease is caused by prions in cattle?
- 5What was Beijerinck's name for the infectious fluid?
- 6Why are viruses not placed in the five kingdoms?
- 7How do lichens help in monitoring pollution?
- 8Differentiate a virus from a viroid.
Show answers (try them yourself first)
- 1T. O. Diener, in 1971.
- 2The capsid.
- 3An alga (phycobiont) and a fungus (mycobiont).
- 4Bovine spongiform encephalopathy (mad cow disease).
- 5Contagium vivum fluidum (infectious living fluid).
- 6They are acellular and active only inside a host cell, so they do not fit cellular classifications.
- 7They are very sensitive to air pollutants such as sulphur dioxide, so their absence signals polluted air.
- 8A virus has a nucleic acid inside a protein coat, whereas a viroid is only free RNA.