Biological Classification: Complete Chapter Revision Notes For Class 11 NEET by PW

Biological Classification includes several names, characteristics, examples, and classification-based concepts that can be easy to mix up during the NEET exam. A quick revision can help you bring these details together and recall how different organisms are grouped.

The Biological Classification Class 11 NEET one-shot revision video by PW covers the chapter from kingdom systems to Monera, Protista, Fungi and Viruses. It also includes topics such as bacteria, photosynthesis in bacteria, parasexual methods, reproduction and classification of fungi, making it useful for revising the chapter in one sitting.

Evolution of Classification Systems

Classification is the process of grouping organisms on the basis of their similarities.

Early Classification

The earliest system classified organisms according to their use to humans:

• Food

• Clothing

• Shelter

Aristotle, the Father of Biology, made the first scientific attempt at classification. Plants were grouped by morphology and habit into herbs, shrubs, and trees. Animals were grouped according to blood:

• Enaima: Animals with red blood and red blood cells

• Anaima: Animals without red blood

Two-Kingdom System

Carolus Linnaeus proposed the Two-Kingdom System, which divided organisms into:

1. Kingdom Plantae

2. Kingdom Animalia

The basis was only the presence or absence of a cell wall. Organisms with a cell wall were placed in Plantae, while those without a cell wall were placed in Animalia. The composition of the wall, such as cellulose, chitin, or peptidoglycan, was not considered.

Plantae included bacteria, cyanobacteria, fungi, algae, bryophytes, pteridophytes, gymnosperms, and angiosperms. Animalia included protozoans, chordates, and non-chordates.

 Euglena had a controversial position because it lacked a cell wall but possessed chloroplasts and performed photosynthesis.

The system grouped prokaryotes with eukaryotes, unicellular with multicellular organisms, and autotrophs with heterotrophs.

 (Memory Tip: The Two-Kingdom System produced a “mixed” classification because organisms with different cellular structures and nutritional modes were grouped together.)

Five-Kingdom System

R. H. Whittaker proposed the Five-Kingdom System in 1969. It considered:

• Cell structure

• Body organisation

• Mode of reproduction

• Mode of nutrition

• Phylogeny

Phylogeny is the study of evolutionary history, including ancestry and common origin.

Yeast remains in Kingdom Fungi, although it is unicellular, because fungi have a separate kingdom.

Six-Kingdom System

The Six-Kingdom System divided Monera into Archaebacteria and Eubacteria. It recognised:

• Domain Archaea

• Domain Prokarya

• Domain Eukarya

The six kingdoms are Archaebacteria, Eubacteria, Protista, Fungi, Plantae, and Animalia. The division was based on the 16S rRNA gene sequence.

Archaebacteria have a pseudomurein cell wall and a monolipid plasma membrane. Other bacteria generally have peptidoglycan and a bilipid membrane.

Kingdom Monera

Kingdom Monera contains all unicellular prokaryotes. Bacteria are cosmopolitan or ubiquitous and occur in soil, water, rocks, deep oceans, hydrothermal vents, and hot sulfur springs.

Cellular Features

Bacteria:

• Lack a nuclear membrane and true nucleus

• Lack membrane-bound organelles

• Possess 70S ribosomes

• Reproduce commonly by binary fission

• Do not show true sexual reproduction

Their major shapes are:

Nutrition And Metabolism

Bacteria show the greatest metabolic diversity. Most are heterotrophic:

• Saprophytic bacteria digest dead matter externally and support mineral cycling.

• Parasitic bacteria obtain food from living hosts.

• Symbiotic bacteria benefit from association with another organism.

Rhizobium fixes atmospheric N₂ into ammonia inside legume root nodules. Frankia is another symbiotic nitrogen-fixing bacterium.

Autotrophic bacteria may be:

• Chemoautotrophic: Use chemical energy

• Photoautotrophic: Use light energy

Photosynthesis In Bacteria

Bacteria do not possess chloroplasts. In photosynthetic bacteria, plasma membrane folds form a chromatophore containing photosynthetic pigments.

• Green bacteria contain bacteriochlorophyll.

• Purple bacteria contain bacteriopurpurin.

Most photosynthetic bacteria perform anoxygenic photosynthesis. They use hydrogen sulfide instead of water and do not release oxygen. Cyanobacteria are the exception: they contain chlorophyll a, use water, and release oxygen.

Chemosynthetic bacteria oxidise inorganic substances such as ammonia, nitrites, nitrates, or ferrous ions. Nitrifying bacteria convert ammonia to nitrite and nitrite to nitrate.

Reproduction And Genetic Variation

In favourable conditions, DNA replicates and binary fission forms two daughter cells. In unfavourable conditions, bacteria may form a resistant endospore for survival.

Bacteria do not undergo true sexual reproduction, but DNA transfer creates variation:

Archaebacteria And Eubacteria

Archaebacteria have pseudo-murein, a monolipid layer, histone proteins, and introns. Eubacteria have peptidoglycan, a bilipid layer, and generally lack histones and introns.

• Methanogens live in rice fields, marshes, and cattle rumen. They produce methane and are obligate anaerobes.

• Halophiles live in high-salt environments.

• Thermoacidophiles live in hot, acidic environments.

• Eubacteria include E. coli, Vibrio cholerae, Salmonella, Lactobacillus, and Rhizobium.

Mycoplasma

Mycoplasma is the smallest bacterium and the smallest living cell. It is the smallest prokaryote and lacks a cell wall and flagella. It is heterotrophic, facultatively anaerobic, and may be saprophytic or parasitic. It is insensitive to penicillin but may be killed by chloramphenicol, sofra­mycin, or neomycin. 

(Memory Tip: Smallest prokaryote → smallest cell → no cell wall → no flagella → survives with or without oxygen → heterotrophic → penicillin-insensitive.)

Cyanobacteria

Cyanobacteria, or blue-green algae, are photoautotrophic and contain chlorophyll a and phycobilins. Phycocyanin is blue, while phycoerythrin is red. They possess a mucilaginous sheath, gas vacuoles, and cyanophycean granules. They are non-motile.

In Nostoc and Anabaena, specialised heterocysts perform nitrogen fixation using nitrogenase. Spirulina is used as single-cell protein. Excessive cyanobacterial growth in polluted water causes an algal bloom.

Useful bacteria include Lactobacillus in curd preparation, Acetobacter in acetic acid production, Streptomyces in antibiotic production, and nitrogen-fixing bacteria in improving soil fertility. Harmful bacteria cause cholera, typhoid, tetanus, and citrus canker.

Kingdom Protista

Ernst Haeckel introduced the term Protista. It contains unicellular eukaryotes but is not a natural group because its members show plant-like, fungal-like, and animal-like features.

Protists possess a true nucleus, membrane-bound organelles, ribosomes, and histone proteins. They may be autotrophic or heterotrophic and may reproduce sexually or asexually.

Major Groups

• Chrysophytes: Diatoms, desmids, and golden algae

• Dinoflagellates: Marine, photosynthetic organisms with two flagella

• Euglena: Mixotrophic protist

• Slime moulds: Fungus-like protists

• Protozoans: Animal-like protists

 Diatoms have silica-containing soap-box-like walls with epitheca and hypotheca. Their deposits form diatomaceous earth, used for polishing and filtration. Diatoms are chief producers in oceans.

Dinoflagellates have two perpendicular flagella and cellulose plates. Some produce light, while Gonyaulax and Gymnodinium may cause poisonous red tides. (Memory Tip: The name dinoflagellate indicates the presence of two flagella.)

Euglena has a flexible proteinaceous pellicle, two flagella, chloroplasts, chlorophyll b, and carotenoids. It performs photosynthesis in sunlight and consumes food in darkness. (Memory 

 Tip: Euglena = pellicle + two flagella + mixotrophic nutrition + chlorophyll b.)

Slime moulds feed saprophytically. Their cells aggregate into a plasmodium under favourable conditions. During unfavourable conditions, fruiting bodies and resistant spores develop.

 Kingdom Fungi

Fungi are cosmopolitan, heterotrophic organisms with cell walls made of chitin. Their nutrition may be saprophytic, parasitic, or symbiotic. Mycorrhiza is an association between fungi and plant roots, while lichen is an association between algae and fungi.

Yeast is unicellular. Most fungi are multicellular and consist of thread-like hyphae. A network of hyphae is called mycelium.

• Aseptate mycelium lacks septa and is multinucleated.

• Septate mycelium has cross-walls and may be monokaryotic or dikaryotic (n + n).

Fungal reproduction may be vegetative, asexual, or sexual. Vegetative reproduction occurs through fission, budding, or fragmentation. Asexual spores include zoospores, sporangiospores, and conidia. Basidiomycetes do not produce asexual spores.

Sexual reproduction includes plasmogamy, karyogamy, zygote formation, and meiosis. Ascomycetes produce ascospores inside asci, while Basidiomycetes produce basidiospores externally on club-shaped basidia.

Lichens

Lichens are dual organisms formed by an alga and a fungus.

• Phycobiont: Algal component

• Mycobiont: Fungal component

The alga performs photosynthesis, while the fungus provides water, minerals, and reproductive support. (Memory Tip: Phycobiont = algal partner; Mycobiont = fungal partner.)

Lichens are air-pollution indicators, especially for SO₂. They are pioneer communities that break down rocks and help in soil formation.

Viruses And Subviral Agents

Viruses are acellular, obligate parasites and are not included in the Five-Kingdom Classification. They are nucleoproteinaceous particles containing either DNA or RNA and a protein coat.

The coat is the capsid, made of capsomeres. Viruses may be DNA or RNA viruses, with single- or double-stranded genetic material.

• Plant viruses are often single-stranded RNA viruses.

• Animal viruses may contain RNA or DNA.

• Bacteriophages infect bacteria and are often double-stranded DNA viruses.

Ivanovsky showed that the tobacco mosaic disease agent passed through bacterial filters. Beijerinck introduced the term virus, and Stanley first crystallised a virus. (Memory Tip: Ivanovsky — filterable agent; Beijerinck — term virus; Stanley — crystallisation.)

Viroids are free single-stranded RNA particles without a protein coat and cause plant diseases. T. O. Diener discovered them in 1979.

Prions are abnormally folded proteinaceous infectious particles without DNA or RNA. They cause animal neurodegenerative diseases such as BSE or mad cow disease and Creutzfeldt–Jakob syndrome.

 Use the Biological Classification Class 11 NEET quick revision video by PW to revisit important classification concepts, characteristics and examples.

After the revision, practise NCERT-based questions to check how well you remember the chapter for NEET. 

NEET Resources  You Might Like: