Notes for NEET 2026
Biological Classification

Since the beginning of civilization, many attempts have been made to classify living organisms. The earliest attempts were based on simple, observable characteristics and practical needs. Aristotle was among the first to use a scientific approach to classify organisms based on morphology. Over time, these methods evolved, and the classification system we use today was influenced by the work of R.H. Whittaker, who proposed the Five Kingdom Classification in 1969, which includes Kingdoms Monera, Protista, Fungi, Plantae, and Animalia.

Topics Covered in Biological Classification

Kingdom Monera, Kingdom Protista, Kingdom Fungi, Kingdom Plantae, Kingdom Animalia, Viruses, Viroids, Prions, and Lichens

Kingdom Monera

The organisms in Kingdom Monera are prokaryotic and include bacteria and archaebacteria. They are the most abundant and diverse group of organisms and can survive in extreme environments such as hot springs, salty regions, and deep-sea vents. Bacteria are categorized based on their shape: cocci (spherical), bacilli (rod-shaped), spirilla (spiral-shaped), and vibrio (comma-shaped).

Key Features of Bacteria

• Prokaryotic: They lack a defined nucleus and membrane-bound organelles.
• Cell Wall: The cell wall is non-cellulosic, composed of polysaccharides and amino acids.
• Nutrition: Bacteria can be autotrophic (photosynthetic or chemosynthetic) or heterotrophic (saprophytic or parasitic).

Types of Bacteria

• Archaebacteria: Found in extreme environments, such as high salt concentrations (halophiles), high temperatures (thermoacidophiles), and anaerobic conditions (methanogens). Their unique cell wall allows them to survive in such conditions. Methanogens are known for producing methane in the guts of ruminants like cows.
• Eubacteria: Also known as 'true bacteria', these organisms have a rigid cell wall and some are motile due to the presence of flagella. Cyanobacteria (blue-green algae) are photosynthetic eubacteria that contain chlorophyll-a, similar to higher plants. Some cyanobacteria, like Nostoc and Anabaena, are capable of nitrogen fixation.

Reproduction in Bacteria

• Asexual Reproduction: Bacteria reproduce mainly by binary fission.
• Spore Formation: Under unfavorable conditions, some bacteria produce spores for survival.
• Sexual Reproduction: A primitive form of sexual reproduction involves DNA transfer between two bacteria through processes like conjugation, transformation, and transduction.

Kingdom Protista

Protista includes all single-celled eukaryotes. These organisms form a link between the kingdoms of plants, animals, and fungi. Being eukaryotic, they have a well-defined nucleus and membrane-bound organelles. Protists can be autotrophic or heterotrophic, and they reproduce both sexually and asexually.

Key Groups of Protists

• Chrysophytes: Includes diatoms and golden algae (desmids). These are primarily found in aquatic environments and are important producers in the food chain. Diatoms have indestructible silica cell walls that leave behind deposits known as diatomaceous earth, used in filtration and polishing.
• Dinoflagellates: Mostly marine, these protists have two flagella and cellulose plates. Certain species like Gonyaulax can cause red tides, releasing toxins that can kill marine life.
• Euglenoids: Freshwater organisms, such as Euglena, that exhibit both plant and animal-like characteristics. They are autotrophic in the presence of sunlight and heterotrophic when light is absent. Their body is flexible due to a protein layer called the pellicle.
• Slime Molds: Saprophytic protists that form a large, multicellular structure called a plasmodium during their life cycle. Under adverse conditions, they form spores with resistant cell walls.
• Protozoans: Heterotrophic protists that are considered to be early relatives of animals. They are divided into four groups: amoeboid, flagellated, ciliated, and sporozoans. Examples include Amoeba, Trypanosoma (causes sleeping sickness), and Plasmodium (causes malaria).

Kingdom Fungi

Fungi are eukaryotic, heterotrophic organisms. Their body is made up of long, thread-like structures called hyphae, which together form a network known as mycelium. Fungi can be saprophytic, parasitic, or symbiotic. They reproduce through both sexual and asexual means, often via spores.

Classification of Fungi

• Phycomycetes: Found in aquatic and damp environments, these fungi produce motile (zoospores) and non-motile (aplanospores) spores. Examples include Mucor and Rhizopus.
• Ascomycetes: Known as sac fungi, they include organisms like yeast and Penicillium. Sexual spores (ascospores) are produced inside sac-like structures called asci. Ascomycetes can be saprophytic or parasitic, and some are used in fermentation and antibiotic production.
• Basidiomycetes: Includes mushrooms, puffballs, and bracket fungi. They reproduce via sexual spores called basidiospores, produced on a structure called a basidium. Examples include Agaricus (mushroom) and Puccinia (rust fungi).
• Deuteromycetes: Known as 'imperfect fungi' because only their asexual stage is known. They reproduce through conidia and include important decomposers and pathogens like Alternaria and Trichoderma.

Symbiotic Fungi

• Lichens: Symbiotic associations between fungi and algae. The fungi provide structure and absorb nutrients, while the algae perform photosynthesis.
• Mycorrhiza: Symbiotic associations between fungi and plant roots, where the fungi help in nutrient absorption, especially phosphorus, for the plants.

Kingdom Plantae

Kingdom Plantae includes all eukaryotic, multicellular, photosynthetic organisms. They have chlorophyll for photosynthesis and their cell walls are primarily made of cellulose. Plants show alternation of generations in their life cycle, alternating between diploid sporophytic and haploid gametophytic stages.

Types of Plants

• Algae: Simple, aquatic, photosynthetic organisms.
• Bryophytes: Known as amphibians of the plant kingdom, they live in moist environments and lack true vascular tissues.
• Pteridophytes: First vascular plants, include ferns and horsetails.
• Gymnosperms: Non-flowering seed plants, like pines and cycads.
• Angiosperms: Flowering plants, which produce seeds enclosed in fruits.

Kingdom Animalia

Kingdom Animalia includes multicellular, eukaryotic organisms that are heterotrophic and lack cell walls. They follow holozoic nutrition, which involves the ingestion of food. Animals reproduce primarily through sexual reproduction, and many show advanced neuromotor mechanisms.

Key Features of Animals

• Heterotrophic Nutrition: Animals depend on plants or other animals for food.
• Complex Organ Systems: Animals possess specialized organ systems for functions like digestion, respiration, and reproduction.
• Growth and Development: Animal life cycles include complex embryonic development and growth.

Viruses, Viroids, Prions, and Lichens

Viruses are acellular, non-living entities outside their host cells, but once inside, they replicate by hijacking the host's cellular machinery. They contain either DNA or RNA as their genetic material but never both. Diseases caused by viruses include smallpox, AIDS, and influenza.

Key Acellular Entities

• Viroids: Discovered by T.O. Diener in 1971, viroids are smaller than viruses and consist only of a short strand of RNA without a protein coat. They cause diseases like potato spindle tuber disease.
• Prions: Abnormal proteins that cause infectious neurodegenerative diseases like mad cow disease (BSE) and Creutzfeldt-Jakob disease (CJD) in humans.
• Lichens: Symbiotic associations between fungi and algae. Lichens are sensitive to pollution and are used as indicators of air quality.

Summary

The biological classification system has evolved over time from Aristotle's early attempts to Whittaker's Five Kingdom Classification. This system divides organisms based on cell structure, body organization, mode of nutrition, reproduction, and phylogeny. Bacteria are placed in Monera, protists in Protista, fungi in Fungi, plants in Plantae, and animals in Animalia. Acellular entities like viruses, viroids, prions, and symbiotic organisms like lichens are not included in the five-kingdom system, but they play crucial roles in biological interactions.

Important Questions for NEET 2026 from Biological Classification

Kingdom Monera

• What are the four major shapes of bacteria?
  The four major shapes of bacteria are: cocci (spherical), bacilli (rod-shaped), spirilla (spiral), and vibrio (comma-shaped).
• What makes Archaebacteria unique compared to other bacteria?
  Archaebacteria have unique cell wall structures that allow them to survive in extreme environments such as high salt areas (halophiles), high temperatures (thermoacidophiles), and anaerobic conditions (methanogens).
• What are eubacteria?
  Eubacteria are the 'true bacteria' characterized by a rigid cell wall and various metabolic functions, including photosynthesis in cyanobacteria.
• How do bacteria reproduce?
  Bacteria reproduce primarily by binary fission. Under unfavorable conditions, they may form spores and can also engage in genetic exchange through processes such as conjugation, transformation, and transduction.
• What is the significance of cyanobacteria in the ecosystem?
  Cyanobacteria, such as Nostoc and Anabaena, play a critical role in nitrogen fixation, converting atmospheric nitrogen into a usable form for plants.
• What is the difference between autotrophic and heterotrophic bacteria?
  Autotrophic bacteria synthesize their own food from inorganic substances (e.g., photosynthetic bacteria), while heterotrophic bacteria obtain nutrients from other organisms or organic matter (e.g., decomposers).
• What type of bacteria live in extreme conditions?
  Archaebacteria are known to live in extreme conditions, such as high temperatures, high salinity, and anaerobic environments.
• What role do mycoplasma play in human health?
  Mycoplasma are the smallest known living cells and some species are pathogenic, causing diseases like pneumonia.
• What are methanogens?
  Methanogens are a type of archaebacteria that produce methane gas as a metabolic byproduct, often found in the intestines of ruminant animals.
• How do bacteria contribute to nutrient cycling in ecosystems?
  Bacteria decompose organic matter, recycling nutrients like nitrogen and phosphorus back into the ecosystem, which is essential for plant growth.

Kingdom Protista

• What are the defining characteristics of protists?
  Protists are primarily single-celled eukaryotes that possess a well-defined nucleus and membrane-bound organelles. They can be autotrophic or heterotrophic.
• What is the ecological role of diatoms?
  Diatoms are key primary producers in aquatic ecosystems, contributing significantly to oxygen production and serving as the base of the food web.
• What are the main groups of protists?
  The main groups of protists include Chrysophytes (e.g., diatoms), Dinoflagellates, Euglenoids, Slime molds, and Protozoans.
• What is a red tide and what causes it?
  A red tide is a phenomenon caused by the rapid multiplication of dinoflagellates, resulting in the discoloration of seawater and the release of toxins harmful to marine life.
• How do Euglenoids exhibit both plant and animal characteristics?
  Euglenoids like Euglena have chloroplasts for photosynthesis (plant-like) but can also ingest food when light is not available (animal-like).
• What is the significance of slime molds in decomposition?
  Slime molds are saprophytic protists that feed on decaying organic matter, playing an important role in nutrient recycling in ecosystems.
• What are the main types of protozoans?
  The four main types of protozoans are Amoeboid protozoans, Flagellated protozoans, Ciliated protozoans, and Sporozoans.
• How do ciliated protozoans feed?
  Ciliated protozoans, like Paramecium, feed by using their cilia to create currents that draw food particles into their gullet for ingestion.
• What is the life cycle of Plasmodium, the malaria parasite?
  Plasmodium undergoes a complex life cycle involving both humans and mosquitoes, including stages of asexual reproduction in humans and sexual reproduction in the mosquito.
• What is the function of heterocysts in cyanobacteria?
  Heterocysts are specialized cells in some cyanobacteria that fix atmospheric nitrogen, providing a source of nitrogen for the organism and its ecosystem.

Kingdom Fungi

• What are the key characteristics of fungi?
  Fungi are eukaryotic, heterotrophic organisms with chitin in their cell walls. They reproduce through spores and can be unicellular or multicellular.
• How do fungi obtain nutrients?
  Fungi absorb nutrients by secreting enzymes that break down organic matter into simpler compounds that can be absorbed.
• What is the mycelium in fungi?
  The mycelium is a network of hyphae (thread-like structures) that forms the vegetative part of a fungus, facilitating nutrient absorption and growth.
• What is the role of fungi in ecosystems?
  Fungi decompose organic matter, recycle nutrients, form symbiotic relationships (e.g., mycorrhizae with plants), and some species are sources of antibiotics.
• How do phycomycetes reproduce?
  Phycomycetes reproduce asexually through spores (zoospores or aplanospores) and sexually by forming zygospores through the fusion of gametes.
• What distinguishes ascomycetes from other fungi?
  Ascomycetes, or sac fungi, produce sexual spores called ascospores in sac-like structures called asci, which are typically found in fruiting bodies.
• What are basidiomycetes commonly known for?
  Basidiomycetes are commonly known as club fungi, which include mushrooms, puffballs, and rusts. They produce basidiospores on basidia.
• What are deuteromycetes and why are they called 'imperfect fungi'?
  Deuteromycetes are fungi known only from their asexual reproductive stages (e.g., conidia). They are called 'imperfect' because their sexual reproductive stages have not been observed.
• How do fungi reproduce sexually?
  Sexual reproduction in fungi involves three stages: plasmogamy (fusion of cytoplasm), karyogamy (fusion of nuclei), and meiosis, resulting in haploid spores.
• What are lichens and what is their ecological importance?
  Lichens are symbiotic associations between fungi and algae. They are important as bioindicators of air quality, and they contribute to soil formation and nutrient cycling.

Kingdom Plantae

• What is the primary characteristic that defines the Plantae kingdom?
  The primary characteristic of Plantae is that they are multicellular, eukaryotic organisms that perform photosynthesis using chlorophyll.
• How do plants demonstrate alternation of generations?
  Plants alternate between a haploid gametophyte phase and a diploid sporophyte phase in their life cycle, with varying dominance in different plant groups.
• What distinguishes bryophytes from vascular plants?
  Bryophytes lack vascular tissues (xylem and phloem) and are generally small, requiring moist environments for reproduction and survival, unlike vascular plants.