Biological Classification Class 11 Biology Chapter 2 CBSE Notes - 2025-26

Biological classification is defined as the process of grouping organisms according to certain similarities.

Linnaeus proposed the two kingdoms of classification, He classified organisms in the animal kingdom which is called Animalia and in the plant kingdom which is called Plantae. There were certain limitations with the classification of two kingdoms as it does not distinguish between eukaryotes and prokaryotes, unicellular and multicellular organisms, and photosynthetic and non-photosynthetic organisms.

Five Kingdoms Classification

RH Whittaker suggested the five-kingdom classification. The classification of these five kingdoms is as follows: Monera, Protista, Fungi, Plantae, and Animalia. The classification was based on the organization of the thallus, the cell structure, mode of nutrition, the phylogenetic relationship, and the mode of reproduction.

Kingdom Monera

The bacteria are one of the main members of Monera.

All organisms found in this kingdom are prokaryotes, It includes blue-green algae together with bacterium. They can be found in terribly extreme habitats such as hot springs, deserts, snow, and deep oceans., All monerans are prokaryotes, i.e., do not have a definite nucleus. They can be autotrophs, heterotrophs moreover as parasites. Cell wall is present.

Archaebacteria

Archaebacteria can be thermoacidophilic (can survive in hot springs), halophilic (survive in high salt concentrations), and methanogenic (Live in swampy areas). Their cell wall structure differs from other monerans. Methanogens are found in the intestines of some ruminants such as buffalo and cows and help to produce biogas from animal dung.

Eubacteria

They are called "true bacteria". They are characterized by the presence of a mobile flagellum and a rigid cell wall. Contains cyanobacteria (blue-green algae). They are similar to plants as they contain chlorophyll a. For this reason, they are called photosynthetic autotrophs.

Cyanobacteria are unicellular, filamentous algae that form colonies which are surrounded by a gelatinous sheath. Some have special structures known as heterocysts that help in nitrogen fixation in place. For example Nostoc.

Chemosynthetic Autotrophs - These bacteria can oxidize certain inorganic substances such as nitrites, nitrates. The energy which is released during the process of oxidation is used in the production of ATP.

Bacteria can reproduce asexually by a process called the binary fission. They can also reproduce through spores if the situation is not favorable. Mycoplasma is also classified found under Monera. They have no cell wall and can easily survive under anaerobic conditions. Mycoplasmas are known to be the smallest living organism.

Kingdom Protista

Single-celled eukaryotes placed under the kingdom Protista. They are mostly aquatic in nature. They are found to be unicellular, eukaryotic organisms. They have a well-defined nucleus that is surrounded by the nuclear membrane. They can move through both cilia and flagella, respiration can be both aerobic and anaerobic, the mode of reproduction can be both sexual and asexual.

They are divided into Chrysophytes, Dinoflagellates, Euglenoids, Slime moulds, and Protozoans.

1. Chrysophytes

It consists of both diatoms and golden algae. They live in both fresh and marine water. They are mostly photosynthetic. Silica is found in their cell wall and this is the reason that their cell wall is indestructible and is responsible for the formation of diatomaceous earth. They are the main producers found in the oceans. They are mostly unicellular flagellates. they are amoeboid.

2. Dinoflagellates

These types of organisms are marine and perform photosynthesis. They can be of different colours, for example, yellow, brown, red, or blue. The appearance of colour is due to the presence of different coloured pigments present in the cells of the dinoflagellates. The cell wall is made up of cellulose. They have 2 flagella, with one flagellum placed longitudinally and the other flagella placed transversely. Some of the examples are- Gonyaulax catenella, Noctiluca scintillans.

3. Euglenoids

Euglenoids are freshwater organisms found in stagnant water; They are surrounded by a protein-rich layer called pellicle. They have one short flagellum and one long flagellum; they can behave as both autotrophic and heterotrophic organisms; for example Euglena.

Most euglenoids have chloroplasts so that they can synthesize their own food. The food reserves found in the euglenoids are the paramylon (carbohydrate).

4. Slime moulds

They are saprophytic in nature, and under favourable conditions form aggregates known as plasmodium.

They form fruiting bodies with spores under unfavourable conditions, they have true cell walls and are extremely resistant to adverse conditions.

5. Protozoan like Protists

They are known to be heterotrophs. They can live as both predators as well as parasites. Protozoans are of four types-

• Amoeboid protozoa are found in freshwater, moist soil, or seawater and have pseudopodia to ingest food particles such as in Amoeba.

• Flagellate protozoa can be of both types i.e free-living or parasitic. This group of organisms is responsible for various parasitic diseases. For example; Trypanosoma causes sleeping sickness.

• Ciliated protozoa have thousands of cilia. The movement of the cilia helps them to move forward or backwards. It also helps in getting nourishment from the outside. For example; Paramecium.

• Sporozoa forms reproductive cells known as spores. They are pathogenic and parasitic. For example, the Plasmodium species causes malaria. Reproduction can be of both types i.e asexual or sexual.

Kingdom Fungi

Fungi are naturally heterotrophic; some feed on dead organic matter and are called saprophytes, and some can feed on living organisms known as parasites. Some fungi can live in mutual relationships with other organisms such as algae, they are called lichens. These types of fungi are called symbionts.

Lichens are symbiotic relationship between fungi and algae and mycorrhiza is the symbiotic relationship between fungi and the roots of higher plants.

Reproduction in fungi can occur by many methods such as budding, fragmentation, and fission. Asexual reproduction occurs by means of spores, which are known as conidia, zoospores, or sporangiospores.

Sexual reproduction takes place through different spores known as oospores, ascospores, and basidiospores.

In fungi, there are three steps in a sexual cycle:

The first step is protoplasmic fusion, known as plasmogamy, then the fusion of nuclei occurs known as karyogamy which leads to the formation of the zygote. Then finally, the meiosis of the zygote leads to the formation of the spores.

The Mycelium is known as the vegetative part of the fungus. The hypha network is called the mycelium. During the sexual cycle of fungi, two haploid hyphae fuse to form diploid cells. The kingdom fungi are divided into different classes.-

• Phycomycetes (Lower Fungi) - Asexual reproduction occurs through motile spores known as zoospores, and immobile spores are known as aplanospores. The spores are produced within the sporangium. The zygospore is formed by the fusion of the gametes. The mycelium is coenocytic (several nuclei) and multicellular. For example, Mucor, Rhizopus, Albugo.

• Ascomycetes (SAC Fungi) - They can be both saprophytic and parasitic. Asexual reproduction occurs through conidia. Sexual reproduction occurs through ascospores; branched and septate mycelium is found in Ascomycetes; for example Penicillium, Aspergillus, Claviceps, etc.

• Basidiomycetes (Club Fungi) - Vegetative reproduction takes place through the process of fragmentation. Sexual organs are absent. Plasmogamy between two vegetative or somatic cells leads to the formation of the basidium.  The basidium undergoes karyogamy and meiosis to form four basidiospores, for example, Agaricus (mushrooms), Ustilago (fire rot), etc.

• Deuteromycetes are commonly known as imperfect fungi because they do not reproduce sexually at any stage of their life cycle. Sexual reproduction occurs through conidia. The mycelium is septate and branched; for example Alternaria, Trichoderma, etc.

• Zygomycetes (conjugation fungi) are a primitive group of fungi. Asexual reproduction takes place by non-motile sporangiospores. For example, Rhizopus.

Kingdom Plantae

It includes all eukaryotic, multicellular, and photosynthetic plants.

The characteristics of the members of Plantae are as follows:

• Most of them are eukaryotic in nature.

• The main pigment present is chlorophyll.

• Its cell wall is made up of cellulose.

• Photosynthesis helps in the synthesis of food. 

• The process of reproduction can be both sexual and asexual.

• They represent the phenomenon of alternation of generations, i.e., diploid sporophytes, and haploid gametophytes.

Kingdom Animalia

These types of organisms are heterotrophic and eukaryotic. Some of the characteristics of the members of Animalia are as follows:

• They are multicellular organisms of various size.

• The organ systems are well developed such as the skeletal system, circulatory system, respiratory system, etc.

• They are found to be bilaterally symmetrical.

• They also have well-developed locomotory organs. 

•  Breathing takes place through gills, book lungs, book gills, skin, lungs, etc. 

•  Membrane-bound cell organelles are present and nucleus is bounded by a nuclear membrane.

•  The circulation takes place through the blood, blood vessels, and the heart. 

• Reproduction takes place through the formation of haploid gametes. The fusion of the gametes forms a diploid zygote that divides to form a new diploid organism. 

•  The kidneys are the most important excretory organ.

Viruses

The viruses are acellular structures and therefore do not find a place in Whittaker’s five kingdom classification. They consist of nucleic acid (either DNA or RNA) that is surrounded by a protein coat. These viruses can grow and multiply only within a host cell. Viruses exist as crystals outside the host cell. They cause disease and severely damage the host. For example; variola causes smallpox, HIV (human immunodeficiency virus) causes AIDS, etc.

Viroids

They are the smallest known infectious structures and consist only of nucleic acid without a protein shell.

Prions:

Prions are infectious agents composed of abnormally folded proteins. Unlike viruses or bacteria, prions lack genetic material. They cause certain neurological diseases by inducing normal proteins in the brain to misfold, leading to brain damage. Notable prion diseases include mad cow disease (bovine spongiform encephalopathy or BSE) in cattle and Creutzfeldt-Jakob disease (CJD) in humans. These diseases are rare but can be severe and are characterised by symptoms such as memory loss, coordination problems, and behavioural changes.

Lichens

They are known to be the symbiotic associations of algae and fungi. The algal components are autotrophic and synthesise and provide food. The fungal component offers protection and shelter.

5 Important Topics of Biology Class 11 Chapter 2 You Shouldn’t Miss!

Importance of Biological Classification Notes Class 11

• Biological Classification Notes PDF helps students understand the basics of how living things are organised and related, which is essential for learning more about biology.

• They show the wide variety of life on Earth and explain the roles of different organisms in nature.

• Good notes make it easier to review important information and prepare effectively for exams.

• Biological Classification Short Notes PDF make complex topics easier to understand by breaking them down into simple points.

• Knowing how to classify and identify organisms is crucial for scientific studies and experiments.

Tips for Learning the Class 11 Biology Chapter 2 Biological Classification

• Focus on the basic ideas such as taxonomy, classification systems, and the five kingdoms of life. Grasping these fundamentals will make it easier to understand more detailed information.

• Study diagrams and charts that show classification systems, such as the hierarchy from kingdom to species. Visuals can help you remember and understand complex structures and relationships.

• Use Biological Classification Notes Class 11 highlighting important points about each kingdom and the characteristics of different organisms. Summarising information in your own words can improve recall.

• Study real-life examples of organisms from each kingdom and understand their classification. This will help you see how classification is applied in the real world.

• Periodically go over your notes and summaries to improve your understanding. Regular revision helps in retaining information better.

• Solve previous years’ exam questions related to biological classification to test your understanding and get familiar with the type of questions asked.

Conclusion

Biological Classification Class 11 Notes groups living organisms into categories based on similarities and differences. This chapter covers the five kingdoms: Monera, Protista, Fungi, Plantae, and Animalia. Monera includes bacteria, Protista covers simple organisms like algae and protozoa, Fungi consists of moulds and mushrooms, Plantae includes plants, and Animalia covers animals. Classification helps scientists organise and study organisms better. Key points of Biological Classification Short Notes PDF include understanding each kingdom's characteristics and how they differ from one another. 

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