NEET UG BIOLOGY Unit 1: DIVERSITY IN THE LIVING WORLD

The Five Kingdom Classification & Atypical Life Forms

A Comprehensive Lecture by Prof. Anil Tyagi, Ph.D.

Welcome, future doctors. Before we can diagnose disease, we must understand the agents that cause it. Before we can appreciate the human body, we must understand the diversity of life from which we evolved. The system we will study today, proposed by Robert H. Whittaker in 1969, was a revolutionary step that organized the chaotic world of living organisms into a logical framework based on specific, scientific criteria. This is not just a list to be learned; it is a story of evolution and complexity to be understood.

Part 1: The Need for a New System & Whittaker’s Criteria

For a long time, biologists used a Two-Kingdom system (Plantae and Animalia). This system was fraught with problems:

Where do bacteria, which have no nucleus, go?
Are fungi, which don’t photosynthesize but have a cell wall, plants?
Where do we put Euglena, which has features of both plants and animals?

Whittaker’s genius was in using a multi-criteria approach to resolve these conflicts. His five-kingdom system was based on the following criteria:

Cell Structure: Prokaryotic vs. Eukaryotic.
Body Organization: Unicellular vs. Multicellular.
Mode of Nutrition: This was a key differentiator.
- Autotrophic: Synthesis their own food (Photosynthesis or Chemosynthesis).
- Heterotrophic: Obtain food from others.
  - Holozoic: Ingesting food.
  - Saprophytic: Absorbing nutrients from decaying matter.
  - Parasitic: Deriving nutrition from a living host.

Based on these criteria, Whittaker proposed the following five kingdoms:

Monera
Protista
Fungi
Plantae
Animalia

For this lecture, we will focus in detail on the first three, along with the atypical life forms that don’t fit neatly into this system: Lichens, Viruses, and Viroids.

Part 2: Kingdom Monera – The Prokaryotic Pioneers

I. Salient Features (The Defining Characteristics):

Cell Type: Prokaryotic. This is their most fundamental feature. They lack a well-defined nucleus and membrane-bound organelles.
Body Organization: Primarily unicellular, though some form colonies or filaments.
Cell Wall: Generally present, composed of complex polysaccharides. A key component in most bacteria is peptidoglycan.
Nutrition: Extremely diverse – autotrophic (both photosynthetic like cyanobacteria and chemosynthetic) and heterotrophic (saprophytic, parasitic, symbiotic).
Reproduction: Primarily asexual through binary fission. They do not undergo mitosis or meiosis. Sexual reproduction is primitive, through conjugation, transformation, or transduction (involving the transfer of genetic material).
Locomotion: Some motile forms use flagella.

II. Classification of Monera into Major Groups:

The Kingdom Monera is vast. For NEET, you must be clear on this primary classification:

1. Archaebacteria (The Ancient Ones)

Habitat: They are the extremophiles, living in the most inhospitable environments reminiscent of primitive Earth.
Cell Wall: Their cell wall lacks peptidoglycan and is made of different polysaccharides and proteins.
Cell Membrane: The lipid structure in their membrane is unique, allowing them to survive extreme conditions.
Major Groups:
- Methanogens: Anaerobic bacteria that produce methane gas. Found in the gut of ruminant animals (like cows) and in biogas plants. Example: Methanobacterium.
- Halophiles: Thrive in extremely salty environments (e.g., the Dead Sea, salt pans).
- Thermoacidophiles: Found in hot springs and volcanic vents, they can withstand both high temperature and high acidity.

2. Eubacteria (The True Bacteria)

Habitat: Found almost everywhere – soil, water, air, inside other organisms.
Cell Wall: Rigid cell wall containing peptidoglycan.
Classification based on Shape:
- Coccus (pl. Cocci): Spherical
- Bacillus (pl. Bacilli): Rod-shaped
- Vibrium: Comma-shaped
- Spirillum: Spiral

A further critical classification is based on the Gram Staining technique, which differentiates bacteria based on their cell wall structure:

Gram-Positive Bacteria: They retain the crystal violet stain and appear purple. They have a thick peptidoglycan layer.
Gram-Negative Bacteria: They do not retain the stain and appear pink/red. They have a thin peptidoglycan layer and an outer lipid membrane.

3. Cyanobacteria (The Photosynthetic Autotrophs)

Previously called: Blue-green algae, but they are prokaryotes, not algae.
Nutrition: They are photosynthetic autotrophs. They contain chlorophyll-a, similar to plants.
Special Structures: Some possess specialized cells called heterocysts for nitrogen fixation.
Importance: They are crucial primary producers and add organic matter and oxygen to the ecosystem. Some can cause water blooms.
Examples: Nostoc, Anabaena, Oscillatoria.

Mycoplasma: A special group within Eubacteria. They are the smallest living cells known and can survive without oxygen. They lack a cell wall, making them resistant to antibiotics like penicillin that target cell wall synthesis. Many are pathogenic. Example: Mycoplasma pneumoniae.

Part 3: Kingdom Protista – The Mosaic Kingdom

I. Salient Features (The “Catch-All” Kingdom):

Cell Type: Eukaryotic. This is the first kingdom with true, membrane-bound nuclei and organelles.
Body Organization: Primarily unicellular, but some are colonial or filamentous. This is a kingdom of mostly unicellular eukaryotes.
Mode of Nutrition: Highly diverse – this is why Protista is a grouping of convenience. It includes photosynthetic autotrophs, heterotrophs, and mixotrophs (like Euglena which can do both).
Locomotion: They use pseudopodia, cilia, or flagella for movement.
Reproduction: Can be both asexual (binary fission, multiple fission) and sexual (syngamy).

II. Classification of Protista into Major Groups:

This classification is primarily based on their mode of nutrition.

1. Chrysophytes (The “Golden” Protists)

Includes: Diatoms and Golden algae (Desmids).
Features: They are photosynthetic, primarily planktonic.
Cell Wall: Diatoms have a unique, siliceous cell wall that fits together like a soapbox. These walls are indestructible and over millions of years, their accumulation forms ‘diatomaceous earth’ which is used in filtration, polishing, and even as a natural pesticide.
Importance: They are the major producers in the ocean.

2. Dinoflagellates

Features: Mostly marine, photosynthetic, and planktonic.
Cell Wall: They have a stiff cellulose plate coat that gives them a protective armor.
Pigments: They appear yellow, green, brown, blue, or red depending on their pigments.
Locomotion: Two flagella, one longitudinal and one transverse, in perpendicular grooves.
The “Red Tide”: Some species (e.g., Gonyaulax) undergo rapid multiplication, making the sea appear red. They release powerful neurotoxins (saxitoxin) that can kill marine life and cause shellfish poisoning in humans.

3. Euglenoids

Features: They are the classic example of mixotrophic nutrition.
Habitat: Mostly found in fresh, stagnant water.
Pigments: They have chlorophyll and can perform photosynthesis in sunlight.
Behaviour: In the absence of light, they behave like heterotrophs, predating on smaller organisms.
Cell Structure: They lack a rigid cell wall. Instead, they have a flexible pellicle. They have two flagella, though one may be reduced.
Example: Euglena.

4. Slime Moulds

Features: They are saprophytic protists. They were once thought to be fungi.
Body Form: The vegetative phase is a free-living, multinucleated, amoeboid mass of protoplasm called a plasmodium. This plasmodium creeps over decaying twigs and leaves, engulfing organic matter.
Reproduction: Under unfavorable conditions, the plasmodium differentiates to form fruiting bodies that bear spores. These spores have true walls and are highly resistant.

5. Protozoans (The Animal-like Protists)

Nutrition: They are heterotrophic and act as predators or parasites.
Classification:
- Amoeboid Protozoans: Use pseudopodia (false feet) for locomotion and capturing prey. Example: Amoeba. Some are parasitic, like Entamoeba histolytica which causes amoebic dysentery.
- Flagellated Protozoans: Use flagella for movement. Many are parasitic. Example: Trypanosoma (causes Sleeping Sickness), Giardia.
- Ciliated Protozoans: Use thousands of cilia for locomotion and steering food into the gullet. They have a more complex structure, with a definite mouth and anus. Example: Paramoecium.
- Sporozoans: This group is entirely parasitic. They lack specific locomotory structures. They have an infectious spore-like stage in their life cycle. Example: Plasmodium (the malarial parasite), a topic of immense medical importance.

Part 4: Kingdom Fungi – The Decomposers

I. Salient Features:

Cell Type: Eukaryotic.
Body Organization: Mostly multicellular (e.g., mushrooms, moulds) with exceptions like yeast (unicellular).
Cell Wall: Composed of a tough, complex sugar called chitin (the same material found in the exoskeletons of arthropods).
Nutrition: They are heterotrophic. Most are saprophytes (absorbing nutrients from dead and decaying matter). Some are parasites (on plants and animals), and others are symbionts (e.g., in lichens and mycorrhiza).
Vegetative Structure: The body is a network of thin, thread-like filaments called hyphae. A network of hyphae is called a mycelium.
Reproduction: Can be vegetative (fragmentation), asexual (through spores like conidia, sporangiospores), and sexual (through spores like ascospores, basidiospores).

II. Classification of Fungi into Major Groups:

The primary classification of fungi is based on their mode of sexual reproduction.

1. Phycomycetes

Habitat: Found on decaying wood, in damp, humid places, or as obligate parasites on plants.
Mycelium: Aseptate and coenocytic (multinucleate).
Asexual Reproduction: By zoospores (motile) or aplanospores (non-motile).
Sexual Reproduction: By zygospore formation (isogamy or anisogamy).
Examples: Mucor (the common bread mould), Rhizopus, Albugo (a parasite on mustard).

2. Ascomycetes (The Sac Fungi)

Features: This is a very large group.
Mycelium: Branched and septate.
Asexual Reproduction: Very common, by conidia produced exogenously on conidiophores.
Sexual Reproduction: The sexual spores are produced endogenously in a sac-like structure called an ascus. The spores are called ascospores. Typically, 8 ascospores are produced per ascus.
Examples:
- Unicellular: Saccharomyces cerevisiae (Baker’s Yeast) – crucial for fermentation.
- Multicellular: Penicillium (source of the antibiotic penicillin), Aspergillus, morels, and truffles (edible delicacies).

3. Basidiomycetes (The Club Fungi)

Features: Includes the most familiar fungi – mushrooms, puffballs, and bracket fungi.
Mycelium: Branched and septate.
Asexual Reproduction: Is less common.
Sexual Reproduction: The sexual spores are produced exogenously on a club-shaped structure called a basidium. The spores are called basidiospores.
The “Fruiting Body”: The basidia are arranged in a large, fleshy structure which is the edible part of the mushroom. This is the reproductive part.
Example: Agaricus (Mushroom), Ustilago (Smut), Puccinia (Rust).

4. Deuteromycetes (The “Imperfect Fungi”)

Concept: This is not a true phylogenetic group. It is a “catch-all” category for those fungi in which the sexual or perfect stage is not known or has been lost.
Importance: Many of these are economically and medically important.
Examples:
- Alternaria
- Colletotrichum
- Trichoderma
- The most famous one: Candida albicans, a human pathogen causing Candidiasis (thrush).

Part 5: The Atypical Life Forms – Beyond the Five Kingdoms

These entities do not fit into the five-kingdom system because they do not exhibit all the characteristics of life independently.

1. Lichens: The Symbiotic Success Story

What are they? Lichens are not a single organism but a symbiotic association between two different organisms: a fungal partner (mycobiont) and an algal partner (phycobiont).
The Symbiosis: The fungus provides shelter, water, and minerals to the alga. The alga, which is photosynthetic, prepares food for both.
Importance:
- Pioneers in Ecological Succession: They are the first to colonize bare rocks, secreting acids that weather the rock and help in soil formation.
- Pollution Indicators: They are highly sensitive to air pollution, especially Sulphur Dioxide (SO₂). The absence of lichens in an area indicates poor air quality.

2. Viruses: The Intracellular Hijackers

Status: They stand at the threshold of living and non-living.
- Non-living characters: Can be crystallized, no protoplasm, no independent metabolism, cannot reproduce outside a host cell.
- Living characters: Possess genetic material (DNA or RNA), can replicate inside a host cell, show mutations, and are specific to their hosts.
Structure: A virus is a nucleoprotein. The genetic material (core) is either DNA or RNA (never both). This is surrounded by a protein coat called a capsid. Capsid is made of small subunits called capsomeres.
Bacteriophages: Viruses that infect bacteria. They have a complex structure with a head and a tail.
Diseases: Cause a multitude of diseases in plants and animals. In humans: Common cold, Influenza, Dengue, AIDS (HIV), Chicken Pox, COVID-19 (SARS-CoV-2).

3. Viroids: The Naked Infectious RNA

Discovery: Discovered by T.O. Diener.
Structure: They are even smaller and simpler than viruses. They consist of only naked RNA without a protein coat. The RNA is of low molecular weight.
Disease Caused: They are known to cause diseases only in plants. The classic example is Potato Spindle Tuber Disease.

Professor Tyagi’s Key Takeaways for NEET Success:

The Criteria are Key: Remember Whittaker’s three main criteria (Cell structure, Body organization, Nutrition). Use them to justify why an organism is placed in a particular kingdom.
Monera = Prokaryotes: This is the simplest and most important distinction. Archaebacteria vs. Eubacteria is a high-yield sub-topic.
Protista is a Mixed Bag: Focus on the distinguishing features of each group – Diatoms (silica walls), Dinoflagellates (red tide, cellulose plates), Euglena (mixotroph), Protozoans (classified by locomotion).
Fungi: Know the Classes by their Spores:
- Phycomycetes: Zygospore
- Ascomycetes: Ascospore (in ascus)
- Basidiomycetes: Basidiospore (on basidium)
- Deuteromycetes: Sexual stage absent
Lichens & Viruses are Favourites: Lichens as symbionts and pollution indicators; Viruses as connecting links and causative agents of disease are very frequently asked.
Distinguish: Virus (DNA/RNA + Protein) vs. Viroid (Only RNA) vs. Prion (Only Protein – not in syllabus but good to know).

This chapter is a high-scoring one. Internalize these concepts, draw the diagrams, and you will build an unshakable foundation for your NEET preparation.

Your journey to becoming a doctor is built one solid concept at a time. Let’s make this one count.

– Prof. Anil Tyagi

The Five Kingdom Classification & Atypical Life Forms : NEET UG Bio Unit 1