Biological Classification ISC Class 11: Domains, Kingdoms and Viruses

What Biological Classification is about

Weightage: Biological Classification belongs to the unit Diversity of Living Organisms, which CISCE weights at 9 of the 70 marks in the ISC Class XI Biology Paper I (Theory). Class XI is examined by the school, so your school sets the paper within this weightage.Source: CISCE ISC Biology syllabus, examination year 2028, Class XI unit weightage

Key concepts

Three domains and the five-kingdom system

Three domains (Woese, 1990), based mainly on rRNA sequences: Archaea (prokaryotes with distinctive membrane lipids and no peptidoglycan), Bacteria (true bacteria) and Eukarya (all organisms with a nucleus). Five kingdoms (Whittaker, 1969): Monera (prokaryotic, unicellular), Protista (eukaryotic, mostly unicellular), Fungi (eukaryotic, heterotrophic, absorptive, chitin walls), Plantae (eukaryotic, multicellular, autotrophic, cellulose walls) and Animalia (eukaryotic, multicellular, heterotrophic, no cell walls). Criteria: cell structure, body organisation, mode of nutrition, reproduction and phylogenetic relationships.

Worked example

Give two drawbacks of the two-kingdom system that the five-kingdom system removed.

Try it first, then show the solutionSolution

1. Prokaryotes (bacteria, cyanobacteria) and eukaryotes were placed together
2. Fungi (heterotrophic, chitin walls) were grouped with green plants; unicellular and multicellular organisms were mixed

Answer: Mixing of prokaryotes with eukaryotes and of fungi with plants

A typical bacterial cell; shapes, nutrition and respiration

A bacterial cell has a rigid cell wall (peptidoglycan), often a capsule or slime layer, a plasma membrane (sometimes infolded as mesosomes), cytoplasm with 70S ribosomes, a nucleoid (circular DNA without a nuclear membrane), extra DNA as plasmids, and may bear flagella and pili. Shapes: coccus (spherical), bacillus (rod), vibrio (comma) and spirillum (spiral). Nutrition: autotrophic (photosynthetic or chemosynthetic) or heterotrophic (saprophytic, parasitic or symbiotic). Respiration: aerobic, obligate anaerobic (killed by oxygen) or facultative anaerobic (with or without oxygen).

Worked example

Name the four shapes of bacteria and give the term for bacteria that obtain energy by oxidising inorganic substances.

Try it first, then show the solutionSolution

1. Coccus, bacillus, vibrio, spirillum
2. Bacteria that oxidise inorganic substances (such as ammonia or nitrite) to make food are chemosynthetic autotrophs

Answer: Coccus, bacillus, vibrio, spirillum; chemosynthetic autotrophs

Gram-positive and Gram-negative bacteria; reproduction

In Gram staining, cells are stained with crystal violet, treated with iodine, washed with alcohol and counterstained with safranin. Gram-positive bacteria have a thick peptidoglycan wall that retains crystal violet and appear purple (e.g. *Bacillus*, *Streptococcus*). Gram-negative bacteria have a thin peptidoglycan layer with an outer membrane of lipopolysaccharide; the dye is washed out and they take up safranin, appearing pink/red (e.g. *Escherichia coli*). Reproduction (definitions only): fission (one cell divides into two); conjugation (transfer of DNA through a pilus between two cells); transduction (transfer of bacterial DNA by a bacteriophage); transformation (uptake of free DNA from the surroundings).

Worked example

Why do Gram-negative bacteria appear pink after Gram staining?

Try it first, then show the solutionSolution

1. Their thin peptidoglycan layer and outer lipid membrane do not hold the crystal violet–iodine complex when washed with alcohol
2. The decolourised cells then take up the pink counterstain safranin

Answer: They lose the violet dye and take up safranin

Archaebacteria, Mycoplasma and economic importance of bacteria

Archaebacteria (brief idea) live in extreme habitats and have cell walls without peptidoglycan and membranes with branched lipids: methanogens (produce methane; in marshes and the guts of ruminants; used in biogas), halophiles (very salty water) and thermoacidophiles (hot, acidic springs). Mycoplasma: (i) no cell wall (so unaffected by penicillin, and of no fixed shape); (ii) the smallest known living cells (about 0.1 µm); (iii) can survive without oxygen; many are plant and animal pathogens. Economic importance: sewage treatment (bacteria digest organic matter), antibiotics (e.g. streptomycin from *Streptomyces*), energy production (methanogens in biogas plants) and curd and cheese (*Lactobacillus* converts milk sugar to lactic acid, curdling the milk; other bacteria ripen cheese).

Worked example

Give three features of Mycoplasma.

Try it first, then show the solutionSolution

1. No cell wall (pleomorphic, resistant to penicillin)
2. Smallest living cells known (about 0.1 µm)
3. Can live without oxygen

Answer: No wall; smallest cells; anaerobic survival

Kingdom Protista

Unicellular eukaryotes with a nucleus and membrane-bound organelles, mostly aquatic. Chrysophytes: diatoms and desmids; cell walls with silica fitting like a soap box; chief producers in oceans; their deposits form diatomaceous earth (e.g. *Navicula*, *Cymbella*). Dinoflagellates: marine, photosynthetic, two flagella in grooves, cellulose plates; blooms of red forms cause red tides (e.g. *Gonyaulax*, *Noctiluca*). Euglenoids: no cell wall but a flexible pellicle; photosynthetic in light, heterotrophic in darkness (e.g. *Euglena*, *Phacus*). Slime moulds: saprophytic; cells form a plasmodium that produces fruiting bodies with resistant spores (e.g. *Physarum*, *Dictyostelium*). Protozoans: heterotrophs: rhizopods (pseudopodia; *Amoeba*, *Entamoeba*), flagellates (flagella; *Trypanosoma*, *Leishmania*), ciliates (cilia; *Paramecium*, *Vorticella*), sporozoans (spore-forming parasites; *Plasmodium*, *Toxoplasma*).

Worked example

Give two characteristics and two examples of dinoflagellates.

Try it first, then show the solutionSolution

1. Mostly marine and photosynthetic, with stiff cellulose plates on the outer surface
2. Two flagella lying in grooves (one longitudinal, one transverse); some cause red tides
3. Examples: *Gonyaulax*, *Noctiluca*

Answer: Cellulose plates and two flagella; *Gonyaulax*, *Noctiluca*

Kingdom Fungi: features, spores and sexual cycle

Fungi are eukaryotic, heterotrophic (saprophytes, parasites or symbionts), absorb food after external digestion, store glycogen and have cell walls of chitin. Most are made of thread-like hyphae forming a mycelium; hyphae may be coenocytic (aseptate, multinucleate) or septate. Asexual spores: zoospores, sporangiospores, conidia; sexual spores: oospores, ascospores, basidiospores. Gametes may be similar (isogamy), differing in size (anisogamy), or a small motile male and large non-motile female (oogamy). The sexual cycle has plasmogamy (fusion of cytoplasm), karyogamy (fusion of nuclei) and meiosis; in ascomycetes and basidiomycetes a dikaryophase (two separate nuclei per cell, n + n) intervenes between plasmogamy and karyogamy.

Worked example

Define plasmogamy, karyogamy and dikaryophase.

Try it first, then show the solutionSolution

1. Plasmogamy: fusion of the protoplasm of two motile or non-motile gametes
2. Karyogamy: fusion of the two nuclei to form a diploid zygote nucleus
3. Dikaryophase: the stage in which each cell has two unfused haploid nuclei (n + n), between plasmogamy and karyogamy

Answer: Cytoplasm fusion; nuclear fusion; n + n stage

Classes of fungi and their importance

Zygomycetes: mycelium aseptate, coenocytic; asexual sporangiospores (non-motile); sexual zygospores from fusion of two gametangia (e.g. *Rhizopus*, *Mucor*). Ascomycetes (sac fungi): septate mycelium; asexual conidia; sexual ascospores in sac-like asci within fruiting bodies (ascocarps) (e.g. *Penicillium*, *Aspergillus*, *Saccharomyces* (yeast), *Neurospora*). Basidiomycetes (club fungi): septate, branched mycelium; usually no asexual spores; sexual basidiospores on club-shaped basidia in basidiocarps (e.g. *Agaricus* (mushroom), *Ustilago* (smut), *Puccinia* (rust)). Deuteromycetes (imperfect fungi): only the asexual stage is known; septate; reproduce by conidia; many are decomposers (e.g. *Alternaria*, *Colletotrichum*, *Trichoderma*). Importance: medicine (penicillin from *Penicillium*), bakery and brewing (yeast), and decomposition of dead matter, recycling minerals.

Worked example

To which class do *Mucor*, *Neurospora*, *Puccinia* and *Trichoderma* belong, and what sexual spores (if any) does each form?

Try it first, then show the solutionSolution

1. *Mucor*: Zygomycetes, zygospores
2. *Neurospora*: Ascomycetes, ascospores in asci; *Puccinia* (rust): Basidiomycetes, basidiospores on basidia
3. *Trichoderma*: Deuteromycetes, no sexual stage known (only conidia)

Answer: Zygomycetes; Ascomycetes; Basidiomycetes; Deuteromycetes (no sexual spores)

Lichens and mycorrhiza

Lichens are symbiotic associations of an alga (phycobiont, making food by photosynthesis) and a fungus (mycobiont, providing shelter, water and minerals). They are pioneers on bare rocks and are very sensitive to air pollution (especially SO₂), so their absence indicates polluted air. Mycorrhiza is a symbiotic association of a fungus with the roots of higher plants; the fungus absorbs water and mineral nutrients, especially phosphorus, and passes them to the plant, which supplies sugars (e.g. *Glomus* with many crop plants; mycorrhizal roots of pine).

Worked example

Why are lichens called pioneers on bare rock, and what does each partner contribute?

Try it first, then show the solutionSolution

1. The algal partner photosynthesises and supplies food; the fungal partner gives shelter and absorbs water and minerals
2. Together they survive on bare rock where neither could live alone; acids they secrete slowly weather the rock, and their dead remains add organic matter, starting soil formation

Answer: They are the first colonisers and begin soil formation; alga gives food, fungus gives shelter, water and minerals

Viruses, viroids and prions

Discovery: Ivanowsky (1892) found that the agent of tobacco mosaic disease passed through bacteria-proof filters; Beijerinck (1898) called it 'contagium vivum fluidum' (infectious living fluid); W. M. Stanley (1935) crystallised TMV, showing it is mainly protein (later shown to be nucleoprotein). Viruses are obligate parasites: inert crystals outside cells, but they use the host's machinery to replicate inside. TMV: rod-shaped, a helical protein coat (capsid) of identical subunits (capsomeres) around single-stranded RNA. Bacteriophage: a polyhedral head containing double-stranded DNA, a hollow tail with sheath, a base plate and tail fibres for attachment. Viroids (Diener, 1971): free RNA without a protein coat; low molecular weight; cause potato spindle tuber disease. Prions: abnormally folded proteins that cause other proteins to misfold; they cause bovine spongiform encephalopathy (BSE, 'mad cow disease') in cattle and Creutzfeldt-Jakob disease (CJD) in humans.

Worked example

Compare the genetic material and shape of TMV and a T-even bacteriophage.

Try it first, then show the solutionSolution

1. TMV: single-stranded RNA in a rod-shaped helical capsid
2. Bacteriophage: double-stranded DNA in a polyhedral head, with a tail and tail fibres

Answer: ssRNA, rod; dsDNA, head and tail

Biological Classification in real life

Each concept, met in an everyday situation. Read the situation, answer the question in your own words, then compare with the model answer. The one-line hook is what to remember.

Topics in this chapter

• Kingdom Monera: Bacteria and Archaebacteria
• Protista, Fungi, Viruses and Lichens

Biological Classification: key facts

Five kingdoms

$\text{Monera, Protista, Fungi, Plantae, Animalia}$

Whittaker (1969).

Three domains

$\text{Archaea, Bacteria, Eukarya}$

Woese, based on rRNA.

Fungal sexual cycle

$\text{plasmogamy}\to (\text{dikaryophase})\to \text{karyogamy}\to \text{meiosis}$

Dikaryophase in ascomycetes and basidiomycetes.

Bacterial growth by fission

$N=N_0\times 2^{t/g}$

g = generation (doubling) time.

Biological Classification: diagrams to practise

Practise each one on paper: draw it from the checklist, then tick off every label.

How the school exam asks Biological Classification

The question patterns to expect, and the method that works for each.

Tricks to remember Biological Classification

Short memory aids. Say each one aloud once, then use it on a question the same day.

• Monera Pass Fungi, Plants, Animals

  Monera, Protista, Fungi, Plantae, Animalia: from simplest to most complex.

  Use it when: Five kingdoms.

• Purple keeps, pink peels

  Gram-positive keeps violet (thick wall); Gram-negative loses it and turns pink.

  Use it when: Gram staining.

• Zygo, Asco, Basidio, Deutero

  Zygospore, ascus (sac), basidium (club), deuteron (second: imperfect, no sexual stage).

  Use it when: Fungal classes.

• Plasma before karyo

  Cytoplasm fuses first (plasmogamy), nuclei later (karyogamy).

  Use it when: Fungal sexual cycle.

• Viroid = RNA alone, prion = protein alone

  Viruses have both; viroids lack protein; prions lack nucleic acid.

  Use it when: Acellular agents.

Biological Classification: exam-level solved questions

Ordered from exam level to HOTS. Cover the solution, write your own, then compare step by step.

Case-study practice (Section D style)

Read the situation, then answer every part. In a school exam set on the board pattern this is a 5-mark question split into 1 + 1 + 1 + 2 marks.

Where the marks are

How a typical answer is split into step marks. Show every step: a right final answer alone rarely earns full marks.

Where students lose marks in Biological Classification

The slips examiners see most often, and what to write instead.

• ✗Mistake: Drawing a nucleus in a bacterium

  Students copy a eukaryotic cell.

  Wrong working

  Bacterial cell with a round, membrane-bound nucleus.

  Right working

  A nucleoid (DNA) with no nuclear membrane.

  ✓Do this: Prokaryote: no membrane-bound organelles.

  Usually costs: 1-2 marks.

• ✗Mistake: Confusing viroids with prions

  Both are 'sub-viral' agents.

  Wrong working

  Prions are RNA molecules.

  Right working

  Viroids are RNA; prions are proteins.

  ✓Do this: Viroid has 'r' for RNA; prion has 'p' for protein.

  Usually costs: 1 mark.

• ✗Mistake: Wrong order in the fungal sexual cycle

  Students put karyogamy first.

  Wrong working

  Karyogamy → plasmogamy.

  Right working

  Plasmogamy → (dikaryophase) → karyogamy → meiosis.

  ✓Do this: Cytoplasm meets before nuclei can.

  Usually costs: 1 mark.

• ✗Mistake: Giving more than asked for protists

  Students write long accounts and miss examples.

  Wrong working

  A paragraph on *Euglena* without a second example.

  Right working

  Two characteristics and two examples, as the syllabus requires.

  ✓Do this: Use a four-point answer.

  Usually costs: 1 mark.

• ✗Mistake: Calling Mycoplasma a virus

  They are very small.

  Wrong working

  Mycoplasma are viruses because they pass through filters.

  Right working

  They are cellular prokaryotes (Monera) without a cell wall.

  ✓Do this: Cells with ribosomes and metabolism are not viruses.

  Usually costs: 1 mark.

Practice questions on Biological Classification

Write each answer on paper first. Open the hint only if you are stuck, and the answer only when you have finished.

1. Q1 · 1 mark · MCQExam level

   Diatoms belong to:
   (a) dinoflagellates
   (b) chrysophytes
   (c) euglenoids
   (d) slime moulds

   Hint

   Silica walls.

   Answer

   (b)

2. Q2 · 1 mark · MCQExam level

   The genetic material of TMV is:
   (a) dsDNA
   (b) ssDNA
   (c) ssRNA
   (d) protein

   Hint

   Plant virus.

   Answer

   (c)

3. Q3 · 1 mark · Very short answerExam level

   Name the sexual spores of Basidiomycetes.

   Hint

   Club fungi.

   Answer

   Basidiospores

4. Q4 · 1 mark · Very short answerExam level

   Name the disease caused by a viroid.

   Hint

   Potato.

   Answer

   Potato spindle tuber disease

5. Q5 · 2 marks · Short answerExam level

   Name the four groups of protozoans with one example each.

   Hint

   Means of movement.

   Answer

   Rhizopods (*Amoeba*), flagellates (*Trypanosoma*), ciliates (*Paramecium*), sporozoans (*Plasmodium*)

6. Q6 · 2 marks · Short answerHarder exam level

   Classify bacteria on the basis of respiration, with an explanation of each type.

   Hint

   Oxygen.

   Answer

   Aerobic (need O₂), obligate anaerobic (killed by O₂), facultative anaerobic (can grow with or without O₂).

7. Q7 · 2 marks · Short answerHarder exam level

   What are the three types of archaebacteria and where are they found?

   Hint

   Extreme habitats.

   Answer

   Methanogens (marshes, ruminant guts), halophiles (very salty water), thermoacidophiles (hot acidic springs)

8. Q8 · 3 marks · Short answerHarder exam level

   Define conjugation, transduction and transformation in bacteria.

   Hint

   How DNA enters.

   Answer

   Conjugation: DNA transfer between two cells through a pilus. Transduction: transfer of bacterial DNA by a bacteriophage. Transformation: uptake of free DNA from the surroundings.

9. Q9 · 2 marks · Short answerHarder exam level

   Give two features and two examples of slime moulds.

   Hint

   Plasmodium.

   Answer

   Saprophytic; under good conditions cells aggregate into a plasmodium that forms fruiting bodies with resistant spores. Examples: *Physarum*, *Dictyostelium*.

10. Q10 · 3 marks · Short answerHarder exam level

    What are lichens and mycorrhiza? Give the benefit to each partner in mycorrhiza.

    Hint

    Fungus with alga; fungus with roots.

    Answer

    Lichen: symbiosis of an alga and a fungus. Mycorrhiza: symbiosis of a fungus with plant roots; the fungus supplies water and minerals (especially phosphorus), the plant supplies sugars.

11. Q11 · 3 marks · Short answerHarder exam level

    State three economic uses of bacteria named in the syllabus.

    Hint

    Sewage, drugs, energy, food.

    Answer

    Sewage treatment, production of antibiotics (e.g. streptomycin), energy production as biogas (methanogens), and making curd and cheese.

12. Q12 · 2 marks · Short answerHarder exam level

    List the five criteria Whittaker used for his classification.

    Hint

    Cell, body, food, reproduction, ancestry.

    Answer

    Cell structure, body organisation (thallus), mode of nutrition, reproduction, phylogenetic relationships

13. Q13 · 5 marks · Long answerHarder exam level

    (a) Draw a labelled diagram of a typical bacterial cell. [2]
    (b) Describe the Gram-staining procedure. [2]
    (c) Give one example each of a Gram-positive and a Gram-negative bacterium. [1]

    Hint

    Crystal violet, iodine, alcohol, safranin.

    Answer

    (a) Capsule, wall, membrane, nucleoid, plasmid, ribosomes, mesosome, flagellum, pili. (b) Stain with crystal violet, fix with iodine, wash with alcohol, counterstain with safranin; Gram-positive stays purple, Gram-negative turns pink. (c) *Bacillus*; *Escherichia coli*.

14. Q14 · 3 marks · Short answerHOTS / competency

    A culture starts with 500 bacteria and doubles every 25 minutes. How many bacteria are present after 2 hours 5 minutes?

    Hint

    Number of doublings.

    Answer

    125 min ÷ 25 = 5 doublings: 500 × 2⁵ = 16 000

15. Q15 · 3 marks · Short answerHOTS / competency

    *Euglena* is sometimes described as a link between plants and animals. Give two plant-like and two animal-like features.

    Hint

    Chloroplasts and pellicle.

    Answer

    Plant-like: chloroplasts and photosynthesis in light; stores paramylon (a carbohydrate). Animal-like: no cellulose wall (flexible pellicle); can feed heterotrophically in darkness and moves by a flagellum.

Test your recall

Answer in your head or on paper first, then open the card. Pulling an answer out of memory makes it stick far better than re-reading.

1. 1.Who proposed the three domains?Show answer

   Carl Woese.

2. 2.Wall material of fungi?Show answer

   Chitin.

3. 3.Smallest living cells?Show answer

   Mycoplasma.

4. 4.Cause of red tides?Show answer

   Dinoflagellates such as *Gonyaulax*.

5. 5.Stage with n + n nuclei in fungi?Show answer

   Dikaryophase.

6. 6.Example of a deuteromycete?Show answer

   *Alternaria* (or *Trichoderma*).

7. 7.Who crystallised TMV?Show answer

   W. M. Stanley (1935).

8. 8.Agent of BSE?Show answer

   Prion.

Quick revision checklist

The night before the exam: can you do each of these without looking?

• ☐Can you list Whittaker's five kingdoms and his criteria?
• ☐Can you draw and label a bacterial cell?
• ☐Can you explain Gram staining?
• ☐Can you give three features of Mycoplasma?
• ☐Can you give two features and two examples of each protist group?
• ☐Can you compare the four classes of fungi?
• ☐Can you define plasmogamy, karyogamy and dikaryophase?
• ☐Can you compare viruses, viroids and prions?

Biological Classification: frequently asked questions