Earlier attempts for classification –
- 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. He divided animals into two groups, those which had red blood and those that did not.
- Linnaeus gave a Two Kingdom system of classification with Plantae and Animalia
Five kingdom classification –
- Proposed by R.H. Whittaker (1969).
- The kingdoms defined by him were named Monera, Protista, Fungi, Plantae and
- The main criteria for classification used by him include cell structure, thallus organisation, mode of nutrition, reproduction and phylogenetic relationships.
Table – Characteristics of the Five Kingdoms
|Cell wall||Noncellular (Polysaccharide+ amino acid)||Present in some||Present (without cellulose)||Present (cellulose)||Absent|
|Body organisation||Cellular||Cellular||Multiceullar/ loose tissue||Tissue/ organ||Tissue/organ/ organ system|
|Mode of nutrition||Autotrophic (chemosynthetic and photosynthetic) and Heterotrophic (saprophyte/ parasite)||Autotrophic (Photosynthetic) and Heterotrophic||Heterotrophic (Saprophytic/ Parasitic)||Autotrophic (Photosynthetic)||Heterotrophic (Holozoic/ Saprophytic etc.)
Now a classification system has evolved which reflects not only the morphological, physiological and reproductive similarities, but is also phylogenetic (based on evolutionary relationships.)
- It consists of only Bacteria.
- Bacteria live in all types of habitat, even in extreme habitats such as hot springs, deserts, snow and deep oceans or as parasite in or on organisms.
- Though the bacterial structure is very simple, they are very complex in behaviour.
- Some of the bacteria are autotrophic (they synthesise their own food from inorganic substrates).
- They may be photosynthetic autotrophic or chemosynthetic autotrophic.
- The majority of bacteria are heterotrophs. (They do not synthesise their own food but depend on other organisms or on dead organic matter for food.)
- Classification of bacteria according to their shape –
- Spherical – Coccus
- rod-shaped – Bacillus
- comma-shaped – Vibrium
- spiral – Spirillum
Bacteria are also classified into – archaebacteria and eubacteria.
Archaebacteria: (Primitive Bacteria)
- These bacteria live in some of the most harsh habitats such as extreme salty areas (halophiles), hot springs (thermoacidophiles) and marshy areas (methanogens).
- Archaebacteria have a different cell wall structure and this feature is responsible for their survival in extreme conditions.
- Methanogens are present in the guts of several ruminant animals such as cows and buffaloes and they are responsible for the production of methane (biogas) from the dung of these animals.
Eubacteria: (True Bacteria)
- These are characterised by the presence of a rigid cell wall, and if motile, a flagellum.
- Cyanobacteria (blue-green algae) have chlorophyll–a similar to green plants and are photosynthetic autotrophs.
- The 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.
Nitrogen fixing bacteria –
- They fix atmospheric nitrogen in specialised cells called heterocysts, g., Nostoc and Anabaena. (heterocyst provide anaerobic condition required for N2 fixatiom)
Chemosynthetic autotrophic bacteria –
- They oxidise various inorganic substances such as nitrates, nitrites and ammonia and use the released energy for their ATP production.
- They play a great role in recycling nutrients like nitrogen, phosphorous, iron and sulphur.
Heterotrophic bacteria –
- They are the mostly important decomposers.
- Some help in making curd from milk, production of antibiotics, fixing nitrogen in legume roots, etc.
- Some are pathogens causing damage to human beings, crops, farm animals and pets.
- Cholera, typhoid, tetanus, citrus canker are well known diseases caused by different bacteria.
Reproduction in bacteria –
- Bacteria reproduce mainly by fission.
- Sometimes, under unfavourable conditions, they produce spores.
- They also reproduce by a sort of sexual reproduction by adopting a primitive type of DNA transfer from one bacterium to the other. (Conjugation)
- These organisms completely lack a cell wall.
- They are the smallest living cells known and can survive without oxygen.
- All single-celled eukaryotes are placed under
- Being eukaryotes, the protistan cell body contains a well defined nucleus and other membrane-bound organelles. Some have flagella or cilia.
- Protists reproduce asexually and sexually by a process involving cell fusion and zygote formation.
Chrysophytes: (diatoms / golden algae /desmids).
- Found in fresh water as well as in marine environments.
- float passively in water currents (plankton). [plankton – passive flow, nekton – active flow]
- Most of them are photosynthetic.
- In diatoms the cell walls form two thin overlapping shells, which fit together as in a soap box.
- The walls are embedded with silica and thus the walls are indestructible. Thus, diatoms have left behind large amount of cell wall deposits in their habitat; this accumulation over billions of years is referred to as ‘diatomaceous earth’.
- Being gritty this soil is used in polishing, filtration of oils and syrups.
- Diatoms are the chief ‘producers’ in the oceans.
- They are mostly marine and photosynthetic.
- They appear yellow, green, brown, blue or red depending on the main pigments present in their cells.
- The cell wall has stiff cellulose plates on the outer surface.
- Most of them have two flagella; one lies longitudinally and the other transversely in a furrow between the wall plates.
- Red dianoflagellates (Gonyaulax) undergo such rapid multiplication that they make the sea appear red (red tides).
- Toxins released by such large numbers may even kill other marine animals such as fishes.
- Most of them are fresh water organisms found in stagnant water.
- Instead of a cell wall, they have a protein rich layer called pellicle which makes their body flexible.
- They have two flagella, a short and a long one.
- Though they are photosynthetic in the presence of sunlight, when deprived of sunlight they behave like heterotrophs by predating on other smaller organisms. Therefore, they are known as connecting link between plant and animal.
- The pigments of euglenoids are identical to those present in higher plants. Example: Euglena
- Slime moulds are saprophytic protists.
- The body moves along decaying twigs and leaves engulfing organic material.
- Under suitable conditions, they form an aggregation called plasmodium which may grow and spread over several feet.
- During unfavourable conditions, the plasmodium differentiates and forms fruiting bodies bearing spores at their tips. The spores possess true walls. They are extremely resistant and survive for many years, even under adverse conditions. The spores are dispersed by air currents.
- All protozoans are heterotrophs and live as predators or parasites.
- There are four major groups of protozoans –
(a) Amoeboid protozoans:
- They move and capture their prey by pseudopodia (false feet).
- Marine forms have silica shells on their surface.
e.g., Amoeba, Entamoeba (Parasite)
(b) Flagellated protozoans:
- either free-living or parasitic.
- They have flagella.
- The parasitic forms cause diaseases such as sleeping sickness.
(c) Ciliated protozoans:
- Aquatic, actively moving organisms.
- Have thousands of cilia.
- They have a cavity (gullet) that opens to the outside of the cell surface.
- The coordinated movement of rows of cilia causes the water laden with food to be steered into the gullet.
- These organisms have an infectious spore-like stage in their life cycle.
e.g., Plasmodium (malaria parasite).
- organisms are heterotrophic.
- Fungi are cosmopolitan and occur in air, water, soil and on animals and plants.
- fungi are filamentous (except yeast which is unicellular)
- Their bodies consist of long, slender thread-like structures called hyphae. The network of hyphae is known as
- Some hyphae are continuous tubes filled with multinucleated cytoplasm – these are called coenocytic hyphae. Others have septae or cross walls in their hyphae.
- The cell walls of fungi are composed of chitin and polysaccharides.
- Most fungi are heterotrophic and absorb soluble organic matter from dead substrates and hence are called saprophytes.
- Those that depend on living plants and animals are called parasites.
- They can also live as symbionts – in association with algae as lichens and with roots of higher plants as mycorrhiza.
- vegetative – fragmentation, fission and budding.
- Asexual reproduction – by spores called conidia or sporangiospores or zoospores.
- sexual reproduction – by oospores, ascospores and basidiospores.
The various spores are produced in distinct structures called fruiting bodies.
The sexual cycle involves the following three steps –
- Plasmogamy – Fusion of protoplasm between two motile or non-motile gametes.
- Karyogamy – Fusion of two nuclei.
- Meiosis – in zygote resulting in 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). However, in other fungi (ascomycetes and basidiomycetes), an intervening dikaryophase (2 nuclei per cell) occur. 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.
Haploid spores →fusion begin →dikaryophase →nuclei fuse →diploid body →meiosis →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.
- Found in aquatic habitats and on decaying wood in moist and damp places or as obligate parasites on plants.
- The mycelium is aseptate and coenocytic.
- Asexual reproduction takes place by zoospores (motile) or by aplanospores (non-motile). These spores are endogeneously produced in sporangium.
- Zygospores are formed by fusion of two gametes. These gametes are similar in morphology (isogamous) or dissimilar (anisogamous or oogamous).
e.g., Mucor,Rhizopus (the bread mould) and Albugo (the parasitic fungi on mustard).
Ascomycetes (Sac fungi)
- multicellular (except Yest)
- Mycelium is branched and septate.
- The asexual spores are conidia produced exogenously on the special mycelium called conidiophores.
- Sexual spores are called ascospores which are produced endogenously in sac like asci. These asci are arranged in different types of fruiting bodies called ascocarps.
- Neurospora is used extensively in biochemical and genetic work.
- Many members like morels and buffles are edible and are considered delicacies.
e.g., yeast, Aspergillus,Penicillium,Claviceps and Neurospora.
Basidiomycetes (mushrooms, bracket fungi, puff balls)
- They grow in soil, on logs and tree stumps and in living plant bodies as parasites, e.g., rusts and smuts.
- The mycelium is branched and septate.
- The asexual spores are generally not found.
- vegetative reproduction commonly by fragmentation.
- The sex organs are absent,
- Plasmogamy is brought about by fusion of two vegetative or somatic cells of different strains or genotypes.
- The resultant structure is dikaryotic which ultimately gives rise to basidium.
- Karyogamy and meiosis take place in the basidium producing four basidiospores. The basidiospores are exogenously produced on the basidium. The basidia are arranged in fruiting bodies called basidiocarps.
e.g., Agaricus (mushroom), Ustilago (smut) and Puccinia (rust fungus).
Deuteromycetes (imperect fungi)
- Commonly known as imperfect fungi because only the asexual or vegetative phases of these fungi are known.
- When the sexual forms of these fungi were discovered they were moved into classes they rightly belong to.
- The deuteromycetes reproduce only by asexual spores known as conidia.
- The mycelium is septate and branched.
- Some members are saprophytes or parasites while a large number of them are decomposers of litter and help in mineral cycling.
e.g., Alternaria, Colletotrichum and Trichoderma.
- Includes all eukaryotic chlorophyll containing organisms.
- Few members are partially heterotrophic such as the insectivorous plants or parasites.
e.g., Insectivorous plants – Bladderwort and Venus fly trap.
Parasitic Plants – Cuscuta.
- cell wall mainly made of cellulose.
- Plantae includes algae, bryophytes, pteridophytes, gymnosperms and angiosperms.
- Life cycle of plants has two distinct phases – the diploid sporophytic and the haploid gametophytic – that alternate with each other. This phenomenon is called alternation of generation.
- They directly (herbivore) or indirectly (carnivore) depend on plants for food.
- They digest their food in an internal cavity and store food reserves as glycogen or fat.
- Their mode of nutrition is holozoic – by ingestion of food.
- They follow a definite growth pattern and grow into adults with definite shape and size.
- Higher forms show elaborate sensory and neuromotor mechanism.
- Most of them are capable of locomotion.
- The sexual reproduction is by copulation of male and female followed by embryological development.
VIRUSES, VIROIDS AND LICHENS
- Some acellular organisms like viruses, viroids and lichens are not included in the five kingdom classification of Whittaker.
- Viruses did not find a place in classification since they are not truly ‘living’.
- The viruses are non-cellular organisms that are characterized by having an inert crystalline structure outside the living cell.
- Once they infect a cell they take over the machinery of the host cell to replicate themselves, killing the host.
- The name virus that means venom or poisonous fluid was given by Pasteur.
- D.J. Ivanowsky (1892) recognised certain microbes as causal organism of the mosaic disease of tobacco. These were found to be smaller than bacteria because they passed through bacteria-proof filters.
- M.W. Beijerinek (1898) demonstrated that the extract of the infected plants of tobacco could cause infection in healthy plants and called the fluid as Contagium vivum fluidum (infectious living fluid).
- W.M. Stanley (1935) showed that viruses could be crystallised and crystals consist largely of proteins. They are inert outside their specific host cell.
- Viruses are obligate parasites.
- In addition to proteins viruses also contain genetic material that could be either RNA or DNA. No virus contains both RNA and DNA.
- A virus is a nucleoprotein and the genetic material is infectious.
- 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 (viruses that infect the bacteria) are usually double stranded DNA viruses.
- The protein coat called capsid made of small subunits called capsomeres, protects the nucleic acid.
- These capsomeres are arranged in helical or polyhedral geometric forms.
- Viruses cause diseases like mumps, small pox, herpes, influenza, AIDS.
- In plants, the symptoms can be mosaic formation, leaf rolling and curling, yellowing and vein clearing, dwarfing and stunted growth.
- Dicovered by T.O. Diener in 1971.
- It was smaller than viruses and caused potato spindle tuber disease.
- It was found to be a free RNA; it lacked the protein coat that is found in viruses, hence the name viroid.
- The RNA of the viroid was of low molecular weight.
- Lichens are symbiotic associations (mutually useful) 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.(Sulphur indicator).
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