Biological Classification — NEET notes

Kingdom Monera — The Prokaryotic World

The most ancient and diverse kingdom — includes all prokaryotes (bacteria and archaea). No membrane-bound nucleus, no organelles, yet they dominate every habitat on Earth from boiling hot springs to your gut.

General Features of Monera

Monerans are the simplest organisms structurally but the most diverse metabolically. They lack a nuclear membrane and membrane-bound organelles.

Kingdom Monera includes all prokaryotes — organisms whose cells lack a membrane-bound nucleus. Their genetic material (DNA) lies free in the cytoplasm in a region called the nucleoid. Despite being 'simple,' they are metabolically the most diverse kingdom — autotrophs, heterotrophs, chemotrophs, you name it.

• Cell type: Prokaryotic — no membrane-bound nucleus or organelles
• Cell wall: Most have a rigid cell wall (composition varies — peptidoglycan in eubacteria)
• Genetic material: Naked circular DNA (no histones, no true chromosomes)
• Reproduction: Mainly binary fission (asexual). Gene transfer via conjugation, transformation, transduction
• Nutrition: Autotrophic (photo/chemo) OR heterotrophic — most metabolically diverse kingdom
• Size: Typically 0.2 to 10 micrometres

Bacterial Shapes

Bacteria come in four main shapes, and NEET expects you to know each one with examples.

Bacteria are classified by shape into four categories. This is the simplest classification and a NEET staple.

• Coccus (plural: cocci) — spherical. Example: *Streptococcus*, *Staphylococcus*
• Bacillus (plural: bacilli) — rod-shaped. Example: *Lactobacillus*, *E. coli*
• Vibrio — comma-shaped. Example: *Vibrio cholerae* (causes cholera)
• Spirillum (plural: spirilla) — spiral/helical. Example: *Treponema pallidum* (causes syphilis)

Archaebacteria

Archaebacteria differ from typical bacteria in cell wall composition and can survive extreme environments — hot springs, salt lakes, and marshy swamps.

Archaebacteria are the most ancient living fossils. They live in the harshest environments on Earth — places that would kill any other organism. Their cell wall lacks peptidoglycan (unlike eubacteria), which is a key NCERT distinction.

• Methanogens — produce methane (CH4). Found in marshy areas and gut of ruminants (cows). Responsible for biogas production
• Halophiles — 'salt-loving.' Live in extremely salty environments like the Great Salt Lake and Dead Sea
• Thermoacidophiles — live in hot acidic environments like hot sulphur springs (high temperature + low pH)

Eubacteria

Eubacteria have a rigid peptidoglycan cell wall, are characterized by Gram staining, and include the vast majority of known bacteria.

Eubacteria ('true bacteria') are the most common bacteria you encounter. Their cell wall contains peptidoglycan (also called murein). They are broadly classified by Gram staining into Gram-positive (thick peptidoglycan layer, retain crystal violet stain) and Gram-negative (thin peptidoglycan layer, lose the stain).

• Gram-positive: Thick peptidoglycan wall. Retain crystal violet stain (appear purple). Example: *Bacillus*, *Streptococcus*
• Gram-negative: Thin peptidoglycan + outer lipid membrane. Lose crystal violet, take up safranin counterstain (appear pink/red). Example: *E. coli*, *Salmonella*
• Motility: Some are motile (flagella), some are non-motile
• Nutrition: Autotrophic (photosynthetic or chemosynthetic) or heterotrophic (saprophytic or parasitic)

Cyanobacteria

Cyanobacteria (formerly 'blue-green algae') are photosynthetic prokaryotes that have chlorophyll a and were responsible for oxygenating Earth's early atmosphere.

Cyanobacteria are photosynthetic prokaryotes — they have chlorophyll a (similar to plants) but are NOT plants (they're bacteria). They were once called 'blue-green algae' but that name is misleading because they lack a true nucleus and are prokaryotic.

• Have chlorophyll a — perform oxygenic photosynthesis (release O2)
• Can be unicellular (*Synechococcus*), colonial (*Gloeocapsa*), or filamentous (*Nostoc*, *Anabaena*)
• Many can fix atmospheric nitrogen (e.g., *Nostoc*, *Anabaena*) — using specialized cells called heterocysts
• Often form blooms in polluted water bodies (algal blooms) — indicator of water pollution
• Some are used as biofertilizers (e.g., *Anabaena* in association with *Azolla* in rice paddies)

Mycoplasma

Mycoplasma are the smallest known free-living organisms, lack a cell wall entirely, and are notorious for causing diseases and contaminating cell cultures.

Mycoplasma are unique among bacteria — they completely lack a cell wall. This makes them pleomorphic (can change shape) and resistant to antibiotics that target cell walls (like penicillin). They are the smallest known cells capable of independent life (0.1 to 0.3 micrometres).

• No cell wall — only a plasma membrane (hence resistant to penicillin and lysozyme)
• Smallest living cells capable of autonomous replication
• Pleomorphic — can assume various shapes due to absence of rigid wall
• Also called PPLO (Pleuro Pneumonia-Like Organisms) — discovered in cattle with pleuropneumonia
• Can be saprophytic or parasitic. Many are pathogenic — cause diseases in plants and animals

Mycoplasma are sometimes called the 'jokers of the microbial world' — they pass through bacterial filters (used to think they were viruses) and can assume any shape.

Kingdom Protista — The Eukaryotic 'Misfits'

The first eukaryotes — a diverse grab-bag of unicellular (mostly) organisms that don't fit neatly into fungi, plants, or animals. Includes chrysophytes, dinoflagellates, euglenoids, slime moulds, and protozoans.

General Features of Protista

Kingdom Protista was created as a 'catch-all' for eukaryotes that didn't fit the other three eukaryotic kingdoms. They show plant-like, animal-like, and fungus-like characteristics.

Protists are eukaryotic organisms — they have a true nucleus and membrane-bound organelles (a huge upgrade from Monera). Most are unicellular, though some form colonies. They reproduce both asexually (binary fission) and sexually (involving cell fusion/zygote formation).

• Cell type: Eukaryotic — true nucleus, membrane-bound organelles (mitochondria, ER, Golgi)
• Body form: Primarily unicellular, some colonial, a few multicellular (kelp)
• Locomotion: Cilia (*Paramecium*), flagella (*Euglena*), pseudopodia (*Amoeba*), or none
• Reproduction: Asexual (binary fission, spore formation) and sexual (by cell fusion forming zygote)
• Habitat: Mostly aquatic (freshwater and marine)
• Linking group: Protists connect prokaryotes to the complex eukaryotic kingdoms (Fungi, Plantae, Animalia)

Chrysophytes

Chrysophytes include diatoms (with silica cell walls) and desmids. Diatoms are the chief producers in oceans and their cell walls don't decay — forming diatomaceous earth.

Chrysophytes include diatoms and desmids (golden algae). Diatoms are among the most important organisms on Earth — they produce a huge fraction of the world's oxygen through photosynthesis.

• Found in freshwater and marine environments
• Diatoms have cell walls made of silica (SiO2) — called frustules. They are essentially embedded in 'glass boxes'
• The frustule has two overlapping halves that fit together like a soap box (one lid fits over the other)
• Diatom cell walls are indestructible — they pile up on ocean floors over millions of years forming diatomaceous earth
• Diatomaceous earth is used in: polishing, filtration, and as an insecticide
• Diatoms are the chief producers in the ocean — responsible for about 20-25% of global photosynthesis

Dinoflagellates

Dinoflagellates are photosynthetic protists with two flagella and cellulose plates. Some produce toxins that cause deadly Red Tides.

Dinoflagellates are marine, photosynthetic protists with a distinctive feature: they have two flagella — one longitudinal and one transverse — that cause them to spin as they swim ('dino' = whirling). Their cell wall has stiff cellulose plates on the outer surface.

• Mostly marine and photosynthetic
• Cell wall made of cellulose plates (not silica like diatoms)
• Two flagella — one transverse (in a groove around the cell), one longitudinal (trails behind)
• Color varies depending on pigments: yellow, green, brown, blue, or red
• Red dinoflagellates (e.g., *Gonyaulax*) multiply rapidly causing Red Tides — the toxins released kill fish in massive numbers
• Some are bioluminescent — produce light at night (glowing waves in oceans)

Euglenoids

Euglenoids like Euglena have chloroplasts (like plants) AND can move with flagella and ingest food (like animals). They lack a rigid cell wall — instead having a flexible pellicle.

Euglenoids are freshwater organisms found in stagnant water rich in organic matter. The most famous is *Euglena* — it has chloroplasts and can photosynthesize in sunlight, but in darkness it becomes heterotrophic (absorbs nutrients from water). This dual mode of nutrition is called mixotrophic.

• Mixotrophic — photosynthetic in light, heterotrophic in darkness
• No cell wall — instead have a protein-rich layer called pellicle (makes them flexible)
• Have two flagella — one short, one long (only the long one is used for locomotion)
• Have a photosensitive eyespot (stigma) — helps detect light for photosynthesis
• Chloroplasts are identical to those of higher plants — suggesting an evolutionary connection
• Found in stagnant, freshwater bodies rich in organic matter

Slime Moulds

Slime moulds are saprophytic protists that form a multinucleate plasmodium during favorable conditions and produce spores (like fungi) during unfavorable conditions.

Slime moulds are saprophytic protists that feed on decaying organic matter. During favorable conditions, they form an aggregation called plasmodium — a mass of protoplasm with many nuclei but no cell walls between them (multinucleate, acellular). When conditions turn bad, they form fruiting bodies that release spores with tough walls.

• Saprophytic — feed on decaying organic matter (leaves, logs, bark)
• Favorable conditions: form a plasmodium — multinucleate, creeping mass that engulfs food
• Unfavorable conditions: form fruiting bodies bearing spores with resistant walls
• Spores are extremely durable — survive harsh conditions and germinate when conditions improve
• Spore walls are tough enough to resist unfavorable conditions for years

Protozoans

Protozoans are animal-like protists — all heterotrophic. NCERT classifies them into four groups based on how they move: Amoeboid, Flagellated, Ciliated, and Sporozoans.

Protozoans are heterotrophic, predatory or parasitic protists. They act as important decomposers and also cause major diseases. NCERT divides them into four groups based on their locomotory structures.

• Amoeboid protozoans — move by pseudopodia ('false feet'). Example: *Amoeba*, *Entamoeba histolytica* (causes amoebic dysentery). Marine forms like *Foraminifera* have silica shells
• Flagellated protozoans — move by flagella. Many are parasitic. Example: *Trypanosoma* (causes sleeping sickness, spread by tsetse fly)
• Ciliated protozoans — move by cilia (thousands of tiny hair-like projections). Most complex protozoans. Example: *Paramecium* (has a gullet for feeding)
• Sporozoans — NO locomotory structures at all. All are parasitic with a spore-forming stage. Example: *Plasmodium* (causes malaria)

Kingdom Fungi — Closer to Animals Than Plants

Fungi are eukaryotic, heterotrophic organisms with chitinous cell walls. Surprisingly, molecular evidence shows they are more closely related to animals than to plants. Includes Phycomycetes, Ascomycetes, Basidiomycetes, and Deuteromycetes.

General Features of Fungi

Fungi are neither plants nor animals. They have chitin cell walls (like insects!), are heterotrophic (absorb nutrition), and their body is made of thread-like hyphae forming a mycelium.

Fungi are eukaryotic, heterotrophic organisms that absorb nutrition from their substrate. They cannot photosynthesize (no chlorophyll). Their cell wall is made of chitin (the same polymer found in insect exoskeletons) — NOT cellulose like plants.

• Cell wall: Made of chitin (a nitrogen-containing polysaccharide) — key NCERT distinction
• Body structure: Filamentous — made of long, thread-like structures called hyphae (singular: hypha). Network of hyphae = mycelium
• Hyphae types: Septate (with cross-walls/septa) or coenocytic (no septa — continuous tube with multiple nuclei)
• Nutrition: Saprophytic (dead matter), parasitic (living hosts), or symbiotic (mutualistic, e.g., lichens, mycorrhizae)
• Storage food: Glycogen (like animals, NOT starch like plants)
• Reproduction: Vegetative (fragmentation, budding), asexual (spores — conidia, sporangiospores, zoospores), sexual (oospores, ascospores, basidiospores)

Sexual Reproduction in Fungi

Fungal sexual reproduction follows a unique three-step process: plasmogamy (cytoplasm fusion), karyogamy (nuclear fusion), and meiosis. Between plasmogamy and karyogamy, a dikaryotic phase exists.

Sexual reproduction in fungi involves three sequential steps. The gap between the first two steps creates a unique dikaryotic stage (n+n) — two nuclei per cell, but they haven't fused yet.

• Step 1 — Plasmogamy: Fusion of the cytoplasm (protoplasm) of two cells. Two haploid nuclei now sit in the same cell but have NOT fused yet
• Step 2 — Karyogamy: Fusion of the two nuclei to form a diploid (2n) zygote nucleus. In many fungi, karyogamy is DELAYED — creating a dikaryotic phase (n+n) that can persist for a long time
• Step 3 — Meiosis: The diploid nucleus undergoes meiosis to produce haploid spores
• Dikaryotic stage (n+n): Two unfused haploid nuclei per cell — unique to fungi. Prominent in Ascomycetes and Basidiomycetes

Phycomycetes

Phycomycetes are the most primitive fungi — with coenocytic (aseptate) hyphae, aquatic or obligate parasites, and asexual reproduction by zoospores or aplanospores.

Phycomycetes ('algal fungi') are primitive fungi found in aquatic habitats, on decaying wood in moist areas, or as obligate parasites on plants. They are the simplest fungi.

• Mycelium: Aseptate (no septa) and coenocytic — continuous multinucleate tube
• Asexual reproduction: By zoospores (motile) or aplanospores (non-motile)
• Sexual reproduction: Forms zygospores by fusion of isogametes or heterogametes
• Examples: *Mucor* (bread mould), *Rhizopus* (common bread mould), *Albugo* (parasitic, causes white rust of mustard)

Ascomycetes

Ascomycetes produce sexual spores (ascospores) inside a sac-like structure called an ascus. Includes yeasts, morels, truffles, Penicillium, and Aspergillus.

Ascomycetes are called 'sac fungi' because their sexual spores (ascospores) are produced inside a sac-like structure called an ascus (plural: asci). They are the largest class of fungi — including some of the most economically important species.

• Mycelium: Branched, septate (with septa)
• Asexual spores: Conidia — produced exogenously (outside) on special hyphae called conidiophores
• Sexual spores: Ascospores — produced endogenously (inside) in a sac called ascus (typically 8 ascospores per ascus)
• Fruiting body: Ascocarp — contains multiple asci
• Examples: *Saccharomyces* (yeast — unicellular!), *Aspergillus*, *Claviceps* (causes ergot of rye), *Neurospora* (used in genetic research), *Penicillium*, Morels, Truffles

Basidiomycetes

Basidiomycetes produce sexual spores (basidiospores) on club-shaped structures called basidia. This group includes mushrooms, bracket fungi, puffballs, and agricultural pests like rusts and smuts.

Basidiomycetes are called 'club fungi' because their sexual spores (basidiospores) are produced on a club-shaped structure called a basidium (plural: basidia). This is the most familiar group — mushrooms, toadstools, bracket fungi, and puffballs are all basidiomycetes.

• Mycelium: Branched, septate
• Asexual spores: Generally absent — this is a key distinction from other classes
• Sexual spores: Basidiospores — produced exogenously (outside) on a club-shaped basidium (typically 4 per basidium)
• Fruiting body: Basidiocarp (e.g., the mushroom cap you see above ground)
• Prominent dikaryotic phase — the secondary mycelium (n+n) is long-lived and forms the visible basidiocarp
• Examples: *Agaricus* (edible mushroom), *Ustilago* (smut), *Puccinia* (rust), bracket fungi, puffballs

Deuteromycetes

Deuteromycetes are called 'Fungi Imperfecti' because their sexual reproduction has never been observed. Once the sexual stage is discovered, they get reclassified into Ascomycetes or Basidiomycetes.

Deuteromycetes are a temporary class — a holding area for fungi whose sexual reproduction has never been observed. Since fungal classification relies heavily on the type of sexual spore, fungi without a known sexual stage are dumped here. Once a sexual stage is discovered, the fungus is moved to Ascomycetes or Basidiomycetes.

• Also called Fungi Imperfecti ('imperfect' = no known sexual reproduction)
• Reproduce only asexually — by conidia
• Septate, branched mycelium
• Saprophytic or parasitic — many cause plant and human diseases
• Examples: *Alternaria* (plant pathogen), *Colletotrichum* (causes anthracnose), *Trichoderma* (used as biocontrol agent)

Comparison of Fungal Classes

A comparative overview of all four classes of fungi based on mycelium type, asexual spores, sexual spores, fruiting body, and examples.

This comparison table is pure gold for NEET — nearly every year, at least one question can be answered directly from this table.

Kingdom Plantae — The Autotrophic Multicellular Kingdom

All multicellular, eukaryotic, photosynthetic organisms with cellulose cell walls. Ranges from simple algae to complex angiosperms. Some exceptions exist — insectivorous plants are partially heterotrophic, parasitic plants lack chlorophyll.

General Features of Plantae

Kingdom Plantae includes all multicellular, photosynthetic eukaryotes. They have cellulose cell walls, chloroplasts with chlorophyll, and show alternation of generations.

Plants are multicellular eukaryotes with cellulose cell walls and chloroplasts containing chlorophyll a and b. They are primarily autotrophic (make their own food via photosynthesis), though a few are partially heterotrophic.

• Cell wall: Made of cellulose (unlike chitin in fungi or peptidoglycan in bacteria)
• Nutrition: Primarily autotrophic (photosynthetic). Exceptions: *Cuscuta* (parasitic, no chlorophyll), insectivorous plants like *Venus flytrap* (supplement nitrogen by trapping insects), *Rafflesia* (holoparasite)
• Storage food: Starch (unlike glycogen in fungi/animals)
• Life cycle: Shows alternation of generations — diploid sporophyte alternates with haploid gametophyte
• Includes: Algae, Bryophytes, Pteridophytes, Gymnosperms, Angiosperms

Plant Classification Overview

A quick overview of the five major plant groups, arranged from simplest (algae) to most complex (angiosperms), showing the evolutionary trend of increasing complexity.

Plants show a clear evolutionary progression from water to land, from simple to complex. Each group represents a major step in plant evolution.

• Algae — simplest; mostly aquatic; no true roots/stems/leaves; thallus body. Examples: *Spirogyra*, *Ulva*, kelp
• Bryophytes — 'amphibians of plant kingdom'; first land plants but need water for reproduction; no vascular tissue. Examples: Mosses, *Marchantia* (liverwort), *Anthoceros* (hornwort)
• Pteridophytes — first plants with vascular tissue (xylem, phloem); still need water for fertilization. Examples: Ferns, *Selaginella*, *Equisetum* (horsetail)
• Gymnosperms — first seed-bearing plants; seeds are naked (not enclosed in fruit); do NOT need water for fertilization (pollen!). Examples: *Pinus* (pine), *Cycas*, *Ginkgo*
• Angiosperms — most advanced; seeds enclosed in fruit; have flowers for reproduction; dominant plant group on Earth. Examples: Rice, wheat, mango, rose

Alternation of Generations

All plants alternate between a diploid sporophyte (produces spores) and a haploid gametophyte (produces gametes). The dominant generation shifts from gametophyte to sporophyte as we move from bryophytes to angiosperms.

Every plant alternates between two life stages: the sporophyte (2n, diploid — produces spores by meiosis) and the gametophyte (n, haploid — produces gametes by mitosis). The key evolutionary trend is a shift from dominant gametophyte (bryophytes) to dominant sporophyte (angiosperms).

• Bryophytes: Gametophyte is dominant, sporophyte is dependent on gametophyte (the green moss plant IS the gametophyte)
• Pteridophytes: Sporophyte is dominant, gametophyte (prothallus) is small and independent but short-lived
• Gymnosperms and Angiosperms: Sporophyte is highly dominant, gametophyte is reduced and dependent on sporophyte
• Trend: Gametophyte becomes progressively smaller and more dependent; sporophyte becomes dominant

Kingdom Animalia — Multicellular Heterotrophs Without Cell Walls

The largest kingdom by species count — all multicellular, eukaryotic, heterotrophic organisms without cell walls. They exhibit a definite body form, growth, and most have locomotion. Detailed classification is covered in Chapter 4.

General Features of Animalia

Animals are multicellular eukaryotes with no cell wall, heterotrophic (holozoic) nutrition, and a definite body form with determinate growth.

Kingdom Animalia is the largest kingdom in terms of species diversity (~1.5 million known species). All animals share some basic features that separate them from every other kingdom.

• Cell type: Eukaryotic — no cell wall (only a plasma membrane). This is a key distinction from plants and fungi
• Nutrition: Heterotrophic — specifically holozoic (ingest food, then digest it internally). Unlike fungi which absorb nutrients externally
• Body form: Multicellular with definite body form (shape) — except sponges which have irregular shapes
• Growth: Determinate — growth stops at a certain point (unlike plants which grow throughout life)
• Storage food: Glycogen (same as fungi, NOT starch like plants)
• Locomotion: Most animals can move (exception: sponges, corals, and some other sessile organisms)
• Reproduction: Mostly sexual (gamete formation and fertilization); some reproduce asexually (budding in *Hydra*)

Animal Classification Preview

A brief preview of how animals are classified based on levels of organization, symmetry, germ layers, coelom, and segmentation. Full details in Chapter 4.

Animal classification uses body plan features as the basis for grouping. These features represent evolutionary milestones — each new feature opened up possibilities for greater complexity.

• Levels of organization: Cellular (sponges) then Tissue (cnidarians) then Organ (flatworms) then Organ system (all higher animals)
• Symmetry: Asymmetrical (sponges) then Radial (cnidarians, echinoderms) then Bilateral (most animals)
• Germ layers: Diploblastic (2 layers: ectoderm + endoderm — cnidarians) vs Triploblastic (3 layers: + mesoderm — all others)
• Coelom: Acoelomate (no body cavity — flatworms) then Pseudocoelomate (false cavity — roundworms) then Coelomate (true cavity — annelids onward)
• Segmentation: Absent in most then Present in annelids, arthropods, chordates
• Notochord: Absent = Non-chordates vs Present = Chordates

NCERT gives only a brief mention of Animalia in Chapter 2 (Biological Classification). The real deep dive into animal classification happens in Chapter 4. For now, just know the general features and classification criteria.

Viruses, Viroids & Lichens — The Misfits of Biology

Viruses don't fit any kingdom — they are non-cellular, obligate intracellular parasites with either DNA or RNA. Viroids are even simpler (just naked RNA). Prions are just misfolded proteins. Lichens are a mutualistic partnership between algae/cyanobacteria and fungi.

Virus Structure

Viruses are non-cellular entities with a nucleic acid core (DNA or RNA, never both) surrounded by a protein coat (capsid). They are obligate intracellular parasites — inert outside a host cell.

Viruses are the ultimate biological paradox — outside a host, they are inert crystal-like particles with no metabolism, no reproduction, no growth. Inside a host cell, they hijack the cell's machinery to replicate explosively. They are not placed in any kingdom because they are not truly 'living' by conventional definitions.

• Non-cellular — no cell membrane, no cytoplasm, no organelles, no ribosomes
• Genetic material: Either DNA or RNA, NEVER BOTH. This is a fundamental rule
• Protein coat: Called capsid — made of protein subunits called capsomeres. Protects the nucleic acid
• Obligate intracellular parasites — can only replicate inside a living host cell. Inert outside
• Host-specific: Plant viruses usually have RNA, animal viruses have DNA or RNA, bacteriophages usually have DNA
• Some viruses have an additional envelope (lipid bilayer from host cell membrane) — e.g., HIV, influenza

TMV and Bacteriophage

Tobacco Mosaic Virus (TMV) is a rod-shaped RNA virus infecting plants. Bacteriophage (e.g., T4 phage) is a complex DNA virus infecting bacteria with a distinct head-tail structure.

Two viruses are NCERT favorites: TMV (the first virus discovered) and bacteriophage (viruses that infect bacteria). They illustrate two very different viral architectures.

• TMV: First virus to be discovered, first to be crystallized (by Stanley), RNA genome, rod-shaped, infects tobacco plants causing mosaic pattern on leaves
• Bacteriophage structure: Icosahedral head (contains DNA) connected to a tail with tail fibers and base plate (for attachment to bacterial cell wall)
• Phages inject their DNA into bacteria while the protein coat stays outside — proven by the famous Hershey-Chase experiment (1952)

Viral Diseases

Viruses cause diseases in all forms of life. Key diseases to remember for NEET include AIDS (HIV), influenza, mumps, smallpox, polio in animals, and mosaic diseases in plants.

Viruses are responsible for some of the most devastating diseases in human history. NCERT mentions several, and NEET regularly tests virus-disease associations.

• Human viral diseases: AIDS (HIV — retrovirus, RNA), influenza, common cold (rhinovirus), mumps, smallpox (variola), polio, measles, herpes, dengue, chikungunya
• Plant viral diseases: Tobacco mosaic disease (TMV), cauliflower mosaic, turnip yellow mosaic, potato leaf roll
• Bacteriophages: Infect and often kill bacteria — basis for phage therapy (alternative to antibiotics)
• HIV (AIDS virus): A retrovirus — has RNA genome + reverse transcriptase enzyme that converts RNA to DNA inside host cell

Viroids

Viroids are infectious agents even simpler than viruses — they are just short, circular, single-stranded RNA molecules with no protein coat at all. Discovered by T.O. Diener (1971).

If viruses are minimal, viroids are ultra-minimal. A viroid is literally a small, circular, single-stranded RNA molecule — no protein coat, no capsid, no envelope. Just naked RNA. Yet they can infect plants and cause diseases.

• Discovered by T.O. Diener (1971) while studying potato spindle tuber disease
• Structure: Short, circular, single-stranded RNA — 5 to 10 times SMALLER than the smallest virus
• No protein coat — just free RNA (this is the key difference from viruses)
• No translation — viroid RNA does NOT code for any protein. It replicates by using host cell enzymes
• Cause diseases only in plants — examples: potato spindle tuber disease, citrus exocortis disease
• Molecular weight: Much lower than even the smallest viral RNA

Prions

Prions are misfolded proteins that can cause other normal proteins to misfold — causing fatal neurodegenerative diseases. They have no DNA or RNA whatsoever.

Prions are the most bizarre 'infectious agents' — they are misfolded proteins with absolutely no nucleic acid (no DNA, no RNA). Yet they are 'infectious' because they can convert normal proteins into the misfolded form, creating a chain reaction in the brain.

• No nucleic acid — just abnormally folded protein. This was revolutionary because the 'central dogma' assumed all infectious agents need nucleic acid
• Discovered by: Stanley B. Prusiner (won Nobel Prize in 1997)
• Mechanism: Prion protein (PrPSc) contacts normal brain protein (PrPC) and converts it to the misfolded form — a domino effect
• Diseases: Bovine spongiform encephalopathy (BSE or 'mad cow disease'), Creutzfeldt-Jakob disease (CJD) in humans, scrapie in sheep
• Cannot be destroyed by normal sterilization — resistant to heat, radiation, and most chemicals

Lichens

Lichens are composite organisms formed by a mutualistic association between a fungus (mycobiont) and an alga or cyanobacterium (phycobiont). They are pioneer colonizers and pollution indicators.

A lichen is NOT a single organism — it's a mutualistic partnership between a fungus (called the mycobiont) and a photosynthetic partner — either a green alga or a cyanobacterium (called the phycobiont). Neither can survive well alone in the same habitat, but together they thrive.

• Mycobiont (fungal partner): Usually an ascomycete. Provides shelter, water, and mineral absorption
• Phycobiont (algal/cyanobacterial partner): Provides food through photosynthesis (supplies carbohydrates to the fungus)
• Mutualism: Both partners benefit — classic textbook example of mutualistic symbiosis
• Pioneer species: Lichens are among the first organisms to colonize bare rocks — they secrete acids that break down rock, initiating soil formation
• Pollution indicators: Lichens are extremely sensitive to SO2 (sulphur dioxide). Their absence from an area indicates air pollution. Used as bioindicators of air quality
• Types by morphology: Crustose (crust-like, flat on rock), Foliose (leaf-like, loosely attached), Fruticose (shrub-like, hanging)

Quick recall

In a lichen, which partner provides shelter and which provides food?

The fungus (mycobiont) provides shelter, water, and minerals. The alga/cyanobacterium (phycobiont) provides food via photosynthesis. It's a 'room and board' arrangement — fungus gives the room, alga provides the food!