Biological Classification

BIOLOGICAL CLASSIFICATION

**Definition:** Biological classification is the systematic arrangement of living organisms into groups and categories based on shared characteristics, evolutionary relationships, and structural features.

**Historical Context:**

**Aristotle** (earliest attempt): Classified plants as trees, shrubs, herbs; animals as red-blooded and non-red-blooded (simple morphological criteria)

**Linnaeus (Two Kingdom System):** Divided all organisms into Kingdom Plantae and Kingdom Animalia

**Limitation:** Did not distinguish between prokaryotes and eukaryotes, unicellular and multicellular organisms, or photosynthetic and non-photosynthetic organisms

Example problem: Fungi and green algae grouped together merely because both had cell walls, ignoring fundamental metabolic differences

**Need for Modern Classification:**

Gross morphology alone proved inadequate

Required criteria: cell structure, cell wall composition, mode of nutrition, habitat, reproduction methods, and evolutionary (phylogenetic) relationships

Many organisms did not fit neatly into Plant or Animal kingdoms

**R.H. Whittaker's Five Kingdom Classification (1969):**

Based on: cell structure, body organisation, mode of nutrition, reproduction, and phylogenetic relationships

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TABLE 2.1: CHARACTERISTICS OF FIVE KINGDOMS

| **Character** | **Monera** | **Protista** | **Fungi** | **Plantae** | **Animalia** |

|---|---|---|---|---|---|

| **Cell Type** | Prokaryotic | Eukaryotic | Eukaryotic | Eukaryotic | Eukaryotic |

| **Cell Wall** | Noncellulosic (polysaccharide + amino acid) | Present in some | Present (chitin) | Present (cellulose) | Absent |

| **Nuclear Membrane** | Absent | Present | Present | Present | Present |

| **Body Organisation** | Cellular | Cellular | Multicellular/loose tissue | Tissue/organ | Tissue/organ/organ system |

| **Mode of Nutrition** | Autotrophic (photosynthetic, chemosynthetic) and Heterotrophic (saprotrophic, parasitic) | Autotrophic (photosynthetic) and Heterotrophic | Heterotrophic (saprotrophic, parasitic) | Autotrophic (photosynthetic) | Heterotrophic (holozoic, saprotrophic) |

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2.1 KINGDOM MONERA

**Definition:** Kingdom Monera contains all prokaryotic organisms; bacteria are the sole members.

**Characteristics:**

**Prokaryotic** cells (no true nucleus or membrane-bound organelles)

Most abundant microorganisms on Earth

Found almost everywhere: soil, extreme habitats (hot springs, deserts, snow, deep oceans)

Many parasites

Reproduce mainly by **binary fission** (asexual)

**Bacterial Shapes (Figure 2.1):**

**Coccus** (cocci, plural): spherical shape

**Bacillus** (bacilli, plural): rod-shaped

**Vibrium** (vibrio, plural): comma-shaped

**Spirillum** (spirilla, plural): spiral/helical shape

**Metabolic Diversity:**

**Autotrophic bacteria:**

Photosynthetic autotrophs: synthesize food from inorganic matter using light energy

Chemosynthetic autotrophs: oxidize inorganic substances (nitrates, nitrites, ammonia) and use released energy for ATP production; play crucial role in nutrient cycling (N, P, Fe, S)

**Heterotrophic bacteria:** vast majority; depend on dead organic matter or living organisms

Saprophytic decomposers (most common, ecologically important)

Parasitic: cause diseases (cholera, typhoid, tetanus, citrus canker)

Beneficial: yogurt/curd production, antibiotic synthesis (Penicillium), nitrogen fixation in legume roots

**Reproduction:**

**Sexual reproduction:** primitive DNA transfer between bacteria

**Asexual reproduction:** binary fission (main method)

**Under unfavourable conditions:** spore formation for survival

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2.1.1 Archaebacteria

**Definition:** Special bacteria living in extreme harsh habitats; differ from other bacteria by unique cell wall structure.

**Types and Habitats:**

**Halophiles:** extreme salty areas (salt marshes, salt lakes)

**Thermoacidophiles:** hot springs (temperature >80°C, acidic pH)

**Methanogens:** marshy areas, anaerobic environments; gut of ruminants (cows, buffaloes)

Produce methane (biogas) from animal dung

Exam point: Commercial importance in biogas production

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2.1.2 Eubacteria (True Bacteria)

**Characteristics:**

Rigid cell wall

Flagellum present if motile

Thousands of species

**Cyanobacteria (Blue-green Algae):**

Photosynthetic autotrophs (contain chlorophyll a like green plants)

Unicellular, colonial, or **filamentous** forms

Freshwater, marine, or terrestrial

Surrounded by gelatinous sheath (colonial forms)

Often form **algal blooms** in polluted water bodies (eutrophication)

**Nitrogen fixation:** specialized cells called **heterocysts** fix atmospheric nitrogen (Example: Nostoc, Anabaena)

Ecologically important: convert N₂ to usable forms

Figure 2.2: Nostoc (filamentous form)

**Chemosynthetic Autotrophic Bacteria:**

Oxidize inorganic substances without sunlight

Example: Nitrifying bacteria (Nitrosomonas, Nitrobacter)

Nutrient cycling role: nitrogen and sulfur cycles

**Heterotrophic Bacteria:**

Most abundant; important decomposers

**Helpful applications:** curd fermentation, antibiotic production, nitrogen fixation in legume nodules

**Pathogens:** cholera (Vibrio cholerae), typhoid (Salmonella typhi), tetanus (Clostridium tetani), citrus canker (Xanthomonas citri)

**Mycoplasma:**

Lack cell wall completely

Smallest living cells known

Can survive without oxygen (facultative anaerobes)

Many pathogenic in animals and plants

Cause plant diseases (witches' broom) and animal diseases (pleuropneumonia)

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2.2 KINGDOM PROTISTA

**Definition:** Kingdom containing all single-celled eukaryotes; primarily aquatic; boundaries not well-defined.

**Characteristics:**

**Eukaryotic:** well-defined nucleus and membrane-bound organelles

Primarily **aquatic** (freshwater and marine)

Forms link between kingdoms: Plants, Animals, and Fungi

Some have flagella or cilia

**Reproduction:** asexual (binary fission, budding) and sexual (involving cell fusion and zygote formation)

Diagrammatic mention: Figure 2.4 (a-d) shows various protistans

**Exam Point:** Includes organisms previously classified in different kingdoms—example: Chlamydomonas (earlier in Plants, algae) and Paramoecium (earlier in Animals) now together in Protista based on eukaryotic unicellular nature.

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2.2.1 Chrysophytes

**Composition:** Diatoms and golden algae (desmids)

**Habitat:** Fresh water and marine environments

**Characteristics:**

Microscopic; **float passively** in water (plankton)

Mostly photosynthetic

Cell wall: two thin overlapping silica-embedded shells (like soap box)

Walls indestructible due to silica

Chief producers (primary producers) in oceans

**Ecological and Economic Importance:**

Over billions of years, accumulated silica shells form **diatomaceous earth** (gritty soil)

Uses: polishing compounds, filtration of oils and syrups, insecticides

Diatom diversity indicates water quality; used as water pollution indicators

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2.2.2 Dinoflagellates

**Characteristics:**

Mostly **marine** and photosynthetic

Colored: yellow, green, brown, blue, or red (based on pigments)

Cell wall: stiff cellulose plates on outer surface

**Two flagella:** one longitudinal, one transverse in furrow between plates

**Photosynthetic pigments:** chlorophyll a and c, and xanthophyll

**Red Tides (Harmful Algal Bloom):**

Red dinoflagellates (Example: Gonyaulax) undergo rapid multiplication

Ocean appears red

Released toxins kill fish and other marine animals

Causes economic loss to fisheries

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2.2.3 Euglenoids

**Habitat:** Freshwater, stagnant water

**Characteristics:**

**No cell wall:** protein-rich layer called **pellicle** (flexible body)

**Two flagella:** one short, one long

Photosynthetic pigments identical to higher plants (chlorophyll a and b)

**Nutrition:** autotrophic in sunlight; heterotrophic (predatory) in darkness

**Example:** Euglena (Figure 2.4b)

**Exam Point:** Demonstrates metabolic flexibility and transition between autotrophy and heterotrophy; evidence of evolutionary link between plants and animals

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2.2.4 Slime Moulds

**Characteristics:**

**Saprophytic protists** (heterotrophic, feed on dead matter)

Body moves along decaying twigs, leaves, engulfing organic material

**Plasmodium:** aggregation of cells under suitable conditions; may grow over several feet

Appearance: colorless, gelatinous mass

**Reproduction under Unfavourable Conditions:**

Plasmodium differentiates

Forms **fruiting bodies** with spores at tips

Spores possess true walls; extremely resistant; survive years under adverse conditions

Dispersed by **air currents**

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2.2.5 Protozoans

**Definition:** Heterotrophic protists living as predators or parasites; believed primitive relatives of animals.

**Four Major Groups:**

**1. Amoeboid Protozoans:**

Habitat: freshwater, seawater, moist soil

Locomotion: **pseudopodia** (false feet)

Marine forms: silica shells

Examples: Amoeba (free-living), Entamoeba (parasitic—causes amoebic dysentery)

**2. Flagellated Protozoans:**

Free-living or parasitic

Possess flagella

Parasitic forms cause sleeping sickness (African trypanosomiasis)

Example: **Trypanosoma** (causes sleeping sickness in humans; transmitted by tsetse fly)

**3. Ciliated Protozoans:**

Aquatic, actively moving (thousands of cilia)

**Gullet:** cavity opening to cell surface

**Coordinated ciliary movement** creates water currents, steering food into gullet

Example: **Paramoecium** (Figure 2.4d)

**4. Sporozoans:**

Spore-like infectious stage in life cycle

**Plasmodium:** malarial parasite

Causes malaria (transmitted by Anopheles mosquito)

Staggering impact on human population

Intracellular parasite in RBCs

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2.3 KINGDOM FUNGI

**Definition:** Heterotrophic eukaryotic organisms with unique morphology, nutrition, and reproduction; show great diversity.

**Characteristics:**

**Heterotrophic:** absorb soluble organic matter (no photosynthesis)

**Eukaryotic:** membrane-bound organelles

Cosmopolitan: air, water, soil, animals, plants

Prefer warm, humid conditions

Cell wall: **chitin and polysaccharides** (NOT cellulose like plants)

**Body Structure:**

**Except yeasts (unicellular):** filamentous

**Hyphae:** long, slender, thread-like structures

**Mycelium:** network of hyphae

**Coenocytic hyphae:** continuous tubes, multinucleated cytoplasm (no cross walls)

**Septate hyphae:** cross walls (septae) dividing into compartments

**Modes of Nutrition:**

**Saprophytes:** decompose dead substrate (majority); ecologically crucial

**Parasites:** live on living plants and animals; cause diseases (wheat rust from Puccinia)

**Symbionts:**

Lichens (with algae)

Mycorrhiza (with plant roots; mutually beneficial)

**Common Examples:**

Bread mould: Rhizopus

Yeast: Saccharomyces (unicellular)

Penicillium (antibiotic source)

Aspergillus

Mushroom: Agaricus

Toadstool

Wheat rust: Puccinia

**Reproduction:**

**Vegetative (Asexual) Reproduction:**

Fragmentation of mycelium

Fission

Budding (especially yeasts)

**Asexual Reproduction (Spore Formation):**

**Spore types:** conidia, sporangiospores, zoospores

Formed in distinct structures: **fruiting bodies**

Spores allow dispersal and survival

**Sexual Reproduction:**

**Three-Step Process:**

1. **Plasmogamy:** Fusion of protoplasm between two motile or non-motile gametes

Gametes: haploid hyphae of compatible mating types

2. **Karyogamy:** Fusion of two nuclei

Results in diploid (2n) cells

**OR** intervening **dikaryotic stage** (n+n, two nuclei per cell)—called **dikaryon** or **dikaryophase**

Later, nuclei fuse to become diploid

3. **Meiosis:** Occurs in fruiting bodies (zygote undergoes reduction division)

Results in **haploid spores**

Spore types: oospores, ascospores, basidiospores

**Exam Point:** Sexual cycle allows genetic recombination; spores are dispersal and survival units.

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2.3.1 Phycomycetes

**Habitat:** Aquatic; decaying wood in moist/damp places; obligate parasites on plants

**Structure:**

**Mycelium:** aseptate and coenocytic (no cross walls)

**Asexual Reproduction:**

**Zoospores:** motile spores produced endogenously in sporangium

**Aplanospores:** non-motile spores

Spores allow aquatic dispersal

**Sexual Reproduction:**

**Zygospore:** formed by fusion of two gametes

**Gamete types:**

**Isogamous:** gametes similar in morphology

**Anisogamous/Oogamous:** dissimilar gametes

Thick-walled zygospore survives unfavourable conditions

**Common Examples:**

**Mucor** (bread mould) (Figure 2.5a)

**Rhizopus** (bread mould; common in kitchens)

**Albugo** (parasitic on mustard; causes white spots)

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2.3.2 Ascomycetes (Sac Fungi)

**Structure:**

Mostly multicellular; rarely unicellular (yeast—Saccharomyces)

**Mycelium:** branched and septate

Examples: Penicillium, Aspergillus, Claviceps, Neurospora

**Nutrition Modes:**

Saprophytic decomposers

Parasitic

Coprophilous (growing on dung)

**Asexual Reproduction:**

**Conidia:** spores produced exogenously (on surface) from specialized hyphae called conidiophores

Conidia germinate to produce mycelium

**Sexual Reproduction:**

**Ascospores:** sexual spores produced endogenously (inside) in sac-like structures called **asci** (singular: ascus)

**Ascocarps:** fruiting bodies containing asci; different morphological types

Reduction division occurs within asci, producing 8 haploid ascospores (typically)

**Noteworthy Members:**

**Penicillium:** antibiotic source; common on bread

**Aspergillus** (Figure 2.5b): decomposer

**Neurospora:** extensively used in biochemical and genetic research (pink bread mould)

**Morels and truffles:** edible, considered delicacies

**Claviceps:** parasitic on grasses

**Exam Point:** Most ascomycetes are beneficial; used in biotechnology and food production.

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2.3.3 Basidiomycetes

**Common Forms:** Mushrooms, bracket fungi, puffballs

**Habitat:** Soil, logs, tree stumps, living plant bodies (some parasitic as rusts and smuts)

**Structure:**

**Mycelium:** branched and septate

**Asexual Reproduction:**

Generally absent

**Vegetative reproduction:** fragmentation (common)

**Sexual Reproduction:**

**No sex organs**

**Plasmogamy:** brought about by fusion of two vegetative/somatic cells of different strains or genotypes

Results in **dikaryotic structure** (n+n)

**Karyogamy:** nuclei fuse in basidium (diploid stage)

**Meiosis:** in basidium produces four haploid basidiospores

**Fruiting Bodies:**

**Basidium:** (plural: basidia) specialized cell bearing spores

**Basidiospores:** four spores, produced exogenously on basidium surface

**Basidiocarps:** fruiting bodies containing basidia (visible mushroom structure)

**Common Examples:**

**Agaricus** (common edible mushroom) (Figure 2.5c)

**Ustilago:** smut (causes crop diseases)

**Puccinia:** rust fungus (causes wheat rust—serious agricultural problem)

Requires two hosts (wheat and barberry) for complete life cycle

Tremendous economic loss

**Exam Point:** Basidiomycetes important in ecosystem (decomposers) and agriculture (pathogens); economically significant edible species.

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2.3.4 Deuteromycetes (Imperfect Fungi)

**Definition:** Fungi with only asexual/vegetative phases known; sexual forms not yet discovered.

**Characteristics:**

Called "imperfect" because sexual reproductive stages absent or unknown

When sexual forms discovered, transferred to appropriate class

Asexual reproduction by conidia from conidiophores

**Examples:**

Penicillium notatum (antibiotic)

Aspergillus (multiple species)

Trichoderma

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2.4 KINGDOM PLANTAE

**Covered in Chapter 3** — plant classification system with detailed categories.

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2.5 KINGDOM ANIMALIA

**Covered in Chapter 4** — animal classification system with detailed categories.

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2.6 VIRUSES, VIROIDS, AND LICHENS

Viruses

**Definition:** Obligate intracellular parasites; genetic material (DNA or RNA) surrounded by protein coat; exist at boundary between living and non-living.

**Characteristics:**

**Acellular:** no true cells

**Genetic material:** DNA or RNA (single or double-stranded)

**Protein coat:** capsid (protective protein shell)

Some viruses have **lipid envelope** (derived from host cell membrane)

Reproduce only inside host cells

Cannot synthesize proteins or ATP independently

Require host cell machinery for replication

**Virus Structure:**

**Capsid:** protein coat surrounding nucleic acid

**Genetic material:** core (DNA or RNA)

**Envelope:** (in some) lipid layer with glycoproteins for host cell recognition

**Classification by Genetic Material:**

**DNA viruses:** bacteriophages, poxviruses, herpes viruses

**RNA viruses:** influenza, HIV, measles, polio

**Replication (General Viral Lifecycle):**

1. **Attachment:** virus binds to specific host cell receptor

2. **Penetration:** genetic material enters host cell

3. **Biosynthesis:** viral DNA/RNA directs host machinery to synthesize viral components

4. **Assembly:** viral proteins and nucleic acids assemble into virions

5. **Release:** new viruses exit host cell (lysis or budding)

Lysis: host cell ruptures (lytic cycle)

Budding: gradual release (enveloped viruses)

**Human Diseases Caused by Viruses:**

**DNA viruses:** chicken pox (Varicella zoster), smallpox (eradicated)

**RNA viruses:** influenza, polio, measles, COVID-19 (SARS-CoV-2), HIV/AIDS

**Retroviruses:** HIV (reverse transcription: RNA → DNA)

**Plant Viruses:**

Tobacco mosaic virus (TMV): causes mosaic disease

Potato virus

Cause significant agricultural losses

**Bacteriophages:**

Viruses infecting bacteria

Used in genetic research and biotechnology

Used therapeutically as phage therapy

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Viroids

**Definition:** Infectious agents smaller than viruses; contain only RNA without protein coat.

**Characteristics:**

**Smallest infectious agents** known

**Genetic material:** small, circular RNA (very small size, ~360-400 nucleotides)

**No protein coat:** naked RNA

Replicate inside plant cells only

Cause plant diseases

**Examples:**

Potato spindle tuber viroid (PSTV)

Citrus exocortis viroid (CEV)

Chrysanthemum stunt viroid (CSV)

Causes economic loss in agriculture (potato, citrus, chrysanthemums)

**Difference from Viruses:**

Viruses: RNA/DNA + protein coat

Viroids: naked RNA only; smaller; plant-specific; no protein synthesis capability

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Lichens

**Definition:** Symbiotic association between fungus and photosynthetic partner (alga or cyanobacterium); mutualistic relationship.

**Composition:**

**Fungal partner (mycobiont):** provides structure, absorbs water and minerals

Usually ascomycete or basidiomycete

**Photosynthetic partner (photobiont):** provides food

Usually green alga (Chlorella, Trebouxia)

OR cyanobacterium (Nostoc, Anabaena)

**Structure:**

Fungal hyphae form outer cortex (protective layer)

Photosynthetic layer below cortex

Inner medulla (loose fungal hyphae)

Basal cortex for attachment to substrate

**Soredia** and **isidia:** asexual reproduction structures

**Growth and Reproduction:**

**Slow growth rate:** millimeters per year

**Reproduction:**

Asexual: soredia (fragments with algal and fungal cells)

Sexual: fungus produces spores; must meet suitable alga for new lichen

**Longevity:** extremely long-lived; some >1000 years old

**Habitat and Ecology:**

Found on rocks, tree bark, soil

Tolerate extreme habitats (deserts, mountains, arctic)

Pioneer species (first to colonize bare rock; initiate soil formation)

Ecological importance: nitrogen fixation (if cyanobacterium present); weathering rock

**Ecological Importance:**

**Bioindicators:** sensitive to pollution (air pollution, heavy metals); lichen distribution indicates air quality

Used in air quality monitoring

Provide food for reindeer (Reindeer lichen)

Antibiotic compounds; source of dyes

**Symbiotic Benefits:**

Fungus: receives carbohydrates from alga/cyanobacterium

Alga/Cyanobacterium: protected by fungal structure; access to water and minerals; nitrogen fixation (cyanobacteria)

Both benefit: true mutualism

**Nutrient Cycling:**

Cyanobacterial partners fix atmospheric nitrogen

Contribute to nitrogen cycling in ecosystems

Rock weathering initiates soil formation

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EXAM-IMPORTANT POINTS

**Classification Criteria:**

Cell type (prokaryotic vs. eukaryotic)

Cell wall composition (cellulose, chitin, peptidoglycan, absent)

Nuclear membrane presence/absence

Body organisation level (cellular, tissue, organ)

Mode of nutrition (autotrophy, heterotrophy type)

Reproduction methods

**Five Kingdom System Advantages:**

Separates prokaryotes (Monera) from eukaryotes

Separates autotrophs (Plantae) from heterotrophs (Animalia, Fungi)

Separates photosynthetic fungi-like organisms (fungi with chitin) from plants (cellulose)

Accommodates unicellular eukaryotes (Protista)

**Evolutionary Links:**

Cyanobacteria (prokaryotes) → chloroplasts (endosymbiotic theory)

Protista (unicellular eukaryotes) → evolutionary link to multicellular kingdoms

Fungi (heterotrophic eukaryotes) → evolutionary ancestor to animals

**MCQ-Type Definitions to Know:**

Saprophyte: organism feeding on dead organic matter

Parasite: organism living in/on host, causing harm

Symbiont: organism in mutually beneficial relationship

Coenocytic: multinucleated, no cross walls

Septate: having cross walls/partitions

Plasmogamy: fusion of cytoplasm

Karyogamy: fusion of nuclei

Dikaryotic: two nuclei per cell (not fused)

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**Key Diagrams to Master:**

Figure 2.1: Bacterial shapes (cocci, bacilli, spirilla, vibrio)

Figure 2.2: Nostoc (filamentous cyanobacterium)

Figure 2.3: Dividing bacterium (binary fission)

Figure 2.4: Protistans (dinoflagellates, Euglena, slime mould, Paramoecium)

Figure 2.5: Fungi (Mucor, Aspergillus, Agaricus)