Biological Classification | ISC Class 11 Biology Notes

This note covers three domains and five kingdoms, bacterial structure and activities, protist groups, fungal structure and reproduction, lichens and mycorrhiza, the main features of plants and animals, and viruses, viroids and prions.

Why do classification systems distinguish domains and kingdoms?

Biological classification groups organisms using shared characteristics and relationships. A domain is a grouping above kingdom; a kingdom is a major category of organisms. Classification considers cell structure, body organisation, nutrition, reproduction and phylogeny, meaning evolutionary relationships.

A prokaryote has no membrane-bound nucleus; a eukaryote has a nucleus enclosed by a membrane. The nucleus contains genetic material. Unicellular means consisting of one cell, while multicellular means consisting of many cells.

Why was the two-kingdom system inadequate?

Linnaeus's two-kingdom system placed organisms in Plantae or Animalia. It did not distinguish prokaryotes from eukaryotes, single-celled from many-celled organisms, or photosynthetic organisms from fungi. A cell wall alone therefore grouped organisms with very different cellular organisation and nutrition.

Autotrophs make food from inorganic substances; heterotrophs depend on other organisms or organic material. Photosynthesis uses light energy to make food. Fungi absorb organic food, whereas green plants are photosynthetic. These differences support their placement in separate kingdoms.

How do the three domains compare?

The domains are Bacteria, Archaea and Eukarya. The three-domain system separates the prokaryotes grouped within Monera into Bacteria and Archaea. Protista, Fungi, Plantae and Animalia belong to Eukarya. Domains should therefore not be treated as alternative names for individual kingdoms.

Peptidoglycan is a cell-wall network of sugars and peptides, short chains of amino acids, the building blocks of proteins. Lipids include membrane fats. Ester and ether linkages are different chemical bonds joining their components.

These membrane differences distinguish Archaea from Bacteria even though both have prokaryotic cells. A shared absence of a membrane-bound nucleus does not erase their differences in chemical composition.

R.H. Whittaker proposed the five-kingdom classification in 1969: Monera, Protista, Fungi, Plantae and Animalia. Changing classification reflects improved understanding of organisms and their relationships, rather than a change in the organisms being classified.

How is a bacterial cell organised and classified by shape or staining?

Monera contains prokaryotic organisms in the five-kingdom system. Bacteria occur almost everywhere, including soil and extreme habitats. Their internal organisation is comparatively simple, but their chemical activities and ways of obtaining food are exceptionally diverse.

What structures occur in a typical bacterial cell?

The cell envelope is the protective covering. In most bacterial cells it comprises an outer glycocalyx, or surface coating, a supporting cell wall and a plasma membrane, which controls the passage of substances into and out of the cell.

The glycocalyx may form a loose slime layer or a thick, tough capsule. The cell wall maintains shape and resists bursting or collapse. Cytoplasm is the semifluid material within the cell, containing its internal components.

DNA, deoxyribonucleic acid, carries hereditary information. The bacterial chromosome occupies a region called the nucleoid, without a surrounding nuclear membrane. Many bacteria also contain plasmids, small circular DNA molecules separate from the main chromosome, which may confer antibiotic resistance, the ability to withstand an antibiotic. Antibiotics are microbial substances that kill or retard the growth of other disease-causing microbes.

Ribosomes make proteins, and inclusion bodies store reserve materials. Motile bacteria have flagella, long structures used for movement. Pili are tubular surface structures; fimbriae are short bristles that help some bacteria attach to surfaces. Neither is a locomotory structure.

How do shape and Gram staining differ as criteria?

The drawing groups spherical cocci, rod-shaped bacilli, spiral spirilla and comma-shaped vibrio. It also labels a spore within a rod and a flagellum extending from a spiral form.

See Fig. 2.1 in your NCERT textbook

Gram staining separates bacteria by their cell-envelope properties and staining response. A stain is a colouring substance used to make cells easier to examine. The terms Gram-positive and Gram-negative refer to this response, rather than to bacterial shape.

Decolourisation removes stain during the procedure; counterstaining applies a contrasting stain afterwards. These responses reflect envelope differences. They do not by themselves tell whether an organism is beneficial or causes disease.

How do bacteria differ in nutrition, respiration and habitat?

Bacteria as a group show extensive metabolic diversity, meaning diversity in their chemical reactions. Some are autotrophs, but the vast majority are heterotrophs. Nutritional classification identifies food and energy sources; respiratory classification concerns how cells release energy.

What are the nutritional and respiratory groups?

Photosynthetic autotrophic bacteria use light energy. Cyanobacteria, also called blue-green algae, contain the photosynthetic pigment chlorophyll a. They may be unicellular, colonial or filamentous. A colony is an association of cells; a filament is a thread-like arrangement.

Cyanobacterial colonies are generally enclosed in a gelatinous sheath. They often form blooms, or dense growths, in polluted water. Some fix atmospheric nitrogen in specialised cells called heterocysts; Nostoc and Anabaena are examples. Nitrogen fixation converts atmospheric nitrogen into forms organisms can use.

Chemosynthetic autotrophic bacteria obtain energy by oxidising inorganic substances, including nitrates, nitrites and ammonia. Oxidation is an energy-releasing chemical change in these reactions. These bacteria help recycle nutrients such as nitrogen, phosphorus, iron and sulphur.

Among heterotrophs, saprophytes obtain food from dead organic matter, while parasites depend on living hosts. A host is the organism supporting a parasite. The majority of heterotrophic bacteria are important decomposers, breaking down organic remains; some are pathogens, organisms that cause disease.

How do archaebacteria and mycoplasma differ?

Archaebacteria have cell walls different from those of other bacteria. Halophiles inhabit extremely salty environments; thermoacidophiles inhabit hot, acidic environments. Their structural and physiological properties enable survival under such conditions.

Methanogens produce methane, a combustible gas. They occur in marshy areas and in the gut of ruminants such as cows and buffaloes. A ruminant is a cud-chewing animal. Methanogens contribute to methane production from cattle dung, making them important in biogas production.

Mycoplasma have three distinctive features: they completely lack a cell wall, are the smallest living cells known, and can survive without oxygen. Many are pathogenic in plants and animals. Wall absence separates them from the usual bacterial cell-envelope pattern.

How do bacteria reproduce, exchange genes and benefit people?

Binary fission is division of one bacterial cell into two daughter cells. Bacteria reproduce mainly by fission. Under unfavourable conditions, they sometimes produce spores, resistant structures that help them persist. Transfer of genetic material must be distinguished from an increase in cell number.

What do the gene-transfer terms mean?

A virus is an infectious agent without cellular organisation that multiplies using a living host cell. Conjugation transfers DNA from a donor bacterium to a recipient through direct cell contact. Transformation is uptake of free DNA from the surroundings. Transduction transfers bacterial DNA through a bacteriophage, a virus that infects bacteria.

These processes can change inherited characteristics without themselves producing daughter cells. Calling all three ordinary cell division loses the distinction between reproduction and gene transfer. In each case, identify how DNA reaches the receiving bacterium.

How does milk become curd?

Lactic acid bacteria, including Lactobacillus, convert milk into curd. An inoculum is material containing microorganisms used to start a culture. Curd added to fresh milk acts as this starter.

1. Add a small amount of curd containing lactic acid bacteria to fresh milk.
2. At a suitable temperature, the bacteria multiply in the milk.
3. During growth, they produce acids that coagulate, or cause the setting of, milk proteins.
4. The proteins are also partly digested, and the milk becomes curd.

Bacteria also contribute to cheese production. The large holes in Swiss cheese result from carbon dioxide gas produced by Propionibacterium sharmanii. Different microorganisms contribute to characteristic cheese textures and flavours.

What happens during biological sewage treatment?

Sewage is municipal wastewater containing organic matter and microorganisms. Physical removal of suspended material precedes biological treatment. Effluent means the liquid flowing out of a treatment stage.

1. Pass the primary effluent into aeration tanks, where agitation and pumped air support aerobic microbes, which use oxygen.
2. Bacteria associated with fungal filaments form flocs, mesh-like masses that consume much of the organic matter.
3. Allow the flocs to settle. This sediment is activated sludge; return a small portion as inoculum to the aeration tank.
4. Transfer most remaining sludge to anaerobic digesters, tanks where microbes break it down without oxygen and produce gases forming biogas.

Biogas is a fuel-gas mixture containing predominantly methane. Methanogens such as Methanobacterium help generate it from organic wastes. Thus microbial activity contributes both to wastewater treatment and to energy production.

Antibiotics are substances produced by some microbes that kill or retard the growth of other disease-causing microbes. Bacteria contribute to antibiotic production as well as food processing. Their importance therefore includes sewage treatment, medicines, fuel, curd and cheese.

What distinguishes the main groups of Protista?

Protista includes single-celled eukaryotes in the five-kingdom scheme, although its boundaries are not well defined. Members are primarily aquatic. They contain a well-defined nucleus and membrane-bound organelles, specialised structures within cells. Some possess flagella or cilia, short hair-like structures.

How do four major protist groups compare?

In these protists, a spore is a reproductive or dispersal unit. A fruiting body is a spore-bearing structure. These terms describe how slime moulds survive and spread when environmental conditions change.

Chrysophytes include diatoms and desmids. Diatoms have two overlapping shell-like wall halves containing silica, a hard mineral material. Their accumulated walls form diatomaceous earth, used for polishing and filtering oils and syrups. Diatoms are chief producers in oceans.

Plankton are organisms carried passively by water currents. Chrysophytes occur in both freshwater and marine environments. Their usual photosynthetic nutrition should not be converted into a claim that every chrysophyte is photosynthetic.

Dinoflagellates have stiff cellulose plates at the cell surface. Cellulose is a structural carbohydrate found in many cell walls. Very often, red dinoflagellates such as Gonyaulax multiply rapidly and colour seawater red, producing red tides. Their toxins may even kill fish.

The majority of euglenoids inhabit stagnant freshwater. Their flexible protein-rich covering is the pellicle. They have a short and a long flagellum. Their photosynthetic pigments are identical to those of higher plants; without sunlight, they feed on smaller organisms.

How do slime moulds survive adverse conditions?

Slime moulds move over decaying leaves and twigs, engulfing organic material. Under suitable conditions they form a spreading mass called a plasmodium. Here the word describes the slime mould's body, not the malaria-causing organism Plasmodium.

Unfavourable conditions lead to fruiting bodies bearing spores at their tips. Slime-mould spores have true walls, resist adverse conditions for many years and disperse in air currents.

How do the four protozoan groups differ?

Protozoans are heterotrophic protists that live as predators or parasites. Predators capture other organisms as food. Protozoans are believed to be primitive relatives of animals. Their locomotory structures and feeding features help distinguish four major groups.

Amoeboid protozoans occur in freshwater, seawater and moist soil. Marine forms have silica shells. Amoeba captures prey with pseudopodia, whereas some forms such as Entamoeba live parasitically. The group is therefore not defined by parasitism alone.

Flagellated protozoans include parasites causing sleeping sickness, such as Trypanosoma. Ciliated protozoans are aquatic and actively moving. In Paramoecium, coordinated rows of cilia steer food-bearing water into a gullet, a feeding cavity opening at the cell surface.

Plasmodium causes malaria and belongs to the sporozoans. An infectious spore-like stage does not make these organisms fungi. Their classification also depends on their cellular organisation and other characteristics, rather than on one reproductive term alone.

What are fungi and how do they reproduce?

Fungi are heterotrophic eukaryotes. Their walls contain chitin, a structural carbohydrate, and other polysaccharides, carbohydrates made from many sugar units. Except for unicellular yeasts, fungi are filamentous. They occur widely and prefer warm, humid conditions.

A hypha is a slender fungal thread; a network of hyphae is the mycelium. Septa are cross walls dividing hyphae. Coenocytic hyphae contain many nuclei in continuous cytoplasm without such divisions.

How are nutrition and reproduction described?

Most fungi absorb soluble organic matter from dead material and are saprophytes. Others are parasites or partners in mutually useful associations called symbioses. Fungi may therefore decompose remains, cause disease or associate with algae and plant roots.

Vegetative reproduction uses parts of the existing body: fragmentation breaks it into pieces, fission divides a cell, and budding forms an outgrowth that develops into a new individual. Asexual reproduction produces spores without fusion of gametes, the cells involved in sexual fusion.

Fungal asexual spores include conidia, produced externally, sporangiospores, produced inside spore cases called sporangia, and zoospores, which are motile spores. Sexual spores include oospores, formed following fertilisation, the fusion of a male gamete with an egg, ascospores, formed inside sacs called asci, and basidiospores, formed externally on structures called basidia.

What do the sexual-reproduction terms mean?

Isogamy is fusion of gametes similar in form and size. Anisogamy involves unequal gametes. Oogamy involves a large non-motile female gamete and a smaller male gamete. The male gamete is not necessarily motile in fungi.

Haploid means having one chromosome set, represented by n. Diploid means having two sets, represented by 2n. Chromosomes carry hereditary information. Meiosis is the division that reduces chromosome-set number from diploid to haploid.

1. Compatible haploid fungal cells or hyphae come together for sexual reproduction.
2. Plasmogamy, fusion of their cell contents, brings the parental nuclei into a shared cell.
3. Karyogamy, fusion of the nuclei, produces a diploid nucleus; in some fungi this fusion is delayed.
4. Meiosis restores the haploid condition and leads to the formation of haploid spores.

In ascomycetes, the sac fungi, and basidiomycetes, fungi bearing sexual spores on basidia, delayed nuclear fusion produces a dikaryophase, a phase with two separate haploid nuclei per cell. Its notation is n + n: each n denotes one haploid nucleus. It differs from one diploid nucleus, 2n.

How do the principal fungal classes compare?

Fungal classes are distinguished using mycelial structure, spore formation and reproductive structures. Zygomycetes, Ascomycetes, Basidiomycetes and Deuteromycetes have different combinations of these features. A zygospore is a thick-walled sexual resting spore formed following fusion of compatible reproductive structures. Spore names identify different reproductive structures and modes of formation.

What are the distinguishing features of each class?

Zygomycetes include bread moulds such as Rhizopus. Their sporangiospores form within sporangia. Their non-motile spores distinguish them from fungi producing motile zoospores.

Ascomycetes, or sac fungi, are mostly multicellular but are rarely unicellular, as in Saccharomyces, a yeast. They include decomposers, parasites and coprophilous forms, which grow on dung. Their conidia develop on specialised hyphae called conidiophores.

An ascus, plural asci, is a sac containing sexual ascospores. Asci occur in fruiting bodies called ascocarps. Conidia form externally, whereas ascospores form internally. Morels and truffles are edible ascomycetes; Neurospora is used in genetic and biochemical research.

Basidiomycetes include mushrooms, bracket fungi, puffballs, rusts and smuts. Fragmentation is common, while asexual spores are generally not found. Sex organs are absent; fusion of compatible ordinary body cells establishes the dikaryotic condition.

A basidium, plural basidia, is the structure in which nuclear fusion and meiosis occur, producing four basidiospores externally. Basidia occur in fruiting bodies called basidiocarps. Agaricus is a mushroom, Ustilago a smut and Puccinia a rust fungus.

Deuteromycetes are called imperfect fungi because only their asexual or vegetative phases are known. When a sexual stage is identified, a fungus can be reassigned, often to Ascomycetes or Basidiomycetes. Many decompose litter and assist mineral cycling.

Why are fungi economically important?

Penicillium provides antibiotics such as penicillin. Baker's yeast, Saccharomyces cerevisiae, ferments bread dough; fermentation is the incomplete breakdown of sugars without using oxygen to release energy; in baker's yeast it produces ethanol and carbon dioxide. Decomposer fungi break down organic litter and return nutrients to circulation. Other fungi cause disease, so their effects are not uniformly beneficial.

How do lichens and mycorrhiza function as partnerships?

A lichen is a mutually useful association between an alga and a fungus. The algal partner is the phycobiont, while the fungal partner is the mycobiont. Their close association forms an integrated body, although the partners have different nutritional roles.

The phycobiont is autotrophic and prepares food. The mycobiont is heterotrophic, provides shelter and absorbs water and mineral nutrients for its partner. A lichen should therefore not be described as a single organism performing both partners' functions independently.

Lichens are very good pollution indicators, organisms whose occurrence helps indicate environmental conditions. They do not grow in polluted areas. Their ecological use follows from their sensitivity, while their nutritional success depends on the complementary activities of the two partners.

How do ectomycorrhiza and endomycorrhiza differ?

Mycorrhiza is a symbiotic association between a fungus and plant roots. The fungal partner absorbs nutrients, including phosphorus, from soil and supplies them to the plant. The plant supplies organic food to the fungus.

A mantle is a surrounding sheath; the cortex is a tissue region within the root. Many members of the fungal genus Glomus form mycorrhiza. Plants with these associations show improved growth, tolerance to salinity and drought, and resistance to root-borne pathogens.

How do Plantae and Animalia complete the five-kingdom comparison?

Plantae includes eukaryotic plants with cell walls mainly made of cellulose. Their chloroplasts are organelles containing photosynthetic pigments. Plants include algae, bryophytes, pteridophytes, gymnosperms and angiosperms. Their detailed classification depends on additional structural and reproductive characteristics.

A few plant members are partially heterotrophic, including insectivorous plants and parasites. Bladderwort and Venus fly trap are insectivorous plants; Cuscuta is a parasite. Treating every plant as nutritionally identical would conceal these differences.

Animalia comprises multicellular heterotrophic eukaryotes whose cells lack walls. Animals depend directly or indirectly on plants for food. Their holozoic nutrition involves ingesting food and digesting it internally. Food reserves include glycogen, a storage carbohydrate, or fat.

Most animals are capable of locomotion, movement from place to place. Their growth produces adults of definite shape and size. Nutrition, cell walls and organisation provide more dependable kingdom comparisons than merely asking whether an organism visibly moves.

Why do viruses link living and non-living characteristics?

A virus is an acellular infectious agent: acellular means lacking cellular organisation. Viruses are inert outside their specific host cells and can be crystallised. Within living cells they use the host's machinery to replicate, meaning to make further virus particles.

Viruses are obligate parasites, dependent on living hosts for multiplication. Their ability to reproduce within cells gives a living characteristic, while their non-cellular organisation and inert state outside cells resemble non-living material. This combination makes simple placement among cellular kingdoms difficult.

What are the components of a virus?

Nucleic acids are DNA and RNA (ribonucleic acid), molecules involved in genetic information. A virus contains genetic material enclosed by a protein coat called the capsid. The capsid consists of smaller units called capsomeres. The genetic material is either DNA or RNA, ribonucleic acid; a virus does not contain both as its genetic material.

TMV stands for tobacco mosaic virus. It has RNA surrounded by a helical protein capsid; helical means arranged spirally. Bacteriophages infect bacteria and are usually double-stranded DNA viruses. Double-stranded describes genetic material made of two associated strands.

The drawing shows a cylindrical arrangement of repeated protein units surrounding the central genetic material. The two labels identify RNA and capsid. Keep the RNA inside the surrounding protein coat.

See Fig. 2.6a in your NCERT textbook

The drawing has an angular head above a collar and an elongated sheath, with tail fibres spreading below. The labels are head, collar, sheath and tail fibres.

See Fig. 2.6b in your NCERT textbook

In the illustrated type of bacteriophage, the head encloses the genetic material and the tail fibres help attachment to a bacterial host. Its head-and-tail arrangement differs from the elongated helical form of TMV.

What did the three scientists establish?

In general, plant viruses have single-stranded RNA. Animal viruses may contain single- or double-stranded RNA or double-stranded DNA. These statements describe patterns; they should not be strengthened into claims that all viruses infecting a particular host have identical genetic material.

How do viroids and prions differ from viruses?

A viroid is an infectious agent consisting of free, low-molecular-weight RNA without a protein coat. Molecular weight refers to molecular mass. T.O. Diener discovered viroids in 1971 while investigating potato spindle tuber disease. Their lack of a capsid distinguishes them from viruses.

A prion is an infectious agent consisting of abnormally folded protein. Protein folding is the arrangement of a protein into its three-dimensional shape. Prions cause certain neurological diseases, meaning diseases affecting the nervous system.

BSE means bovine spongiform encephalopathy, commonly called mad cow disease in cattle. CJD means Creutzfeldt-Jakob disease in humans; variant CJD is the analogous human disease discussed with BSE. Both belong among prion diseases, not bacterial infections.

The decisive comparison is the infectious material: a virus has nucleic acid and a protein coat, a viroid has RNA without that coat, and a prion is abnormal protein. Similar infectious effects therefore do not imply identical composition.

Glossary

• Domain — A classification category above kingdom, separating Bacteria, Archaea and Eukarya.
• Prokaryote — An organism whose cells lack a nucleus enclosed by a nuclear membrane.
• Autotroph — An organism that makes its own organic food from inorganic starting substances.
• Peptidoglycan — A network of sugars and short peptides forming the characteristic bacterial wall material.
• Plasmid — A small DNA molecule separate from the main chromosome in many bacteria.
• Transformation — Uptake of free DNA from the surroundings by a bacterial cell.
• Pellicle — A flexible protein-rich covering present instead of a cell wall in euglenoids.
• Pseudopodium — A temporary cytoplasmic projection used for movement and food capture by amoeboid organisms.
• Mycelium — The network of slender hyphae that forms the body of a filamentous fungus.
• Plasmogamy — Fusion of the cell contents of compatible cells during fungal sexual reproduction.
• Dikaryophase — A fungal phase in which each cell contains two separate haploid nuclei.
• Mycorrhiza — A symbiotic association between a fungus and roots of a plant.
• Capsid — The protein coat surrounding viral genetic material, built from smaller capsomere units.
• Viroid — An infectious low-molecular-weight RNA agent that lacks the protein coat of viruses.
• Prion — An infectious abnormally folded protein associated with certain diseases of the nervous system.

Common errors and misconceptions

• Misconception: Archaea and Bacteria are the same domain because both lack a nucleus. Correct: Both are prokaryotic, but differences including walls and membrane lipids distinguish the domains.
• Misconception: Every bacterium has a cell wall. Correct: Mycoplasma completely lack a wall, unlike the usual bacterial envelope pattern.
• Misconception: Conjugation, transformation and transduction are three kinds of cell division. Correct: They transfer genetic material; binary fission increases bacterial cell number.
• Misconception: Every protist is photosynthetic. Correct: Protozoans are heterotrophs, while euglenoids change their nutritional behaviour when deprived of sunlight.
• Misconception: A dikaryotic cell has one diploid nucleus. Correct: It contains two separate haploid nuclei, written n + n; a diploid nucleus is 2n.
• Misconception: Imperfect fungi have permanently lost sexual reproduction. Correct: Deuteromycetes groups fungi whose sexual phase is unknown; discovery can lead to reassignment.
• Misconception: All bacteriophages contain double-stranded DNA. Correct: Bacteriophages are usually double-stranded DNA viruses; the qualification must be retained.
• Misconception: Viroids and prions are simply small bacteria. Correct: Viroids are infectious RNA without a coat; prions are abnormal infectious proteins.

Exam-style questions with model answers

Q1. State two features that distinguish mycoplasma from a typical walled bacterium. [2 marks]

1. Mycoplasma completely lack a cell wall, so they lack the rigid wall present in a typical bacterial cell.
2. They are the smallest living cells known, distinguishing them by size as well as by their wall absence.

Q2. Define conjugation, transformation and transduction in bacteria, identifying the route of DNA transfer in each. [3 marks]

1. Conjugation is transfer of DNA from a donor bacterium to a recipient through direct contact between the two cells.
2. Transformation is uptake by a bacterial cell of free DNA present in its surroundings, without direct donor-cell contact being the transfer route.
3. Transduction is transfer of bacterial DNA by a bacteriophage, a virus that infects bacteria and acts as the carrier of the material.

Q3. Compare Gram-positive and Gram-negative bacteria with respect to peptidoglycan thickness, outer membrane, retention of the primary violet stain after decolourisation, and final colour after counterstaining. [4 marks]

1. Gram-positive bacteria have a thick peptidoglycan layer in their cell wall, whereas Gram-negative bacteria have a thin peptidoglycan layer.
2. Gram-positive bacteria lack an outer membrane beyond this layer; Gram-negative bacteria possess an additional outer membrane outside it.
3. After decolourisation, Gram-positive bacteria retain the primary violet stain, whereas Gram-negative bacteria lose that stain at this stage.
4. Following counterstaining, Gram-positive bacteria appear purple or violet, while Gram-negative bacteria appear pink or red because they take the contrasting stain.

Q4. Give the locomotory structure and two examples for each group: rhizopods, flagellates, ciliates and sporozoans. For sporozoans, describe the typical feeding stage. [4 marks]

1. Rhizopods move with pseudopodia, temporary cytoplasmic projections also used in feeding. Two examples of this group are Amoeba and Entamoeba.
2. Flagellates move using flagella, long locomotory structures. Two examples belonging to this group are Trypanosoma and Giardia.
3. Ciliates move through coordinated beating of cilia, short hair-like structures on the cell. Two examples are Paramoecium and Balantidium.
4. Typical feeding stages of sporozoans lack locomotory cilia, flagella and pseudopodia. Two examples of the group are Plasmodium and Monocystis.

Q5. Explain fungal sexual reproduction in five points: compatible partners, plasmogamy, the dikaryophase, karyogamy and meiosis. Define n, n + n and 2n in your answer. [5 marks]

1. Compatible haploid cells or hyphae come together. Haploid means one chromosome set, represented by n; compatibility permits the partners to take part in sexual reproduction.
2. Plasmogamy is fusion of the partners' cell contents. This brings their nuclei into a shared cell, but does not necessarily fuse those nuclei immediately.
3. In ascomycetes and basidiomycetes, a dikaryophase intervenes. Each cell contains two separate haploid nuclei, represented by n + n, rather than one diploid nucleus.
4. Karyogamy is fusion of the two parental nuclei. It produces a diploid nucleus, represented by 2n, meaning that the nucleus contains two chromosome sets.
5. Meiosis reduces the chromosome-set number and leads to haploid spores. Thus the haploid condition is restored after the diploid stage of sexual reproduction.

Q6. Explain the division of labour in a lichen, naming both partners, and distinguish ectomycorrhiza from endomycorrhiza by hyphal position. Give four points. [4 marks]

1. The algal partner of a lichen is the phycobiont. It is autotrophic and prepares food used within the association.
2. The fungal partner is the mycobiont. It provides shelter and absorbs water and mineral nutrients for the algal partner.
3. In ectomycorrhiza, the fungus forms a mantle around the plant root and grows between root cells without entering them.
4. In endomycorrhiza, fungal hyphae extend into root tissues and penetrate cortical cells, distinguishing their position from that in ectomycorrhiza.

Q7. Give five points on viruses: their behaviour outside and inside host cells, their protein coat, their genetic material, and one contribution each of Ivanowsky and Stanley. [5 marks]

1. Viruses are inert outside their specific living host cells. Once inside a suitable cell, they use its machinery to replicate, explaining their dependence on a host.
2. The protein coat is the capsid, made of smaller units called capsomeres. It surrounds and protects the viral nucleic acid rather than forming a cellular wall.
3. Viral genetic material is either DNA, deoxyribonucleic acid, or RNA, ribonucleic acid. A virus does not contain both as its genetic material.
4. Ivanowsky recognised the agent responsible for tobacco mosaic disease and found that it passed through bacteria-proof filters, indicating that it was smaller than bacteria.
5. Stanley showed that viruses could be crystallised and that their crystals consisted largely of protein, providing evidence of their unusual behaviour outside living cells.

Q8. Distinguish a virus, a viroid and a prion by composition, giving one named example or associated disease for each. [3 marks]

1. A virus contains DNA or RNA enclosed in a protein coat called a capsid. Tobacco mosaic virus is an example, containing RNA.
2. A viroid consists of infectious low-molecular-weight RNA without the protein coat characteristic of a virus. It causes potato spindle tuber disease.
3. A prion consists of abnormally folded infectious protein. Bovine spongiform encephalopathy, commonly called mad cow disease in cattle, is a prion disease.

Key takeaways

• The three-domain system separates Bacteria, Archaea and Eukarya, while the five-kingdom system groups prokaryotes within Monera.
• Bacterial shape, nutrition, respiration and Gram-staining response are different classification criteria and should be identified separately.
• Binary fission increases bacterial cell number; conjugation, transformation and transduction are routes for transferring genetic information.
• Protists are primarily aquatic eukaryotes, with important differences in covering, nutrition, locomotion and spore formation.
• Fungal classification uses mycelial organisation and reproductive structures; external conidia differ from ascospores formed inside asci.
• Plasmogamy joins cell contents, karyogamy joins nuclei, and an intervening dikaryophase retains two separate haploid nuclei.
• Lichens associate algae with fungi; mycorrhiza associates fungi with plant roots, with partners performing complementary roles.
• Viruses contain nucleic acid and protein, viroids lack the protein coat, and prions consist of abnormal infectious protein.

Test yourself

Which two domains contain prokaryotic organisms?

Bacteria and Archaea contain prokaryotes; the domain Eukarya contains eukaryotic organisms.

What distinguishes a heterocyst from an ordinary descriptive name for a bacterial shape?

A heterocyst is a specialised nitrogen-fixing cell in some cyanobacteria, not a bacterial shape category.

What makes diatomaceous earth useful for polishing and filtration?

It consists of accumulated silica-containing diatom walls, forming a gritty material suitable for these uses.

How does a euglenoid obtain food when deprived of sunlight?

It behaves as a heterotroph, feeding on other smaller organisms instead of relying on photosynthesis.

Where are ascospores and basidiospores formed?

Ascospores form inside sac-like asci, whereas basidiospores form externally on basidia.

Why can a fungus be moved out of Deuteromycetes?

Discovery of its sexual stage permits identification of the class to which it belongs.

What did Beijerinck demonstrate using infected tobacco extract?

He demonstrated that extract from infected tobacco could transmit infection to healthy plants.

Which infectious agents are associated with BSE and CJD?

Prions, consisting of abnormally folded infectious protein, are associated with these neurological diseases.