Biological Classification NEET Notes

Complete Biological Classification Notes for Class 11 Biology | NEET Revision and NCERT Solutions

Systems of Classification

Artificial Classification System: This system was proposed by Linneaus. It was based on vegetative characters and androecium structure.

Natural Classification System: This system was used by George Bentham and Joseph Dalton Hooker. This system was based on natural affinities among organisms. In this system, both external and internal features were considered.

Two Kingdom Classification; as proposed by Linneaus: Animal Kingdom, Plant Kingdom

Five Kingdom Classification; as proposed by R.H. Whittaker (1969): This is the most accepted system of classification. The five kingdoms are as follows:

Monera, Protista, Fungi, Plantae and Animalia

The main criteria for classification used by Whittaker:

1. Cell Structure

2. Thallus Organization

3. Mode of Nutrition

4. Reproduction and

5. Phylogenetic Relationships

Kingdom Monera

The bacteria are kept under the Kingdom Monera. They are prokaryotic and possess cell wall. The cell wall is composed of polysaccharides and amino acids. Bacteria can be autotrophic and heterotrophic. The autotrophic bacteria can be chemosynthetic or photosynthetic. The heterotrophic bacteria can be saprophytic or parasitic.

Based on Their Shape, Bacteria Are Classified into Four Types

Spherical bacteria are called Coccus (pl.: cocci),
Rod-shaped bacteria are called Bacillus (pl.: bacilli),
Comma-shaped bacteria are called Vibrium (pl.: vibrio) and
Spiral shaped bacteria are called Spirillum (pl.: spirilla)

Archaebacteria: The archaebacteria live in some of the harshest habitats. The different structure of their cell wall helps them in surviving in extreme conditions. Based on their habitats, the archaebacteria are classified as follows:

Halophiles: They live in extremely salty areas.
Thermoacidophiles: They live in hot spring.
Methanogens: They live in marshy areas. They also live in the guts of the ruminant animals. They are responsible for the production of methane from the dung of these animals.

Eubacteria: They are also called the ‘true bacteria’. They possess a rigid cell wall, and a flagellum (in motile bacteria). The cyanobacteria are also called ‘blue-green algae’ because they contain chlorophyll. The cyanobacteria can be unicellular or filamentous. They can live solitary or in colonies. The colony of cyanobacteria is usually surrounded by a gelatinous sheath. Some of the cyanobacteria are capable of nitrogen-fixation, e.g. Nostoc and Anabaena.

Heterotrophic: These are the most abundant in nature. Most of them have economic significance for human beings. While many of them are beneficial for humans, many others are quite harmful.

Reproduction in Bacteria

Binary fission is the usual mode of reproduction in bacteria. Under unfavorable conditions, they reproduce by spore formation. They also reproduce by adopting a primitive type of DNA transfer from one bacterium to another. This is like sexual reproduction.

Kingdom Protista

They are eukaryotes and unicellular. Some of them have flagella or cilia for locomotion. Reproduction is asexual and sexual. Sexual reproduction is by a process of cell fusion and zygote formation. Kingdom Protista is divided into following groups:

Chrysophytes

The diatoms and golden algae (desmids) are included in this group. They are found in freshwater and marine habitats. They are microscopic and float passively in water currents and hence are considered as planktons. Organisms which cannot swim against the current are called planktons. Most of the chrysophytes are photosynthetic
The cell walls in diatoms form two thin overlapping shells; which fit together as the two parts of a soapbox. The cell walls are embedded with silica and hence are indestructible. Due to this, the diatoms leave behind a large amount of cell wall deposits in their habitat. The cell wall accumulation of diatoms; over billions of years is called ‘diatomaceous earth’. This soil is gritty and hence is used in polishing, filtration of oils and syrups. Diatoms are the main ‘producers’ in the oceans.

Dianoflagellates

They are usually marine and photosynthetic. Depending on the main pigments in their cells; they appear yellow, green, brown, blue or red. Stiff cellulose plates are present on the outer surface of cell wall in dianoflagellates. Two flagella are present in most of them. One flagellum lies longitudinally and another transversely in a furrow between the wall plates.

Euglenoids

Most of them live in freshwater habitat in stagnant water. Cell wall is absent in them and instead there is a protein rich layer; called pellicle. The pellicle makes their body flexible. Two flagella; one short and another long; are present. They are photosynthetic; but behave as heterotrophs in the absence of sunlight. Example: Euglena.

Slime Moulds

These are saprophytic. They body moves along decaying twigs and leaves and feeds on organic material. Under favorable conditions, they form an aggregation called plasmodium. The plasmodium may grow and spread over several feet. Under unfavorable conditions, the plasmodium differentiates and forms fruiting bodies. The fruiting bodies bear spores at their tips. True walls are present on the spores. The spores are extremely resistant and survive for many years. The spores are dispersed by air currents.

Protozoans

They are heterotrophs and live as predators or parasites. The protozoans are classified into following four major groups:

Amoeboid protozoans: The amoeboid protozoans live in freshwater, sea water or in moist soil. They produce pseudopodia for locomotion and for capturing food. The marine forms have silica shells on their surface. Some of them are parasites, e.g. Entamoeba histolytica.
Flagellated protozoans: They are either free-living or parasitic. Flagella is present for locomotion. Many of them are parasites, e.g. Trypanosoma.
Ciliated protozoans: They are aquatic. Cilia are present for locomotion. A cavity (gullet) is present which opens to the outside of the cell surface. The coordinated movement of cilia facilitates the entry of food-laden water into the gullet. Example: Paramoecium
Sporozoans: The sporozoans have an infectious spore-like stage in their life cycle. Example: Plasmodium

Kingdom Fungi

The fungi are filamentous; except yeast (unicellular). The body consists of long, slender thread-like structures; called hyphae. The network of hyphae is called mycelium. Some hyphae are continuous tubes which are filled with multinucleated cytoplasm. Such hyphae are called coenocytic hyphae. The other type of hyphae has septae or cross-walls. The cell wall of fungi is composed of chitin and polysachharides.
Most of the fungi are heterotrophic and are saprophytes. Some are parasites. Some of the fungi also live as symbionts. Some of the symbiont fungi live as lichens; in association with algae. Some of the symbiont fungi live as mycorrhiza; in association with roots of higher plants.

Reproduction in Fungi

Reproduction by vegetative means takes place by fragmentation, fission and budding. Some fungi reproduce asexually by forming spores which are called conidia or sporangiospores or zoospores. Sexual reproduction is by oospores, ascospores and basidiospores. The spores are produced in distinct structures called fruiting bodies.
The sexual cycle involves three steps which are as follows:

Plasmogamy: This involves fusion of protoplasm between two motile or non-motile gametes.

Karyogamy: This involves fusion of two nuclei.

Formation of haploid spores: This happens because of meiosis in zygote.

During sexual reproduction, two haploid hyphae of compatible mating types come together and fuse. In some fungi, the fusion of two haploid cells immediately results in a diploid cell (2n). In other fungi (ascomycetes and basidiomycetes), an intervening dikaryotic stage occurs. In this stage, two nuclei are present in each cell. This condition is called dikayron. The parental nuclei fuse at a later stage and the cells become diploid. Reduction division in the fruiting bodies leads to the formation of haploid spores.

Based on morphology of mycelium, mode of spore formation and fruiting bodies; Kingdom Fungi is divided into following classes:

Phycomycetes

The members of phycomycetes are found in aquatic habitats and on decaying wood in moist and damp places. They can also be found as obligate parasites on plants.

Mycelium: Aseptate and coenocytic.

Reproduction:

Asexual reproduction is by zoospores (motile) or by aplanospores (non-motile). These spores are endogeneously produced in sporangium.
Zygospores are formed by fusion of two gametes. The gametes can be similar (isogamous) or dissimilar (anisogamous).i on mustard).

Examples: Mucor, Rhizopus and Albugo (the parasitic fungi)

Ascomycetes

They are commonly known as sac-fungi. They are unicellular or multicellular. They are saprophytic, decomposers, parasitic or coprophilous. Those growing on dung are called coprophilous.

Mycelium: Branched and septate.

Reproduction:

Asexual spores are exogenously produced on the
special mycelium called conidiophores. Sexual spores are called ascospores. They are produced endogenously in sac like asci. These asci are arranged in various kinds of fruiting bodies called ascocarps.

Examples: Aspergillus, Claviceps, Neurospora, yeast, penicillium, morels, baffles, etc.

Basidiomycetes

They grow in soil, on logs and tree stumps. Some of them also grow in living plant bodies as parasites. Mushrooms, bracket fungi or puffballs are the commonly known forms.

Mycelium: Branched and septate.

Reproduction:

Asexual spores are usually absent. Vegetative reproduction by fragmentation is common. Sex organs are absent. But plasmogamy is brought about by fusion of two vegetative cells of different strains. The resultant structure is dikaryotic which finally forms the basidium. Karyogamy and meiosis are responsible for formation of four basidiospores in a basidium. The basiodiospores are exogenously produced. Basidia are arranged in fruiting bodies called basidiocarps.

Examples: Agaricus (mushroom), Ustilago (smut) and Puccinia (rust).

Deuteromycetes

They are usually known as imperfect fungi because only the asexual or vegetative phase of them is known. Many deuteromycetes are decomposers, while some members are parasites.

Mycelium: Branched and septate.

Examples: Alternaria, Colletotrichum and Trichoderma.

Kingdom Plantae

All eukaryotes which have chloroplast are kept under this kingdom. Most of them are autotrophic; but some are heterotrophic. Cell wall is mainly composed of cellulose.
The life cycle of plants has two distinct phases; the diploid saprophytic and the haploid gametophytic phase. These phases alternate with each other. The lengths of the haploid and diploid phases vary among different groups of plants. This phenomenon is called Alternation of Generation.

Kingdom Animalia

All multicellular eukaryotes which lack cell wall and are heterotrophs are kept under this kingdom. The animals indirectly or directly depend for food on plants. They follow the
holozoic mode of nutrition. Holozoic nutrition involves ingestion of food and use of internal cavity for digestion of food. Most of the animals are capable of locomotion. Sexual reproduction is by copulation of male and female which is followed by embryological development.

Viruses, Viroids and Lichens

Virus is non-cellular organism. It is characterized by an inert crystalline structure outside the living cell. Once a virus infects a living cell, it begins to replicate and kills the host in the process. Virus is considered as a borderline case between living and non-living.
The name virus was given by Pasteur D. J. Ivanowsky (1892). M. W. Beijerinek (1898) demonstrated that the extract of infected tobacco plants could cause infection in healthy plants. He called the fluid from the infected plant as Contagium vivum fluidum (infectious living fluid). W. M. Stanley (1935) demonstrated that viruses could be crystallized and the crystals mainly consist of proteins.
Viruses also contain genetic material; which can be either RNA or DNA. Both RNA and DNA cannot be present in the same virus. Plant infecting viruses usually have single-stranded RNA and animal infecting viruses usually have double-stranded RNA or double-stranded DNA. Bacteriophages (bacterial viruses) usually have double-stranded DNA.
The protein coat on the virus is called capsid. It is made up of small subunits called capsomeres. The capsid protects the nucleic acid. The capsomeres are arranged in helical or polyhedral geometric forms.
Mumps, small pox, herpes, influenza and AIDS are examples of viral diseases in humans. In plants; viral infection leads to the symptoms; like mosaic formation, leaf rolling and curling, yellowing and vein clearing, dwarfing and stunted growth.

NCERT Solutions

This chapter takes the student deeper into the field of biological classification. The taxonomy of the five kingdoms is explained in simple terms. They learn about Kingdom Monera, Kingdom Protista, Kingdom Fungi, Kingdom Plantae, and Kingdom Animalia. Students will also study about ancient classification methods employed by scientists such as Aristotle and Linnaeus, as well as how the system has changed through time.

1. Discuss how classification systems have undergone several changes over a period of time?

Solution:

Aristotle was the first person to introduce scientific classification. He used simple morphological characters to classify plants into trees, shrubs and herbs. He also divided animals into two groups, those which had red blood and those that did not.

Linnaeus introduced two-kingdom classification which includes Plantae and Animalia i.e., plants and animals respectively. But this classification did not classify eukaryotes and prokaryotes, unicellular and multicellular organisms, photosynthetic (green algae) and non-photosynthetic (fungi) organisms. Hence this system was found to be less significant as it did not include many more characteristics.

Thus, classification of living organisms underwent several changes. R.H Whittaker introduced five-kingdom classification which includes Monera, Protista, Fungi, Plantae and Animalia. Some characteristics that were included in this classification are – cell structure, body organization, nutrition mode, mode of reproduction and phylogenetic relationship to classify the organisms. After this, the three-domain system was proposed which divided Kingdom Monera into two domains, leaving the remaining eukaryotic kingdoms in the third domain and thereby a six kingdom classification.

2. State two economically important uses of:

(a) heterotrophic bacteria

(b) archaebacteria

Solution:

a) Heterotrophic bacteria are used in the production of vitamins, antibiotics, production of cheese and curd. They help in fixing nitrogen and are used in the formation of Humus.

b) Archaebacteria is used in the production of Biogas. They are used in bioleaching of mines.

3. What is the nature of cell-walls in diatoms?

Solution:

In diatoms, cell walls are embedded with silica imparting characteristic patterns onto the walls and are indestructible. Because of this diatoms leave behind large amount of cell wall deposits in their habitat which accumulates to form ‘diatomaceous earth’.

4. Find out what do the terms ‘algal bloom’ and ‘red-tides’ signify

Solution:

Algal blooms are found in polluted water. They are the excessive growth of algae, especially blue-green algae (Cyanobacteria). Their growth results in pollution of water. They inhale carbon dioxide and expel out Oxygen. Rapid multiplication of red-pigmented dinoflagellates such as Gonyaulax imparts a red colour to the sea, this phenomenon is called as red tides. These algae produce toxins which kills fish and other aquatic entities.

5. How are viroids different from viruses?

Solution:

• Viroids are short infectious agents having a single-stranded RNA without protein coat whereas viruses have a single-stranded or double-stranded RNA encapsulated with a protein coat.

• Viroids are very small in size compared to viruses.

• Viroids infect only plants whereas viruses infect both plants, animals and microorganisms.

6. Describe briefly the four major groups of Protozoa

Solution:

Four major groups of Protozoa are as follows

Amoeboid protozoans: Found in aquatic environment; they move and catch their prey using pseudopodia.

Flagellated protozoans: These protozoans are either free-living or parasitic. Their locomotory structure is flagella.

Ciliated protozoans: They live in an aquatic environment and presence of cilia makes them move actively.

Sporozoans: They include diverse organisms, producing infectious spores in their lifecycle. Their spore-like phase helps them to get transferred from one to another host.

7. Plants are autotrophic. Can you think of some plants that are partially heterotrophic?

Solution:

Insectivores and carnivores plants are partially heterotrophic, these beings are green and autotrophic but for

their nitrogen supply, they prey and digest small entities. Ex; Utricularia, Drosera, Nepenthes.

8. What do the terms phycobiont and mycobiont signify?

Solution:

Lichens are the symbiotic association of fungi and algae. Phycobiont is algae part and mycobiont is fungi part of the association. Mycobiont provides a structural covering that protects algae from the unfavorable condition. Similarly Phycobionts prepares food by the process of photosynthesis, which will be utilized by both the organisms.

9. Give a comparative account of the classes of Kingdom Fungi under the following:

(i) mode of nutrition (ii) mode of reproduction

Solution:

Phycomycetes( Includes Oomycetes and Zygomycetes
- Mode of Nutrition
  - Ooymycetes are mostly parasitic (obtain their their nourishment from the protoplasm of plants or animals. e.g. Pytopthora infestans
- Mode of Reproduction

10. What are the characteristic features of Euglenoids?

Solution:

Characteristic features of Euglenoids are as follows:

• Absence of a cell wall

• Their body is flexible due to the presence of protein-rich layer called pellicle.

• Two flagella are found that are of different length.

• They are autotrophic in the presence of sunlight and heterotrophic in the absence of sunlight.

11. Give a brief account of viruses with respect to their structure and nature of genetic material. Also name four common viral diseases.

Solution:

Viruses are the infectious agents which are crystalline in structure when found outside the host cell. Genetic material will be either DNA or RNA (never both) which are present inside the protein core. The virus that infects plants has single-stranded RNA whereas viruses that infect animals are either single or double stranded DNA or RNA. The capsid is their protein coat that inturn is made of smaller subunits known as capsomeres, guarding the nucleic acid. Common viral diseases are –Influenza, AIDS, Herpes and Rabies

12. Organise a discussion in your class on the topic – Are viruses living or nonliving?

Solution:

Living Characters

• They are host-specific

• Presence of genetic material

• Ability to multiply

• They have antigenic properties

• They are obligate parasites

• Mutations occur

Non-living Characters

• The cellular structure is absent

• They can be stored in bottles like crystals

• Energy storing or energy liberating systems are absent

• They cannot grow or multiply outside the host.

The classification of plant and animal species was first based on straightforward variations in physical traits, which further paved the way for classification that was supported by concrete scientific investigation and study. The body organisation, cell structure, manner of nourishment, reproduction, and phylogenetic links were the primary factors in the five-kingdom classification. Bacteria were placed in the Kingdom Monera in this kingdom classification even though their spread is global.

These organisms have a remarkably diverse metabolic repertoire. Single-celled eukaryotes are included in the Kingdom Protista, whereas species of Kingdom Fungi have a wide range of habitats and structural types. Kingdom Animalia is made up of heterotrophic, multicellular, eukaryotic organisms without cell walls, while Kingdom Plantae is made up of all eukaryotic organisms that possess chlorophyll.