Biological Classification

Written and reviewed by Anjali Sharma, Biology faculty at UniDrill (M.Sc. Botany, Banaras Hindu University · 8 years teaching NCERT Biology). Based on NCERT Biology Class XI, Chapter 2.

Is a mushroom a plant?

For about two thousand years, biologists said yes. It grows in soil, it doesn't move and it has no mouth. Today we know a mushroom is closer to you than to a mango tree. This chapter is the story of how classification got that wrong, how it was fixed, and the five kingdoms CUET expects you to know cold.

The big idea: classification kept splitting

Every system before the modern one failed for the same reason: it grouped living things by how they look, not by how they are built or how they live. Each new system added a criterion the old one missed, and one big group split into smaller, more honest ones.

Why two kingdoms weren't enough. Linnaeus put everything that didn't move into Plantae. That one group ended up holding bacteria (no nucleus), fungi (can't make food) and green plants (make their own food), which are three completely different ways of being alive. The two-kingdom system could not separate prokaryotes (no membrane-bound nucleus) from eukaryotes (true nucleus), unicellular from multicellular organisms, or photosynthetic organisms (green algae) from non-photosynthetic ones (fungi).

Whittaker's five criteria. R.H. Whittaker's 1969 fix was to classify using five criteria together: cell structure, body organisation, mode of nutrition, reproduction and phylogenetic relationships (evolutionary relatedness). Learn this list exactly. It is a favourite for "which of these was NOT a criterion" questions.

The table below holds the whole chapter in one view. If you can rebuild it from memory, you can answer most kingdom-level questions.

Beyond the textbook. For the exam: five kingdoms, plus the note that a three-domain system divides Monera into two domains, giving effectively six kingdoms. In reality: the three-domain system (Carl Woese, 1990) came from comparing ribosomal RNA, not from looking at cells. It showed that archaebacteria are not "old bacteria". They are as different from bacteria as bacteria are from us. Modern biology is built on the three domains, but CUET asks the five-kingdom version.

Kingdom Monera: life without a nucleus

Monera is the kingdom of bacteria, the most abundant living things on Earth. They live everywhere: hot springs, deep oceans, snow, your gut. All of them are prokaryotes, with no nuclear membrane and a cell wall that is not made of cellulose.

Bacteria come in four shapes, which NTA likes to test as a match-the-column: coccus (spherical), bacillus (rod), vibrium (comma) and spirillum (spiral). They reproduce mainly by fission, form spores when conditions turn bad, and can swap DNA in a primitive form of sexual reproduction.

Archaebacteria: the extremists. They survive where almost nothing else can, and their unusual cell wall structure is what makes this possible. Three groups, each named for where it lives:

Why it matters. Methanogens in a cow's gut make methane, which the cow burps out, so cattle are a significant source of this greenhouse gas. The same microbes power the biogas plants you meet in Class XII (Microbes in Human Welfare). Same organism, two chapters, two question types.

Eubacteria: the "true" bacteria. They have a rigid cell wall and, if they move, a flagellum. They are grouped by how they get energy:

Cyanobacteria photosynthesise like plants, using chlorophyll a. They can be unicellular, colonial or filamentous, often wrapped in a jelly-like sheath, and they form blooms in polluted water. Some, like Nostoc and Anabaena, have special cells called heterocysts that fix nitrogen from the air. Chemosynthetic autotrophs make food without sunlight. They oxidise inorganic substances such as nitrates, nitrites and ammonia and use the released energy to make ATP. In doing so they recycle nitrogen, phosphorus, iron and sulphur. Heterotrophic bacteria are the most common. They are the decomposers, and they are behind curd, antibiotics and nitrogen fixation in legume roots. Some cause disease: cholera, typhoid, tetanus, and citrus canker in plants.

Pause and predict: cyanobacteria used to be called "blue-green algae". Which kingdom are they actually in?

Check your answer: Monera. They are prokaryotes with no nucleus, so they can't be algae (algae are eukaryotic plants). The old name stuck because they are green and photosynthetic, which is exactly the "classify by looks" mistake this chapter is about.

Mycoplasma: the cell with no wall. Mycoplasma are the smallest living cells known. They have no cell wall at all, can survive without oxygen, and many cause disease in plants and animals.

Why it works this way. Penicillin kills bacteria by stopping them from building their cell wall. Mycoplasma has no wall to attack, so penicillin does nothing to it. Remember "no wall, so penicillin can't touch it" and you will never mix up Mycoplasma with archaebacteria, which have an unusual wall, not a missing one.

Kingdom Protista: the misfits

Protista holds single-celled eukaryotes. They are mostly aquatic, have a true nucleus and membrane-bound organelles, may move with flagella or cilia, and reproduce both asexually and sexually (by cell fusion and zygote formation). They form a bridge between bacteria and the multicellular kingdoms.

Beyond the textbook. For the exam: Protista is one kingdom with five groups. In reality: Protista is a leftover category that holds every eukaryote that isn't a plant, animal or fungus. A diatom and an amoeba are less related to each other than a mushroom is to a tiger, and modern classification has broken Protista into several separate lineages. That is why its groups seem to have nothing in common: they don't.

Beyond the textbook. For the exam: write Gonyaulax. In reality: the species behind many toxic red tides, once called Gonyaulax catenella, has been renamed Alexandrium catenella. Its toxins build up in shellfish and can cause paralytic shellfish poisoning in people who eat them. If you see "Alexandrium" in a news report, it is the same organism.

Euglenoids: plant or animal? Both. Botanists and zoologists once both claimed Euglena for their own kingdom, because it photosynthesises with the same pigments as higher plants yet feeds like an animal in the dark. Organisms like this are exactly why a "misfits" kingdom was needed.

The four protozoan groups differ in how they move:

Kingdom Fungi: they dissolve their food

Fungi can't make food and don't swallow it. They release enzymes onto their food, digest it outside the body and absorb the result. That makes them heterotrophs: saprophytes (on dead matter), parasites, or symbionts, as partners in lichens (with algae) and mycorrhiza (with plant roots).

Except for unicellular yeast, a fungus is a network of thin threads called hyphae, and a mass of hyphae is a mycelium. Hyphae may be continuous tubes full of nuclei (coenocytic, no cross-walls) or divided by cross-walls (septate). The cell wall is made of chitin and polysaccharides.

Beyond the textbook. For the exam: Fungi is a separate kingdom from Plantae. In reality: fungi are more closely related to animals than to plants. Like animals they store food as glycogen, not starch, and chitin is the same material insect exoskeletons are made of. So the opening question was literally true: the mushroom on your pizza is your distant cousin.

How fungi reproduce. Three ways: vegetative (fragmentation, fission, budding), asexual (spores called conidia, sporangiospores or zoospores) and sexual (spores called oospores, ascospores or basidiospores).

Sexual reproduction always follows the same three steps: plasmogamy (cytoplasm of two cells fuses), karyogamy (the two nuclei fuse, giving a 2n zygote) and meiosis (giving haploid spores). The twist is the gap between steps 1 and 2. In ascomycetes and basidiomycetes the nuclei don't fuse straight away, so for a while each cell carries two separate nuclei (n + n). That stage is the dikaryon (dikaryophase).

The cycle in one line: two hyphae (n and n) → plasmogamy (cytoplasm fuses) → dikaryon (n + n, only in ascomycetes and basidiomycetes) → karyogamy (nuclei fuse, 2n) → meiosis → spores (n).

The four classes of fungi. They are separated by three things: whether the hyphae have cross-walls, which spores they make, and where the spores form. This table is the most-tested thing in the chapter.

Memory hooks for the examples. Phycomycetes: "MR A" (Mucor, Rhizopus, Albugo). Basidiomycetes: the mushroom, the smut and the rust (Agaricus, Ustilago, Puccinia). Deuteromycetes: imperfect fungi "ACT" without sex (Alternaria, Colletotrichum, Trichoderma).

In basidiomycetes, sex organs are absent. Two ordinary body cells from different strains fuse (plasmogamy) to give a dikaryotic mycelium, and karyogamy and meiosis then happen inside a club-shaped basidium, which bears four basidiospores on its outside.

Deuteromycetes are called "imperfect" because only their asexual stage is known. When someone finds one reproducing sexually, it is moved into ascomycetes or basidiomycetes. Many deuteromycetes are decomposers of leaf litter and help recycle minerals.

Pause and predict: a fungus has septate hyphae, makes conidia, and nobody has ever seen it reproduce sexually. Which class?

Check your answer: Deuteromycetes. Septate hyphae plus conidia could also mean an ascomycete, but with no known sexual stage it stays "imperfect" until one is found.

Where you've met them already. Penicillium gave us penicillin. Yeast raises bread dough. Neurospora was used in the experiments behind the "one gene, one enzyme" idea. Puccinia causes rust disease in wheat. Trichoderma returns in Class XII as a biocontrol agent that protects crops from other fungi.

Plantae and Animalia, in brief

Plantae are eukaryotes with chlorophyll and a cellulose cell wall: algae, bryophytes, pteridophytes, gymnosperms and angiosperms. Not all are fully autotrophic: bladderwort and Venus flytrap are insectivorous, and Cuscuta is a parasite. Plant life cycles alternate between a diploid sporophyte and a haploid gametophyte (alternation of generations). Chapter 3 covers this in detail.

Animalia are multicellular, heterotrophic eukaryotes with no cell wall. They feed by ingestion (holozoic nutrition), store food as glycogen or fat, and reproduce mostly sexually. Chapter 4 covers them.

Outside the kingdoms: viruses, viroids, prions and lichens

Whittaker's system has no place for viruses, because viruses are not cells. Outside a host cell a virus is an inert crystal; inside one, it takes over the cell's machinery to make copies of itself.

A virus is a protein coat (the capsid, built from units called capsomeres in a helical or polyhedral shape) around genetic material that is either RNA or DNA, never both. Plant viruses usually carry single-stranded RNA. Bacteriophages (viruses that infect bacteria) usually carry double-stranded DNA. Animal viruses can carry single- or double-stranded RNA, or double-stranded DNA. In humans viruses cause mumps, smallpox, herpes, influenza and AIDS; in plants they cause mosaic patterns, leaf rolling and curling, yellowing, vein clearing and stunted growth.

Beyond the textbook. For the exam: viruses are non-cellular and inert outside a living cell. In reality: whether viruses are alive is still argued. They have genes and they evolve, but they can't make energy or copy themselves alone. A useful way to put it: a virus is alive only while it is inside a cell.

Viroids and prions: even simpler. Strip a virus down and you get a viroid, a small, low-molecular-weight RNA with no protein coat. Keep the protein and lose the genes and you get a prion, an abnormally folded protein about the size of a virus that makes normal proteins misfold too. Prions cause BSE (mad cow disease) in cattle and Creutzfeldt–Jakob disease (CJD) in humans.

Pause and predict: a viroid and a prion are both "simpler than a virus". What does each one lack?

Check your answer: a viroid lacks the protein coat (it is bare RNA); a prion lacks any genetic material (it is only protein). They are opposite halves of a virus, a neat way to keep them apart in a match-the-column question.

Lichens: a partnership. A lichen isn't one organism. It is an alga and a fungus living so closely they look like one. The algal partner (phycobiont) is autotrophic and makes the food. The fungal partner (mycobiont) is heterotrophic, gives shelter and absorbs water and minerals. Lichens don't grow in polluted areas, so a city whose trees carry no lichens probably has dirty air. That is why they are called pollution indicators.

Exam traps

60-second revision

Where this chapter connects

• Previous: The Living World gives the taxonomic hierarchy (species → kingdom) that this chapter applies.
• Next: Plant Kingdom expands the one paragraph on Plantae into algae, bryophytes, pteridophytes, gymnosperms and angiosperms.
• Class XII link: Microbes in Human Welfare brings back methanogens (biogas), Penicillium (antibiotics), yeast (fermentation) and Trichoderma (biocontrol).