BiologyISC Class 11 · Biological Classification

Kingdom Protista and Fungi, with Viruses, Viroids, Prions, Lichens and Mycorrhiza

Updated 17 October 2026 · Fact-checked

Protista are unicellular eukaryotes (diatoms, dinoflagellates, euglenoids, slime moulds, protozoans). Fungi are heterotrophic eukaryotes with chitinous cell walls, grouped as Phycomycetes, Ascomycetes, Basidiomycetes and Deuteromycetes. Viruses, viroids and prions are non-cellular infectious agents; lichens and mycorrhiza are symbiotic associations.

Notice that plasmogamy and karyogamy are separate steps, with a dikaryon (n + n) between them, and meiosis restores haploid spores.

Illustration drawn for QuestionBuddy.

Understand Protista, Fungi, Viruses and Lichens

Kingdom Protista. Unicellular eukaryotes, a link between plants, animals and fungi. Chrysophytes include diatoms and desmids (siliceous walls leave diatomaceous earth). Dinoflagellates are marine photosynthetic forms, some causing red tides. Euglenoids (Euglena) have a protein-rich pellicle, are photosynthetic in light and become heterotrophic in its absence. Slime moulds are saprophytic protists that form a plasmodium. Protozoans are heterotrophs: amoeboid (Amoeba), flagellated (Trypanosoma, which causes sleeping sickness), ciliated (Paramecium) and sporozoans (Plasmodium, which causes malaria).

Kingdom Fungi. Heterotrophic, with a body of thread-like hyphae forming a mycelium, a cell wall of chitin and polysaccharides, and reproduction by spores. Sexual cycle: plasmogamy (fusion of protoplasts), forming a dikaryon (n + n), then karyogamy (fusion of nuclei) forming the diploid zygote (2n), followed by meiosis giving haploid spores. Classes: Phycomycetes (Rhizopus, Mucor, Albugo), Ascomycetes (Penicillium, Aspergillus, Neurospora, yeast; ascospores in asci), Basidiomycetes (mushrooms, Puccinia, Ustilago; basidiospores) and Deuteromycetes, the imperfect fungi in which the sexual stage is not known (Alternaria, Colletotrichum, Trichoderma).

Lichens and mycorrhiza. Lichens are symbiotic associations of algae (phycobiont) and fungi (mycobiont); the alga makes food and the fungus gives protection and absorbs water; they are good indicators of air pollution (sulphur dioxide). Mycorrhiza is an association between fungi and the roots of higher plants (for example Pinus): the fungus helps absorb minerals and water.

Viruses. Non-cellular, with a nucleic acid (DNA or RNA, never both) enclosed in a protein coat (capsid) and metabolically active only inside a host cell (obligate parasites). D. J. Ivanowsky (1892) showed that the sap of diseased tobacco plants stayed infectious after filtering; M. W. Beijerinck called the fluid contagium vivum fluidum; W. M. Stanley crystallised the virus (1935). Tobacco mosaic virus contains RNA; bacteriophages contain double-stranded DNA.

Viroids and prions. Viroids are free RNA molecules without a protein coat (potato spindle tuber disease, T. O. Diener). Prions are infectious proteins (mad cow disease, BSE).

Protista, Fungi, Viruses and Lichens: key facts

Protista: Chrysophytes (diatoms)
Dinoflagellates
Euglenoids
Slime moulds
Protozoans; Unicellular eukaryotes.
Fungal classes: $P h y co m y ce t es : R hi z o p u s ∣ A sco m y ce t es : P e ni c i l l i u m ∣ B a s i d i o m y ce t es : A g a r i c u s ∣ D e u t er o m y ce t es : A l t er na r ia$; Spores: sporangiospores, ascospores, basidiospores.
Sexual cycle of fungi: 1.Plasmogamy
2.dikaryon (n + n)
3.karyogamy (2n)
4.meiosis
5.haploid spores; Dikaryotic stage in Asco- and Basidiomycetes.
Non-cellular agents: Virus nucleic acid + protein coat
Viroid free RNA
Prion infectious protein; All are infectious.

Virus	nucleic acid + protein coat
Viroid	free RNA
Prion	infectious protein

Protista, Fungi, Viruses and Lichens in real life

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

A lichen on a city tree

Suppose a student sees a healthy lichen on trees in a hill station but none on trees along a busy industrial road.

Explain what a lichen is and what the observation suggests.

Try it first, then show the solutionSolution

A lichen is a symbiosis between an alga (makes food by photosynthesis) and a fungus (protects, absorbs water and minerals).
Lichens are very sensitive to pollutants such as sulphur dioxide in the air.
Their absence along the industrial road suggests that the air is polluted.

Answer: Lichens are algal-fungal associations used as indicators of air pollution.

Memory hook: No lichens usually means polluted air.

Tricks to remember Protista, Fungi, Viruses and Lichens

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

A-B-P
Ascomycetes: sacs (asci); Basidiomycetes: club (basidia); Phycomycetes: algal-like.
Use it when: Fungal classes and spores.
Plasmo, Karyo, Meiosis
Fusion of protoplasts, then nuclei, then meiosis.
Use it when: Order of the fungal sexual cycle.

Protista, Fungi, Viruses and Lichens: solved examples

Ordered from board level to HOTS. Try each one before you open the solution.

Example 1: Cell wall of fungi · 1 mark · MCQBoard level

The cell wall of fungi is made of:
(a) cellulose
(b) chitin
(c) peptidoglycan
(d) lignin

Try it first, then show the solutionSolution

Fungal walls contain chitin and polysaccharides.

Answer: (b) chitin

Example 2: Viruses, viroids and prions · 3 marks · Short answerBoard level

Distinguish between viruses, viroids and prions, giving one example of each.

Try it first, then show the solutionSolution

Virus: nucleic acid (DNA or RNA) in a protein coat; tobacco mosaic virus. (1 mark)
Viroid: free RNA without a protein coat; potato spindle tuber viroid. (1 mark)
Prion: an infectious protein; the agent of mad cow disease. (1 mark)

Answer: Nucleic acid + protein coat; free RNA; infectious protein.

Example 3: Ploidy in the fungal life cycle · 4 marks · Short answerHarder board level

In a fungus with n = 8, give the chromosome number of the nuclei at (i) the spore, (ii) the dikaryon after plasmogamy, (iii) the zygote after karyogamy and (iv) the spores formed after meiosis. Name the fungal class in which the dikaryon is prolonged.

Try it first, then show the solutionSolution

(i) Spore: haploid, n = 8. (0.75 mark)
(ii) Dikaryon: two haploid nuclei in one cell, n + n (8 + 8). (0.75 mark)
(iii) Zygote after karyogamy: diploid, 2n = 16. (1 mark)
(iv) Spores after meiosis: haploid, n = 8. (0.75 mark)
The dikaryon is prolonged in Ascomycetes and Basidiomycetes. (0.75 mark)

Answer: 8; 8 + 8; 16; 8; Ascomycetes and Basidiomycetes.

Example 4: Classes of fungi · 4 marks · Short answerHarder board level

Place these fungi in their classes and name the spores formed by each: Rhizopus, Penicillium, Agaricus and Alternaria.

Try it first, then show the solutionSolution

Rhizopus: Phycomycetes (sporangiospores, zygospores). (1 mark)
Penicillium: Ascomycetes (ascospores in asci). (1 mark)
Agaricus (mushroom): Basidiomycetes (basidiospores on basidia). (1 mark)
Alternaria: Deuteromycetes (imperfect fungi; only asexual spores known, conidia). (1 mark)

Answer: Phycomycetes, Ascomycetes, Basidiomycetes, Deuteromycetes respectively.

Example 5: Symbiotic associations · 5 marks · Long answerHOTS / competency

Define lichen and mycorrhiza, name the partners in each, state the benefit to each partner, and explain why lichens are used as pollution indicators.

Try it first, then show the solutionSolution

Lichen: a symbiotic association between an alga (phycobiont) and a fungus (mycobiont). (1 mark)
The alga makes food by photosynthesis; the fungus provides shelter and absorbs water and minerals. (1 mark)
Mycorrhiza: an association between a fungus and the roots of a higher plant (such as Pinus); the fungus helps absorb minerals and water and the plant supplies carbohydrates. (1.5 marks)
Lichens absorb substances directly from the air and are very sensitive to pollutants such as sulphur dioxide, so their absence indicates pollution. (1.5 marks)

Answer: Lichen: alga + fungus; mycorrhiza: fungus + root; both mutualistic; lichens indicate air quality.

Where students lose marks in Protista, Fungi, Viruses and Lichens

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

Mistake: Confusing viruses with cells
Saying viruses have both DNA and RNA.
Do this: A virus contains either DNA or RNA, never both.
Usually costs: 1 mark.
Mistake: Wrong ploidy in the fungal cycle
Calling the dikaryon diploid.
Do this: The dikaryon has two separate haploid nuclei (n + n); karyogamy gives 2n.
Usually costs: 1 to 2 marks.
Mistake: Classifying Penicillium as Basidiomycetes
Mixing the examples.
Do this: Learn one example for each class: Rhizopus, Penicillium, Agaricus, Alternaria.
Usually costs: 1 mark.

Exam tips

Give the order plasmogamy, karyogamy and meiosis for the fungal life cycle.
Use one named example for each protozoan group.
Write that viruses are obligate parasites.
State that lichens indicate air pollution.

Practice questions on Protista, Fungi, Viruses and Lichens

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

Q1 · 1 mark · MCQBoard level
Malaria is caused by a protozoan of the genus:
(a) Amoeba
(b) Plasmodium
(c) Paramecium
(d) Euglena
Hint
Sporozoans cause malaria.
Answer
(b) Plasmodium
Q2 · 2 marks · Short answerBoard level
What are dinoflagellates and what is "red tide"?
Hint
Marine photosynthetic protists.
Answer
Dinoflagellates are mostly marine, photosynthetic protists; in large numbers they make the sea appear red, called a red tide, and some release toxins.
Q3 · 3 marks · Short answerHarder board level
Describe Euglena's dual mode of nutrition.
Hint
It depends on the light.
Answer
In sunlight Euglena is photosynthetic; in the absence of light it behaves as a heterotroph, feeding on other small organisms. It has a protein-rich pellicle instead of a cell wall, which gives it flexibility.
Q4 · 3 marks · Short answerHarder board level
What is the contribution of Ivanowsky, Beijerinck and Stanley to virology?
Hint
Recall the sequence of discoveries.
Answer
Ivanowsky (1892) showed that sap from diseased tobacco plants remained infectious after filtration; Beijerinck called the fluid contagium vivum fluidum; Stanley (1935) crystallised the virus, showing it as a chemical, mostly protein.
Q5 · 3 marks · Assertion-reasonHarder board level
Assertion (A): Deuteromycetes are called imperfect fungi. Reason (R): Only the asexual or vegetative phases are known for them. Choose: (a) both true and R explains A (b) both true, R does not explain A (c) A true, R false (d) A false, R true.
Hint
Recall why they are called imperfect.
Answer
(a) Both are true and R explains A; when the sexual stage is discovered they are shifted to other classes.
Q6 · 4 marks · Case-basedHOTS / competency
Case study: Suppose a student studies a mushroom growing on a decaying log and a bread mould on stale bread.
(i) Name the class of fungi to which the mushroom belongs. (1 mark)
(ii) Name the spore type formed on the basidia. (1 mark)
(iii) Name the class of the bread mould Rhizopus. (1 mark)
(iv) State the mode of nutrition of both. (1 mark)
Hint
Both feed on dead organic matter.
Answer
(i) Basidiomycetes. (ii) Basidiospores. (iii) Phycomycetes. (iv) Saprophytic heterotrophic nutrition.

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.Cell wall of diatoms?Show answer
Siliceous.
2.Fungal class with ascospores?Show answer
Ascomycetes.
3.What is a viroid?Show answer
Free RNA without a protein coat.
4.Partners in a lichen?Show answer
Alga and fungus.

Protista, Fungi, Viruses and Lichens: frequently asked questions

Are viruses living or non-living?

They show characters of both: they have genetic material and replicate, but only inside a host cell, and they have no cellular structure or independent metabolism.

Why are fungi not classified as plants?

Fungi lack chlorophyll, are heterotrophic, and have cell walls of chitin rather than cellulose.

What do prions cause?

Prions cause neurodegenerative diseases such as mad cow disease (BSE) and, in humans, Creutzfeldt-Jakob disease.