NSEJS Biology: Diversity of Living Organisms - Biological Classification

The Earth is home to an incredible variety of living organisms, ranging from tiny bacteria to large plants and animals. Scientists have identified nearly 1.8 million species, and many more are yet to be discovered. Studying each organism individually would be impossible, which is why biological classification plays such an important role in biology.

Biological classification is the process of grouping organisms based on their shared characteristics. It helps scientists identify, compare, and study living organisms in a systematic way while also revealing their evolutionary relationships. 

In the NSEJS Biology syllabus, this chapter introduces the evolution of classification systems, Whittaker's Five-Kingdom Classification, and the unique features of different kingdoms, making it one of the most important topics for building a strong foundation in biology.

Also Read: 

• NSEJS Syllabus
• NSEJS Previous Question Papers

Biological Diversity and Classification

The growth of fungal colonies on bread left exposed for a few days demonstrates the omnipresence of unseen organisms. Classification involves organising organisms into convenient categories based on shared characteristics. 

For example, animals are generally heterotrophic, multicellular, eukaryotic, and reproduce sexually, while plants are typically autotrophic, eukaryotic, predominantly multicellular, possess a cell wall, and contain chloroplasts. 

Evolution of Classification Systems

The current understanding of biological classification is based on the Five-Kingdom System of Classification, which evolved through various earlier systems. This system categorises organisms into Kingdom Monera, Kingdom Protista, Kingdom Fungi, Kingdom Plantae, and Kingdom Animalia.

Earliest Non-Scientific Classification

The earliest classification was non-scientific, based purely on the uses of different organisms to humans, such as for food (vegetables, fruits), clothing (fibres), shelter (timber), security (dogs), and transport (horses).

Aristotle's First Scientific Classification

The first scientific classification was proposed by Aristotle, often considered the 'Father of Biology.' He used simple morphological characters as criteria.

• Plants: Classified into Herbs (green, soft-stemmed), Shrubs (bushy), and Trees (large, woody).

• Animals: Classified based on the presence or absence of red blood:

• Enaima: Organisms with red blood present.

• Anaima: Organisms with red blood absent (Memory Tip: 'A' signifies absence).

Two-Kingdom System

The Two-Kingdom System of Classification was proposed by Carolus Linnaeus, primarily using morphological characters and the presence or absence of a cell wall.

• Kingdom Plantae: Included all organisms with a cell wall, such as all bacteria, blue-green algae, algae, fungi, and all true plants.

• Kingdom Animalia: Included all organisms where the cell wall was absent, such as protozoans and all true animals.

Demerits of the Two-Kingdom System: This system had significant drawbacks:

• Prokaryotes and Eukaryotes Grouped: Prokaryotic organisms (e.g., bacteria) were placed alongside eukaryotic organisms (e.g., fungi, plants).

• Unicellular and Multicellular Organisms Grouped: Unicellular organisms (e.g., bacteria) were grouped with multicellular organisms (e.g., large plants).

• Heterotrophs and Autotrophs Grouped Together: Heterotrophic organisms (e.g., fungi) were grouped with autotrophic organisms (e.g., plants).

• Ignored Cell Wall Composition: It overlooked diverse chemical compositions of cell walls (e.g., peptidoglycan, chitin, cellulose).

Intermediate Classification Systems

To address these limitations, Haeckel proposed the Three-Kingdom System, introducing Kingdom Protista. Further evolution led to the Four-Kingdom System, which introduced Kingdom Monera.

The Five-Kingdom System (R.H. Whittaker, 1969)

In 1969, R.H. Whittaker proposed the widely accepted Five-Kingdom System of Classification, based on five key criteria:

1. Cell Structure (prokaryotic vs. eukaryotic)

2. Body Organisation (unicellular vs. multicellular; tissue, organ, organ system levels)

3. Mode of Nutrition (autotrophic vs. heterotrophic)

4. Reproduction (asexual vs. sexual)

5. Phylogenetic Relationships (evolutionary connections)

Application of Criteria:

• Cell Structure: All prokaryotic organisms are placed in Kingdom Monera. All other four kingdoms (Protista, Fungi, Plantae, Animalia) comprise only eukaryotic organisms.

• Body Organisation & Cell Type:

• Kingdom Monera: Unicellular prokaryotes.

• Kingdom Protista: Unicellular eukaryotes.

• Fungi, Plantae, Animalia: Primarily multicellular eukaryotes.

• Mode of Nutrition: Varies across kingdoms (autotrophic in plants, some bacteria; heterotrophic in fungi, animals, most bacteria).

Kingdom Monera (Detailed Discussion)

Carl Woese further classified Kingdom Monera into two domains: Archaea (Archaebacteria) and Eubacteria (True bacteria).

General Characteristics of Monera (Bacteria):

• They are the sole members of Kingdom Monera and the most abundant and cosmopolitan organisms.

• Despite simple cell structure, they exhibit complex metabolic diversity.

Bacterial Shapes (Morphology)

The four basic shapes of bacteria are:

• Coccus: Spherical (e.g., Diplococcus, Streptococcus, which causes pneumonia).

• Bacillus: Rod-shaped; some can form a central endospore.

• Spirillum: Spiral-shaped, often with terminal flagella.

• Vibrio: Comma-shaped.
  (Memory Tip: Bacterial names often indicate their shape, e.g., Lactobacillus is rod-shaped due to 'bacillus'.)

Mode of Nutrition in Bacteria

• Heterotrophic: The majority of bacteria are heterotrophic, being parasitic (causing diseases) or saprophytic (decomposers on dead matter).

• Autotrophic: Some can synthesise their own food:

• Photoautotrophic: Use light energy (e.g., Cyanobacteria with chlorophyll a).

• Chemoautotrophic: Oxidise inorganic substances for energy.

Archaebacteria

Archaea are known as extremophiles (Memory Tip: Think "Khatron ke Khiladi" (daredevils)), thriving in extreme conditions.

• Thermoacidophiles: Survive in extremely hot and acidic environments (e.g., hot springs with pH around 2). (Memory Tip: "Thermo-" (high temp), "acid-" (acidic), "-phile" (loving).)

• Methanogens: Generate methane (CH4), found in marshy areas and the gut of ruminant animals, aiding in biogas production. (Memory Tip: "Methano-" (methane), "-gen" (generating).)

• Unique Cell Wall Composition: Their distinct cell wall structure, made of pseudomurein with branched lipids and ether linkages, provides additional stability in harsh environments compared to Eubacteria.

Eubacteria (True Bacteria)

Eubacteria are true bacteria and the most abundant type.

• Mode of Nutrition: Primarily heterotrophic, with some being autotrophic.

• Cyanobacteria (Blue-green algae): Photoautotrophic Eubacteria with chlorophyll a, enabling photosynthesis.

• General Properties and Roles:

• Nitrogen Fixation: Cyanobacteria (e.g., Anabaena, Nostoc) fix atmospheric nitrogen into ammonia. They use specialised heterocysts, which prevent oxygen entry to protect the nitrogenase enzyme.

• Nutrient Cycling: Chemoautotrophic bacteria oxidise inorganic substances (e.g., nitrates, phosphates) to generate ATP and cycle elements.

• Diverse Heterotrophic Roles: Used in industry (e.g., Lactobacillus for curd), antibiotic production (e.g., Neomycin), and symbiotic nitrogen fixation (Rhizobium). Some are pathogenic, causing diseases like cholera, typhoid, tetanus (Clostridium tetani), and citrus canker (Xanthomonas citri).

Mycoplasma

Mycoplasma is the smallest living cell, approximately 0.3 microns in length.

• Lack of Cell Wall: Mycoplasma lacks a cell wall, making it pleomorphic (changes shape). (Memory Tip: They are called "Jokers of the Plant Kingdom" because they constantly change their form.)

• They can survive without oxygen and are pathogenic in plants (e.g., Little Leaf) and animals (e.g., pneumonia-like diseases).

Kingdom Protista: Introduction & General Characteristics

Protista is a diverse kingdom of eukaryotic, mostly unicellular organisms, considered an evolutionary link between prokaryotes and multicellular eukaryotes.

Five Major Categories of Protists:

1. Chrysophytes

2. Dinoflagellates

3. Euglenoids

4. Slime Moulds

5. Protozoans

Chrysophytes

Found in freshwater and marine environments, they are photosynthetic. They include Diatoms and Desmids (Golden Algae).

• Diatoms: Known as "Chief Producers of the Oceans." Their cell wall forms two overlapping shells like a soapbox (Hypotheca and Epitheca), embedded with silica, making them indestructible and gritty. Accumulated deposits form Diatomaceous Earth (Kieselguhr), used for polishing and filtration.

Dinoflagellates

Mostly marine and photosynthetic, they exhibit various colours due to pigments. They have a stiff cellulosic plate cell wall and two flagella (one longitudinal, one transverse).

• Red Tides: Rapid multiplication of certain red dinoflagellates (e.g., Gonyaulax) causes the sea to appear red. Gonyaulax produces potent saxitoxins that kill aquatic animals and can harm humans.

Euglenoids

Mostly found in freshwater, particularly stagnant water. They lack a cell wall, possessing a flexible, protein-rich pellicle instead. They have two flagella and are mixotrophic: photosynthetic in sunlight and heterotrophic in darkness. They are considered a connecting link between plants and animals.

Slime Moulds

These are saprophytic protists.

• Vegetative Stage (Plasmodium): Under favourable conditions, individual cells aggregate to form a large, multinucleate mass called Plasmodium. (Memory Tip: Under 'favourable conditions' (like a birthday party), they form Plasmodium.)

• Reproductive Stage: Under unfavourable conditions, the Plasmodium forms fruiting bodies with spores. These spores have true walls, are extremely resistant, and are dispersed by air currents. (Memory Tip: Under 'unfavourable conditions' (like police arriving at a party), they form fruiting bodies.)

Protozoans

Considered "Primitive Relatives of Animals" due to their heterotrophic nature and absence of a cell wall. They are classified into four groups based on locomotion:

1. Amoeboid Protozoans: Move and capture food using pseudopodia (false feet). Examples: Amoeba, parasitic Entamoeba (causes amoebic dysentery).

2. Flagellated Protozoans: Possess flagella for movement. Many are parasitic, e.g., Trypanosoma (causes sleeping sickness).

3. Ciliated Protozoans: Have thousands of cilia for movement and directing food into a gullet. Example: Paramecium (slipper-like shape, with a macronucleus and micronucleus). (Memory Tip: Think of cilia like many hairs, flagella like a few legs.)

4. Sporozoans: Lack locomotory organs. All are parasites with an infectious spore-like stage. Example: Plasmodium (malarial parasite, causes malaria).

Kingdom Fungi: General Characteristics

Fungi are eukaryotic, heterotrophic organisms that obtain nutrients by absorption.

• Cell Wall: Made of chitin and polysaccharides.

• Unicellular Exception: Yeast (Saccharomyces) is the only unicellular fungus (used in bread and alcohol production).

• Body Structure: Composed of thread-like structures called hyphae, which form a network called mycelium. This provides a large surface area for nutrient absorption.

• Reproduction:

• Asexual: Primarily by fragmentation.

• Sexual: Involves three stages:

1. Plasmogamy: Fusion of protoplasm.

2. Dikaryophase (n+n condition): In some fungi (Ascomycetes, Basidiomycetes), two haploid nuclei remain separate after plasmogamy.

3. Karyogamy: Fusion of the two nuclei to form a diploid (2n) nucleus. Followed by meiosis to produce haploid spores.

• Hyphal Structure:

• Aseptate and Coenocytic: No cross-walls, multinucleated cytoplasm (e.g., Phycomycetes).

• Septate and Branched: Cross-walls divide hyphae into compartments (e.g., Ascomycetes, Basidiomycetes, Deuteromycetes).

• Habitat: Thrive in warm and humid conditions. (Instructional Reasoning: Refrigeration inhibits fungal growth; human fungal infections occur in warm, moist areas.)

Fungal Classes: Detailed Characteristics & Examples

Fungi are classified into four major groups based on the structure of their mycelium and their mode of reproduction. Understanding the features of each class is important for the NSEJS Biology exam.

1. Phycomycetes:

• Mycelium: Aseptate and coenocytic.

• Habitat: Aquatic, decaying wood, or obligate plant parasites.

• Asexual Reproduction: By zoospores (motile) or aplanospores (non-motile), produced endogenously.

• Examples (Memory Tip: RAM): R-Rhizopus (Bread Mould), A-Albugo (causes white rust on mustard), M-Mucor.

2. Ascomycetes (Sac Fungi):

• Mycelium: Branched and septate.

• Organisms: Unicellular (Yeast) and multicellular (Penicillium).

• Lifestyle: Saprophytic, decomposers, parasitic, or coprophilous (grow on dung).

• Dikaryophase: Prolonged.

• Examples (Memory Tip: NASA): N-Neurospora (used in genetic research), A-Aspergillus, S-Saccharomyces (Yeast), A-Claviceps (produces LSD).

3. Basidiomycetes (Club Fungi):

• Mycelium: Branched and septate.

• Common Forms: Mushrooms (Agaricus), bracket fungi, puffballs.

• Habitat: Soil, logs, tree stumps, plant parasites (rusts and smuts).

• Asexual Spores: Generally absent.

• Vegetative Reproduction: By fragmentation.

• Sexual Reproduction: Sex organs absent; plasmogamy between vegetative cells leads to a prolonged dikaryophase.

• Examples (Memory Tip: 'Aaj main usse pareshan raha'): Aaj-Agaricus (Mushroom), Uss-Ustilago (Smut), Pareshan-Puccinia (Rust, causes wheat rust).

4. Deuteromycetes (Imperfect Fungi):

• Mycelium: Branched and septate.

• "Imperfect Fungi": Named because only asexual (conidia) and vegetative phases are known; sexual stages are not observed.

• Reproduction: Only by asexual spores called conidia, produced exogenously.

• Role: Important decomposers and aid in mineral cycling.

• Examples (Memory Tip: CUT): C-Colletotrichum, U-Alternaria, T-Trichoderma.

Organisms Not Included in Whittaker's Five-Kingdom Classification

Some entities like viruses, viroids, prions, and lichens were not assigned to any of the five kingdoms by Whittaker.

Lichens

A symbiotic association between algae (phycobiont) and fungi (mycobiont). They are not classified into a separate kingdom because their individual components are already classified.

• Fungal Component (Mycobiont): Provides shelter and absorbs water and minerals.

• Algal Component (Phycobiont): Performs photosynthesis, producing food.

• Ecological Significance: Lichens are pollution indicators, highly sensitive to sulfur dioxide (SO2), and cannot grow in polluted areas.

Why Viruses, Viroids, and Prions are Not Classified in Five Kingdoms

These entities lack a cellular structure and are devoid of their own protoplasm and cellular organisation. Since a cell is the fundamental unit of life, they are not considered truly "living" organisms and are thus excluded from the five-kingdom system.

Prions

Proteinaceous infectious particles that are misfolded proteins.

• Nature: Unlike correctly folded, functional proteins, misfolded prions accumulate in the brain, causing neurodegenerative diseases.

• Diseases Caused: Mad Cow Disease (Bovine Spongiform Encephalopathy - BSE) in cattle and Creutzfeldt-Jakob Disease (CJD) in humans.

Viroids

Discovered by T.O. Diener (1971).

• Characteristics: Considerably smaller than viruses, composed solely of a naked RNA molecule with low molecular weight. They lack a protein coat.

• Disease Caused: Potato Spindle Tuber Disease (PSTV), causing spindle-shaped potato tubers.

Viruses

Described as connecting links between living and non-living things.

• Nature: Exist as inert particles outside a host cell and are obligate intracellular parasites, meaning they require a host cell for replication. They are non-cellular entities.

• Replication Mechanism: Upon entering a host cell, viruses overtake the host's cellular machinery to replicate.

• Structure: Composed of:

1. Genetic Material: Either DNA or RNA, but never both simultaneously.

2. Protein Coat (Capsid): Encapsulates genetic material, made of subunits called capsomeres.

• Size: Generally smaller than bacteria.

• Naming: The term 'Virus' was coined by Pasteur, meaning 'poisonous fluid'.

• Historical Discoveries:

• D.J. Ivanowsky (1892): Discovered the causative agent of Tobacco Mosaic Disease (TMD) was smaller than bacteria.

• M.W. Beijerinck (1898): Identified the TMD agent as "Contagium vivum fluidum" (infectious living fluid).

• W.M. Stanley (1935): Successfully crystallised viruses, demonstrating they are largely composed of proteins.

Plant Viral Diseases

Common symptoms include mosaic formation, leaf rolling and curling, yellowing and vein clearing, and dwarfing and stunted growth.

Human Viral Diseases

• H - Herpes

• A - AIDS

• N - Influenza

• S - Hepatitis

• M - Mumps

• R - Rabies

• P - Polio

Biological classification is the foundation of biology because it helps us understand the incredible diversity of life in a simple and systematic way. From the earliest classification systems to R.H. Whittaker's Five-Kingdom Classification, this chapter explains how scientists group organisms based on their characteristics and evolutionary relationships.

For NSEJS Biology, it is important to understand the key features of Monera, Protista, and Fungi, along with the differences between archaebacteria and eubacteria, the major groups of protists, fungal classes, and the reasons why viruses, viroids, and prions are excluded from the five-kingdom system. 

Regular revision of these concepts, examples, and classifications will strengthen your conceptual understanding and help you answer both objective and application-based questions with confidence in the NSEJS exam.