Biological Classification

Introduction:

There have been many attempts to classify living organisms. It was done instinctively not

using criteria that were scientific but borne out of a need to use organisms for our own use –

for food, shelter and clothing.

Aristotle was the earliest to attempt a more scientific basis for classification. He used simple

morphological characters to classify plants into trees, shrubs and herbs & divided animals

into two groups, those which had red blood and those that did not.

During Linnaeus' time a Two Kingdom system of classification with Plantae and Animalia

kingdoms was developed that included all plants and animals respectively, which was used

till very recently & did not distinguish between the eukaryotes and prokaryotes, unicellular

and multicellular organisms and photosynthetic (green algae) and non-photosynthetic (fungi)

organisms.

Classification of organisms into plants and animals was easily done and was easy to

understand, inspite, a large number of organisms did not fall into either category. Hence the

two kingdom classification used for a long time was found inadequate.

It was felt to include, other characteristics like cell structure, nature of wall, mode of

nutrition, habitat, methods of reproduction, evolutionary relationships, etc. besides gross

morphology,

Hence classification systems for the living organisms have undergone several changes over

time.

Five Kingdom Classification:

Whittaker (1969) proposed the Five Kingdom Classification.

He named them Monera, Protista, Fungi, Plantae and Animalia where the main criteria for

classification used by him include cell structure, thallus organisation, mode of nutrition,

reproduction and phylogenetic relationships.

The classification of living organisms according to Whittaker into five kingdoms namely are as

follows:

KINGDOM: MONERA

Monera consists of primitive organisms. The organisms are very small and single celled.

They consists of prokarytotes which includes species like the bacteria, archaebacteria,

cyanobacteria and mycoplasma.

Example: bacteria, oscillatoria, nostoc, etc.

KINGDOM: PROTISTA

Protista are single-celled eukaryotes and are mainlky aquatic. It includes diatoms, golden

algae, euglena and protozoans like amoeba, paramoecium, plasmodium etc. They are mostly

marine and photosynthetic.

Example: paramecium

KINGDOM: MYCOTA

Kingdom Mycota or Kingdom Fungi consists of network of thread-like structures called as

mycelium. The bodies consists of long, thread-like structures which are called hyphae.

These organisms are mostly saprophytes or parasites and also symbionts. This kingdom of

fungi also includes lichens, mycorrhiza, etc.

Example: Aspergillus

KINGDOM: METAPHYTA

Kingdom Metaphyta or Kingdom Plantae are eukaryotic, mutlicellular plants, They contain

chlororphyll pigment, which helps them prepare their own food by the process of

photosynthesis. This kingdom includes all types of plants like herbs, shrubs, trees, flowering

and non flowering plants.

Example: rose plant, mango tree, algae, bryophytes, etc.

KINGDOM: METAZOA

Kingdom Animalia or Kingdom Metazoa are heterotrophic, eukaryotic, multicellular

organisms. They lack cell wall. This kingdom includes all types of animals.

Example: lion, peacock, etc.

Merits of Five Kingdom Classification

This system of classification is more scientific and natural.

It is the most accepted system of modern classification as the different groups of animals are

placed phylogenetically.

The prokaryotes are placed in a separate kingdom as they differ from all other organisms in

their organization.

As the unicellular organisms are placed under the kingdom protista, it has solved many

problem related to the position of organisms like euglena.

The fungi totally differ from other primitive eukarytotes hence, placing the group fungi in a

status of kingdom is justifiable.

The kingdom Plantae and Animalia shows the phylogeny of different life styles, in the five

kingdom classification, they are more homogeneous group than the two kingdom

classification.

This system of classification clearly indicates cellular organization and modes of nutrition,

the character which have appeared very early in the evolution of life.

Demerits of Five Kingdom Classification

This system of classification has drawbacks with reference to the lower forms of life.

The Kingdom of Monerans and the Protists include diverse, heterogenous forms of life. In

both the kingdoms there are autoptrophic and hetertrophic organisms. They also include

organisms which have cells with cell wall and cells without cell wall.

All the organisms of these three kingdoms do not originate from a single ancestor.

Multicellular organisms have originated from protists several times.

Organisms like the unicelluar green algae like volvox and chlamydomonas have not been

included under the Kingdom Protista because of their resemblance to other greeen algae.

The general organization of the slime moulds are completely different from the members of

protists.

In this system of classification viruses have not been given proper place.

Kingdom Monera:

All the organisms of this kingdom are prokaryotes. Complex structure was the basis of

classification of organisms, many centuries ago.

Monera are considered as the most primitive group of organisms. They include various types

of bacteria and blue-green algae.

Monerans are most abundant of all organisms, due to their versatility of their habitat. It is

estimated that a single drop of water contains 50 billion bacteria.

The kingdom is divided into two groups Archaebacteria and Eubacteria.

All the organisms of this kingdom are prokaryotes.

These cells do not have nuclear membrane, the chromosome is a single and circular, they

also lack membrane bound cellular organelles.

This kingdom includes bacteria, cyanobacteria, mycoplasma etc.

They are unicellular organisms and do not have specific mode of nutrition. They can be

either aerobic or anaerobic.

These organisms have cell wall which is made up of peptidoglycans. The cell organelles are

not membrane bound. Cell organelles like endoplasmic reticulum, mitochondria are absent.

Reproduction is by spore formation and binary fission.

Bacteria are grouped under four categories based on their shape: the spherical Coccus (pl.:

cocci), the rod-shaped Bacillus (pl.: bacilli), the comma-shaped Vibrium (pl.: vibrio) and the

spiral Spirillum (pl.: spirilla)

Though the bacterial structure is very simple, they are very complex in behaviour. Compared to

many other organisms, bacteria as a group show the most extensive metabolic diversity. Some of

the bacteria are autotrophic, i.e., they synthesise their own food from inorganic substrates. They

may be photosynthetic autotrophic or chemosynthetic autotrophic. The vast majority of bacteria are

heterotrophs, i.e., they do not synthesise their own food but depend on other organisms or on dead

organic matter for food.

Kingdom Monera has been classified into two groups - Archaebacteria and Eubacteria.

Archaebacteria are microbes that live in extreme and harsh conditions, they are known as

extremophiles. These bacteria lack cell wall, their cell membrane is made up of different lipids, and

their ribosomes are similar to that of eukaryotes. Archaebacteria are of three major groups of

bacteria based on their habitat i.e., thermophiles, halophiles and methanogen

Some extremophiles live in boiling water like geysers of Yellowstone National Park and and in

volcanoes. These are known as 'thermophiles'.

Some extremophiles live in extremely salty water, the salt loving bacteria are known as

'halophiles'.

Some bacteria are present in the guts of ruminants and are responsible for production of methane

gas from their dung. These bacteria are known as 'methanogens'.

Eubacteria are true bacteria. The characteristic feature is the presence of rigid cell wall and if

present a motile flagelllum that aids in locomotion. These organisms are characterized based on

their nutrition and their shapes.

Classification based on Shape

Bacteria can be classified in four groups based on shape : Spherical or round shaped bacteria

are called cocci, Rod-shaped are bacilli, Comma-shaped bacteria are vibrio and spiral

shaped bacteria are spirilla.

Classification Based on Mode of Nutrition:

Based on the mode of nutrition bacteria are broadly classified into Autotrophic and Heterotrophic.

Autotrophic bacteria - Bacteria which prepare their own food are autotrophic.

(Example Cyanobacteria)

Heterotrophic bacteria - Bacteria which are dependent on other organisms for their food are

heterotrophic.(Example Escherichia coli)

Autotrophic bacteria can be Chemosynthetic or Photosynthetic.

Chemosynthetic bacteria are those which prepare their food with the help of inorganic substrates.

Photosynthetic bacteria are autotrophic bacteria which prepare their own food by the process of

photosynthesis.

Eubacteria are characterised by the presence of a rigid cell wall, and if motile, a flagellum. The

cyanobacteria (also referred to as blue-green algae) have chlorophyll ‘a’ similar to green plants and

are photosynthetic autotrophs.

Cyanobacteria are unicellular, colonial or filamentous, marine or terrestrial algae.

The colonies are generally surrounded by gelatinous sheath. They often form blooms in polluted

water bodies.

Cyanobacteria multiply by fragmentation, hormogones and akinetes. Hormogones are small

trichome segments which separate naturally in filamentous forms due to death of an intervening cell

or presence of heterocyst. Akinetes are thick-walled resting spores.

Some of these organisms can fix atmospheric nitrogen in specialised cells called heterocysts, e.g.,

Nostoc and Anabaena.

Cyanobacteria protect the soil from erosion. They are also used in reclamation of saline & alkaline

soil.

Chemosynthetic autotrophs:

Oxidize various inorganic substances like nitrates/nitrites, ammonia and use released energy for

their ATP production.

Heterotrophic bacteria:

Mostly decomposer

Helpful in making curd from milk

Produce antibiotics

Symbiotically associated with leguminous plant and fix nitrogen.

Some are pathogen causing diseases like cholera, typhoid, and tetanus.

Bacteria reproduce mainly by fission, also produce spore in unfavorable condition.

Reproduce sexually by transfer of DNA form one bacteria to other, the process called

conjugation.

Mycoplasma :

They are tiny organisms found in soil and sewage water. Some are parasites. They are the smallest

among living organisms (other than viruses) varying in size from 0-125 to 0-150mm in diameter.

They exhibit a great deal of variation in their shape and form.

They are unicellular, gram negative, non-motile prokaryotes which lack a distinct cell wall.

Some examples of mycoplasmas are pathogenic parasites in humans, animals and plants. The most

familiar example is Mycoplasma pneumoniae which causes the disease pneumonia.

Kingdom Protista:

All are unicellular and eukaryotic.

Mostly aquatic, can live in moist places.

Forms a link between plants, animals and fungi.

The cell contain nucleus and membrane bound organelles.

May be ‘a photosynthetic protistan’ to one biologist may be ‘a plant’ to another.

Being eukaryotes, the protistan cell body contains a well defined nucleus and other

membrane-bound organelles.

Some have flagella or cilia.

Protists reproduce asexually (binary fission, multiple fission, budding, sporulation & cyst

formation) and sexually by a process involving cell fusion and zygote formation.

Chrysophytes :

Includes diatoms and golden algae (desmids)

Found in freshwater or marine water.

Mostly planktonic ( passive swimmer)

Photosynthetic – pigments are chlorophyll ‘a’ & chlorophyll ‘c’ and fucoxanthin rich

carotenoids.

Cell walls overlap to fit together like a soap box.

Cell wall contains silica hence indestructible.

Their accumulation forms ‘Diatomaceous Earth’.

Reproduction is by binary fission. Size will decrease progressively with each generation

which is restored by the formation of auxospore (rejuvenescent cells).

Used in polishing, tooth pastes, filtration of oils and syrups.

Diatomaceous earth enhances night visibility of paints.

Diatoms are the chief ‘producers’ in the oceans.

Dinoflagellates :

Marine, photosynthetic.

Cell wall has stiff cellulose plates.

Appears yellow, green, brown, blue or red depending on the pigments.

Have two flagella – one longitudinal and other transversely in a furrow between wall plates.

Red Dinoflagellates (Gonyaulax) form red tides.

The common mode of multiplication is by binary fission which may be longitudinal, oblique

or transverse. Spore, cysts & sexual reproduction also occur.

Euglenoids : Unicellular flagellates

Mostly fresh water form found in stagnant water.

Instead of cell wall they have protein rich layer ‘pellicle’ which makes body flexible.

They have two flagella one short and one long

Euglenoids show metaboly i.e. wriggling type of movement due to passage of contraction &

expansion from one end to the other and also flagellar locomotion.

Photosynthetic in presence of sunlight but become heterotrophs if they do not get sunlight.

Sexual reproduction is not known. Multiplication commonly occurs through longitudinal

binary fission. Cyst formation occurs during unfavourable conditions.

e.g. Euglena

Slime Moulds :

Saprophytic Protists

Form aggregates to form plasmodium grow on decaying twigs and leaves.

Plasmodium forms fruiting bodies bearing spores at their tips.

Spores have true walls which are extremely resistant and survive for many years.

Slime Mold: Physarum polycephalum Vomit Slime Mold: Fuligo septica

Protozoans :

All protozoans are heterotrophs and live as predators or parasites.

Believed to be primitive relatives of animals.

These are divided into four major groups on the basis of locomotery organelle they have.

Amoeboid protozoans :

Move and Catch prey using pseudopodia, e.g., Amoeba.

Many forms have silica shells on their surface.

Some of them are parasitic e.g. Entamoeba.

Flagellated protozoans :

Either free living or parasitic.

They have flagella.

Cause disease like sleeping sickness e.g., Trypanosome.

Ciliated protozoans :

These are aquatic, actively moving organisms due to presence of thousands of cilia. e.g.,

Paramecium.

They have a cavity called gullet that opens to outside the cell.

Sporozoans :

Lack any locomotery organelle.

All members are parasitic.

Have infective spore like stage in life cycle, e.g., Plasmodium which causes Malaria.

Kingdom Fungi:

With the exception of yeasts which are unicellular all others are multicellular and

filamentous.

Consists of long slender thread like structure called hypha.

Non chlorophyllous

Network of hyphae called mycelium.

Uninucleated or multinucleated (coenocytic)

Cell wall made of complex polysaccharide called chitin.

Grow in warm and humid places.

Saprophytic, parasitic, symbiotic (Lichen)

Reproduce asexually by spores conidia sporangiospores or zoospores.

Sexual reproduction is by oospores, ascospores and basidiospores.

Sexual cycle involves the following thee steps:

Plasmogamy: fusion of protoplasms between two motile or non-motile gametes.

Karyogamy: fusion of two nuclei

Meiosis: zygote undergoes meiosis resulting haploid spores.

When a fungus reproduces sexually, two haploid hyphae of compatible mating types come

together and fuse.

In some fungi the fusion of two haploid cells immediately results in diploid cells (2n).

In other fungi (ascomycetes and basidiomycetes), an intervening dikaryotic stage (n + n i.e.

two nuclei per cell) occurs; such a condition is called a dikaryon and the phase is called

dikaryophase of fungus. Later, the parental nuclei fuse and the cells become diploid. The

fungi form fruiting bodies in which reduction division occurs, leading to formation of

haploid spores.

The morphology of the mycelium, mode of spore formation and fruiting bodies form the

basis for the division of the kingdom into various classes.

Classification of Fungi:

Fungi are classified based on – (i) morphology of mycelium (ii) mode of spore formation (iii)

fruiting bodies and (iv) presence or absence of sexual stage.

Phycomycetes :

Found in aquatic habitat, on decaying wood in moist and damp places.

Some of them are obligate parasite on plants.

Mycelium is aseptate and coenocytic

Asexual reproduction by zoospores (motile) or by aplanospores (nonmotile).

Spores are produced endogenously in sporangium.

Zygospore produced by fusion of gametes.

e.g., Mucor, Rhizopus, Albugo.

Ascomycetes :

Commonly known as ‘sac fungi’.

Unicellular (yeasts) or multicellular (e.g. Penicillium)

Saprophytic, decomposers, parasitic or coprophilous.

Mycelium branched and septate.

Asexual spores are called conidia produced exogenously on the conidiophores. Conidia on

germination produce mycelium.

Sexual spores are called ascospores produced endogenously in ascus produced inside

fruiting body called Ascocarp.

e.g., Aspergillus, Neurospora

Basidiomycetes

Common known forms called mushrooms, bracket fungi or puffballs.

Mycelium septate and branched.

Asexual spores generally are not found.

Vegetative reproduction by fragmentation.

Sexual reproduction by fusion of vegetative or somatic cells of different strains to form

basidium produced in basidiocarp.

Basidium produces four basidiospores after meiosis.

e.g., Agaricus, Ustilago.

Deuteromycetes :

Called as ‘Fungi Imperfecti’ as sexual form (perfect stage) is not known for them.

Once sexual form is discovered the member is moved to Ascomycetes or Basidiomycetes.

Mycelium is septate and branched.

Are saprophytic, parasitic or decomposers.

e.g., Alternaria, Colletotrichum.

Kingdom Plantae:

Kingdom Metaphyta or Kingdom Plantae are eukaryotic, mutlicellular plants, They contain

chlororphyll pigment, which helps them prepare their own food by the process of photosynthesis.

This kingdom includes all types of plants like herbs, shrubs, trees, flowering and non flowering

plants.

Chlorophyll containing multcellular eukaryots.

Cell wall made of cellulose.

Life cycle shows alternation of generation having

gametophytic (n) and sporophytic(2n) phase

Kingdom Animalia:

Heterotrophic multicellular eukaryots.

Lack cell wall.

Depend on plants directly or indirectly.

Stores food reserves as fat or glycogen.

Holozoic nutrition.

Capable of locomotion.

Follow a definite growth pattern.

Reproduction is sexual .

Viruses, Viroids & Lichens:

Viruses:

They did not find a place in Whitaker classification. Take over the machinery of host cell on

entering it but as such they have inert crystalline structure, due to which it is difficult to call

them living or non-living.

Pasteur gave the term ‘Virus’ i.e., poisonous fluid.

D. J. Ivanowsky (1892) found out that certain microbes caused Tobacco Mosaic Disease in

tobacco plant.

M. W. Beijerinek (1898) called fluid as ‘Contagium vivum fluidum’ as extracts of infected

plants of tobacco could cause infection in healthy plants.

W. M. Stanely (1935) showed viruses could be crystallised to form crystals of protein which

are inert outside their specific host.

Structure of Virus:

Its a nucleoprotein made up of protein called Capsid. Capsid is made up of capsomeres

arranged in helical or polygeometric forms. Have either DNA or RNA as genetic material

which may be single or double stranded.

In general, viruses that infect plants have single stranded RNA and viruses that infect

animals have either single or double stranded RNA or double stranded DNA.

Bacterial viruses or bacteriophages have double stranded DNA.

Viruses cause diseases like mumps, small pox, herpes and influenza. AIDS in humans is also

caused by a virus.

In plants, the symptoms can be mosaic formation, leaf rolling and curling, yellowing and

vein clearing, dwarfing and stunted growth.

Viriods:

In 1971 T.O. Diener discovered a new infectious agent that was smaller than viruses and

caused potato spindle tuber disease.

Viroids are nucleic acid species of relatively low molecular weight and unique structure that

cause several important diseases of cultivated plants.

Similar nucleic acid species may be responsible for certain diseases of animals and humans.

Viroids are the smallest known agents of infectious disease. Unlike viral nucleic acids,

viroids are not encapsidated.

Despite their small size, viroids replicate autonomously in cells of susceptible plant species.

Known viroids are single-stranded, covalently closed circular, as well as linear, RNA

molecules with extensive regions of intramolecular complementarity; they exist in their

native state as highly base-paired rods.

Lichens:

Lichens are symbiotic associations i.e. mutually useful associations, between algae and

fungi.

The algal component is known as phycobiont and fungal component as mycobiont, which

are autotrophic and heterotrophic, respectively.

Algae prepare food for fungi and fungi provide shelter and absorb mineral nutrients and

water for its partner.

So close is their association that if one saw a lichen in nature one would never imagine that

they had two different organisms within them.

Lichens are very good pollution indicators – they do not grow in polluted areas.

Crustose lichens

The thallus forms a crust over the substrate and is firmly attached to it. There is an upper

cortex, at least in early development, but no lower cortex and the medulla is in direct contact

with the substrate and commonly grows into it to some extent. Consequently, the lichen

normally cannot be collected intact without collecting a portion of the substrate along with

it.

There are various terms to describe the nature of the thallus surface (quite apart from sexual

reproductive structures such as apotheciaor asexual reproductive structures such as soralia

or isidia). The surface may be smooth or lumpy ('warted') and frequently the surface is

areolate, i.e. composed of small, separate islands of thallus seated on an underlying

prothallus or hypothallus. These areoles may be clearly separate ('dispersed') or be closely

contiguous and often represent break-up of the thallus surface ('cracked-areolate'). If the

surface is cracked but not broken up into discrete areoles, it is said to be 'rimose'.

Sometimes the surface is pruinose.

Foliose lichens

These are some of the largest and perhaps most complex lichens. The thallus generally

forms flat, leaf-like lobes, with differentiated layers of tissue, the upper and lower cortices,

forming the upper and lower surfaces. The lobes are commonly, but not always, appressed to

the substrate surface, but can be lifted away. The lower cortex is often differently coloured,

frequently brown or black and usually bears rhizines. In Peltigera the lower surface is

ecorticate.

In foliose lichens with multiple branches of the thallus that may stand away from the

substrate, the differentiated lower cortex distinguishes them from fruticose lichens, e.g.

Evernia prunastri, in which the thallus lobes are white beneath, and Pseudevernia

furfuracea, in which the undersides are black when mature.

Fruticose lichens

The thallus is extended up into a tufted or pendant branched structure, the branches being

covered by a single cortex. In fruticose lichens with flattened branches, e.g. Ramalina spp.,

the cortex extends round both surfaces of the branch. Consequently, they differ from foliose

lichens with branched, aerial lobes such as Evernia.