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Thursday, August 5, 2021


Since the dawn of civilisation, 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. He also divided animals into two groups, those which had red blood and those that did not. In Linnaeus' time a Two Kingdom system of classification with Plantae and Animalia kingdoms was developed that included all plants and animals respectively.

R.H. Whittaker (1969) proposed a Five Kingdom Classification. The kingdoms defined by him were named Monera, Protista, Fungi, Plantae and Animalia. The main criteria for classification used by him include cell structure, body organisation, mode of nutrition, reproduction and phylogenetic relationships. Table 2.1 gives a comparative account of different characteristics of the five kingdoms.

1.Need of Classification
There have been many attempts to classify living organisms since ancient times. Aristotle was the earliest to attempt a scientific basis of classification. He used simple morphological characters to classify plants as trees, shrubs and herbs. He also classified animals into two groups,i.e„ enaima (with red blood) and anaima (without red blood). A need for proper system of classification was always felt.
Living organisms need to be classified because of the following reasons: The study of one or two organisms is not sufficient to know the essential features of the group. All kinds of organisms do not occur in one locality. Classification helps in knowing the relationship among-est different groups of organisms. It helps in knowing the evolutionary relationship between organisms.
Types of Classification System
Depending upon the types of system of classification, organisms are classified into following kingdoms
1. Two Kingdom Classification System
Linnaeus (the father of taxonomy system) divided all the living organisms into two kingdoms in 1758.
These are Plantae and Animalia.
Features of Kingdom-Plantae
The characteristic features of this kingdom are
 Cell wall is present. A big central vacuole is present. Absorb inorganic nutrieiAs from outside. Unlimited growth and well defined growing points. Autotrophic mode of nutrition, reserve food is starch. No locomotion (except in some lower algae). Absence of excretory organs, nervous system, sense organs and muscular system. Slow response to external stimuli.
Features of Kingdom-Animalia
The characteristic features ofthis kingdom are
(a) Absence of cell wall.
(b) Inorganic crystals are not present in their cells.
(c) Central vacuole is not present.
(d) Heterotrophic mode of nutrition.
(e) Growth is limited and well defined growing points are not present.
(f) Reserve food as glycogen.
(g) Excretory organs, nervous system and sense organs are present.
(h) Locomotion is present.
(i) Muscular system is present.
(j) Show quick response to external stimuli.
Objections Against Two Kingdom Classification System
The two kingdom system of classification was accepted for a long time. However, some difficulties arised from this classification as several new living organisms have been discovered.
Some of these difficulties are mentioned below
(a) The first formed organisms were neither plants nor animals.
(b) Fungi do not show similarity with structure, physiology and reproductive system of plants.
(c) It is not easy to recognise the lower organisms as plants or animals. For example, Euglena has mixotrophic (dual) mode of nutrition, while sponges are fixed, branched and irregular creatures like plants.
(d) Slime moulds, a group of fungi, are wall-less in vegetative phase. They develop cell wall in the reproductive phase. Slime moulds can neither be placed in fungi, nor plants.
(e) Lichens are formed by the symbiotic association of an alga and a fungus. They neither resemble plants nor animals.
(f) Prokaryotes do not have an organised nucleus. They have single envelope organisation, absence of spindle apparatus, meiosis and sexual reproduction.
Eukaryotes have a well-defined nucleus, a double envelope organisation, spindle apparatus, meiosis and sexual reproduction.
On the other hand, viruses have no protoplasm and metabolic machinery of their own. Therefore, all of these cannot be kept in a single group.
(g) Unicellular algae like diatoms, euglenoids, dinoflagellates and Protozoa resemble each other.

2. Three Kingdom Classification System
Ernst Haeckel in 1866, classified living organisms into three kingdoms-Plantae, Protista and Animalia. The new kingdom-Protista included all those organisms, which lack the capability of tissue differentiation. These are algae, fungi and Protozoa. Later, kingdom-Protista was reserved only for unicellular organism.

Limitations of Three Kingdom Classification System: 
(a) Prokaryotes and eukaryotes are not separated.
(b) Both unicellular and multicellular organisms are kept in Protista.
3.Four Kingdom Classification System
The four kingdom classification system included Monera in addition to Protista, Plantae and Animalia. Studies with electron microscope made it clear that bacteria and related organisms have a different nuclear structure as compared to others. They areprokaryotes, thus kingdom-Monera was created by Copeland (1956). Fungi continued to remain with Plantae in this system.

4. Five Kingdom Classification System
This classification was proposed by RH Whittaker, in 1969. Before 1969, the classification systems for the living organisms have undergone several changes overtime. He created fungi, as separate kingdom. The main criteria for classification used by Whittaker
(i) Cell structure (ii) Modes of nutrition
(iii) Thallusorganisation (iv) Reproduction
(v) Phylogenetic relationships.
Merits of Five Kingdom Classification System
Merits of five kingdom classification system are
(a) Euglena and other transition types which had been included both amongst plants and animals are given proper place under kingdom—Protista.
(b) Fungi have their own biochemical, physiological and structural organisation. They have never been related to plants. In this system of classification fungi are separately placed.
(c) A separate kingdom of prokaryotes include Monera has been created. Monerans differ from all other organisms in their cellular, reproductive and physiological organisations.
(d) The five kingdom classification system is based on cellular organisation, the mode of nutrition and complexity of structure. These were the basic factors used in earliest two kingdom system of classification.
(e) The system shows the gradual evolution of early organisms into plants and animals.
(f) The plant and animal kingdoms are more homogenous than, they were in the two kingdom system of classification.

Demerits of Five Kingdom Classification System
(a) Animal protozoans have been included in kingdom—Protista, which also includes unicellular plants. They show different modes of nutrition.
(b) Yeasts are though, unicellular eukaryotes, do not belong to kingdom—Protista.
(c) Chlorella and Chlamydomonas, though unicellular included under the kingdom-Plantae. They should be kept in Protista.
(d) Euglena like organisms and slime moulds with flexible life style may need the creation of an intermediate kingdom of Protista.
(e) Viruses and viroids are not kept in proper place in this system.

5. Six Kingdom Classification System
It was introduced by Carl Woese a Professor in the Department of Microbiology, University of Illinois in 1990. This system is also named as three domain system as in it organisms are classified into three domains, i.e., Archaea, Bacteria and Eukarya.
It mainly used basic principles of five kingdom system but divides the Monera into two domains Archaebacteria, Eubacteria and other eukaryotes in third kingdom.

Kingdom Monera
The kingdom-Monera includes all prokaryotes such as bacteria, mycoplasma, Actinomycetes and cyanobacteria (blue-green algae). Bacteria are the sole members of the Kingdom Monera. They are the most abundant micro-organisms. Bacteria occur almost everywhere. Hundreds of bacteria are present in a handful of soil. They also live in extreme habitats such as hot springs, deserts, snow and deep oceans where very few other life forms can survive. Many of them live in or on other organisms as parasites.This group includes all kinds of bacteria, having a prokaryotic cell. The cell does not contain a nucleus. There are different shapes of bacteria present; spherical- cocci, rod-shaped- bacillus, comma- vibrio and spiral- spirilla. 

 Archaebacteria- These bacteria are present in the harshest environmental conditions such as salty, marshy and in hot springs. They are known as halophiles, methanogens, thermoacidophiles, respectively/ Methanogens are present in the gut of ruminants and produce biogas
Super kingdom (prokaryote)Archaebacteria: These bacteria are special since they live in some of the most harsh habitats such as extreme salty areas (halophiles), hot springs (thermoacidophiles) and marshy areas (methanogens). Archaebacteria differ from other bacteria in having a different cell wall structure and this feature is responsible for their survival in extreme conditions. Methanogens are present in the gut 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.
The characteristics of this domain are: They are most primitive prokaryotes. They are found in stressed environment, such as high salt content (Great salt lake, the dead sea), edge of the ocean, hot sulphur springs, volcanic walls, etc. Their cell walls lack peptidoglycan. In most cases, the wall composed of non-cellulosic polysaccharides and some proteins. In some members, there is no cell wall. This feature of having different cell walls is responsible for their survival in extreme condition.
Most of the archaebacteria are chemoautotrophs.

Types of Archaebacteria
Archaebacteria are of following three types
Methanogens: These are stricdy anaerobes. They live anaerobically in gut of several ruminants such as cows, buffaloes, goat, etc. These bacteria help in fermentation of cellulose. They produce almost 65% of atmospheric methane. Example Methanobacterium,Methanobacillus, Methanosarcina and Methanococcus.
Halophiles:  These are found in extreme saline environments like salt lakes, salt marshes, salt pans, salt solutions, etc. They are mosdy anaerobes. They contain a chemical called halorhodopsin to pump in chlorides into the cell to prevent cellular dehydration. Halobacterium develops purple membrane having photoreceptor pigment bacteriorhodopsin. In light, it acts as a proton pump and helps in synthesise of ATP. The formation of ATP is a survival mechanism under anaerobic condition. Examples Halobacterium and Halococcus.
Thermoacidophiles:  These archaebacteria can live in both extreme heat and acidic pH (around 2) environment. Under anaerobic conditions, these organisms oxidisesulphur to sulphuric acid.

The members of this kingdom have peptidoglycan cell wall, naked DNA in coiled form, glycogen food reserves.Eubacteria are prokaryotic microorganisms consisting of a single cell lacking a nucleus and containing DNA is a single circular chromosome. The sap vacuole is not present and 70S ribosomes are present. Eubacteria can be either gram-negative or gram-positive, they have economic, agricultural, and medical importance. They include E. coli, Lactobacilli, and Azospirillum. The members of this kingdom are bacteria, mycoplasma, Actinomycetes, rickettsiae, spiro- chaetes, cyanobacteria, firmicutes.
The four kingdoms ofthis domain are
(a) Protista
(b) Fungi
(c) Plantae
(d) Animalia
Eubacteria are mostly heterotrophs, which take food from an outer source. Most heterotrophs decompose dead material or parasites that live on or in a host. Other eubacteria are autotrophs by making their own food; they are either chemosynthetic or photosynthetic. The most important autotrophic eubacteria are cyanobacteria.

Eubacteria Structure: Are eubacteria unicellular or multicellular? The eubacteria number of cells is only one. They are single prokaryotic cells. There is no such thing as eukaryotic bacteria. The structures found in eubacterial cells are either external or internal to the cell wall. Structures external to the cell wall may be flagella, fimbriae, axial filaments, glycocalyx, or pili. Each of these structures has its distinctive function where some eubacteria have flagella to facilitate their movement.

Flagella are long filaments that facilitate the mobility of bacteria. Flagella consist of three main parts: a portion consisting of flagellin protein called the filament (i.e. the long outermost part), the middle hook, and the basal body that attaches the bacterial plasma membrane and cell wall. Fimbriae and pili are hairlike structures similar to flagella but thinner. Pili or pilus are thin projections used for conjugation between bacteria for reproductionafter the bacteria are brought together by pili, the DNA moves from one cell to another so new features as antibiotic resistance are transferred between cells.

Glycocalyx surrounds some eubacterial cells. It is a viscous polymer composed of polypeptides or polysaccharides and functions to protect the bacteria. It is commonly known as the capsule. The capsule is considered to be one of the virulence factors of some bacteria since it enables the bacteria to resist phagocytosis by the immune system. Glycocalyx coat can also help bacteria to attach firmly to the host cell such as Vibrio cholera that produces glycocalyx to attach to the intestinal cells.

The cell wall of bacteria consists chiefly of a network of peptidoglycan. It may be associated with other substances or present alone. The peptidoglycan network consists of disaccharide portions connected together by polypeptides forming a lattice to protect the bacteria. Some antibiotics such as penicillins and cephalosporins interfere with the structure of bacterial cell walls leading to cell lysis and rupture. Structures internal to the cell wall include cell membrane, cytoplasm, DNA, plasmid, and ribosomes.

Like eukaryotic cells, prokaryotic internal structures, such as those of eubacteria, are surrounded by a cytoplasmic membrane consisting mainly of phospholipids. However, the cell membrane of prokaryotes is less rigid than that of many eukaryotes. Additionally, a group of antibiotics called polymyxins can also damage plasma membranes leading to the destruction of bacterial cells. The cytoplasm is the substance inside plasma membranes and acts as a media for the internal structures of the cell. In eubacteria, the cytoplasm contains DNA that is not normally seen in the eukaryotic cytoplasm. Prokaryotic cells contain cytoskeletons in the cytoplasm that aids in the growth, reproduction, and maintenance of the cell’s shape. The nucleoid or DNA of prokaryotic eubacteria differs from that of eukaryotes, it consists of a long, single, circular double-stranded DNA, which contains all the bacterial genetic information.

Bacterial cells mostly contain a small structure of DNA double-strand called a plasmid that is distinct from the bacterial chromosome. Plasmids replicate independently from the bacterial chromosomes. Plasmids are the main structures used in biotechnological applications recently because they can replicate independently.Like eukaryotes, prokaryotes have ribosomes for the synthesis of proteins.

What is the difference between Bacteria And Eubacteria? 
Bacteria is a term that was previously used to include all bacteria. Soon, two groups emerged: eubacteria or true bacteria and archaebacteria or archaea. Eubacteria and archaebacteria are the only prokaryotes found on earth. They have a common progenitor cell but different evolutionary lines. Even though the archaea are structurally similar to bacteria, they are different when examined at a molecular level.

Eubacteria are classified into several phyla. Each bacterial phylum includes species characterized by specific features. Examples are as follows:
Proteobacteria includes most of the gram-negative bacteria. They are thought to have arisen from photosynthetic ancestors. Proteobacteria are classified into five classes: alphaproteobacteria, betaproteobacteria, gammaproteobacteria, deltaproteobacteria, and epsilonproteobacteria.

Cyanobacteria are characterized by a blue-green pigment. They perform photosynthesis as plants and algae. Many of these bacteria can fix nitrogen in the soil, therefore, they are important in agricultural fields.

Chlorobi phylum consists of photosynthetic bacteria. Members of this phylum are green sulfur bacteria. Chlorobi reduces carbon dioxide during photosynthesis using organic compounds such as carbohydrates and acids. Members of this phylum diverse greatly with their rods, spiral, cocci, or budding forms.

Chloroflexi members are green nonsulfur bacteria, such as Chloroflexi can perform photosynthesis.

Chlamydiae members are pathogenic gram-negative cocci that have a unique cycle of development. They are transmitted from human to human by direct contact or respiratory airborne routes.

Planctomycetes are budding gram-negative bacteria. Even though their DNA is similar to bacteria, their cell wall is similar to archaea. Additionally, some of them contain organelles similar to those of eukaryotes.
Bacteroidetes are anaerobic bacteria that inhabit the human intestinal tract or oral cavity or intestinal tract. They are present in feces and may cause infection due to surgery or puncture wounds.

Fusobacteria are anaerobic bacteria; their cell shape is either pleomorphic or spindle.

Spirochaetes are coiled resembling metal springs. They are flagellated. Their flagella facilitate their movement using axial filaments. Spirochaetes are usually present in the human mouth.

Biological Importance Of Eubacteria
The world is filled with different eubacterial species and our bodies contain different species of eubacteria, which are biologically important in our life. Our body is only of the eubacteria habitats forming our normal flora. Normal flora causes no harm to us and they are beneficial to our bodies. For example, they defend our bodies against pathogenic eubacteria, others can produce biologically important substances such as B vitamins as well as vitamin K. Several species of eubacteria are used in the mass production of chewable or tablet vitamins since bacterial species provide an inexpensive, safe, and non-toxic source of vitamins. Streptomyces hygroscopicus is of particular importance in the pharmaceutical industry since their different strains can produce about 200 different types of antibiotics. Eubacteria decompose organic matter and dad leaves into carbon-dioxide and nutrients such as nitrogen They contribute to keeping the balance of all ecosystems.

Eubacteria – Bacteria
The term Bacteria was proposed by Ehrenberg in 1829. They have widespread distribution be it air, water or soil. They can survive in extreme range of temperatures like up to 78°C and -190°C.
Important characteristics of bacteria are: Bacteria are found in all kinds of habitats. They are prokaryotic microorganisms. They are unicellular. Cell wall contains peptidoglycan. An organised nucleus in absent. Extra chromosomalself replicating DNA segments called plasmids occur in most of the bacteria.Mitochondria, plastids, Golgi apparatus,endoplasmic reticulum and other membrane covered cell organelles are absent.

The size of bacterial cell ranges from 1-10 Jim in length and from 0.7-1.5 flm in width.
The bacteria possess the following forms: Coccus (PI. cocci) bacteria are oval or spherical cells without flagella. The spheres occur as single cells (Monococcus), a pair of cells (Diplococcus), in groups of four cells (Tetracoccus), as chain of cells (Streptococcus) or in sheets (Staphylococcus).
A few cocci may also occur in cube-like arrangements of 8 or more cells (Sarcina). Bacillus (PI. bacilli) bacteria are rod-shaped cells which many occur singly (Monobacillus), in pairs (Diplobacillus), in chains (Streptobacillus) or as a layer (suck) with many cells called Palisade bacillus. Spirillum (PI. spirilla) bacteria are cells, which are twisted, like a screw. They occur as free single cells, e.g., Spirillum, Spirochaete, etc.
(iv) Vibrio are cells which are curved, C-shaped or comma-shaped, e.g., Vibrio cholerae.

A bacterial cell is covered by mucilage. It is differentiated into cell wall, plasma membrane, cytoplasm, nucleoid, plasmids, inclusion bodies, flagella, pilli and fimbriae. Membrane bound organelles are absent. 
Bacteria show both autotrophic and heterotrophic mode of nutrition, i.e., mixotrophic.
On the basis ofmodeofnutrition, bacteria are of two types:

Autotrophic Bacteria
These are offolbwing two types: Photosynthetic These bacteria have green – sunlight trapping pigment called bacteriochlorophyil.
These are found at the bottom of ponds and lotus. Bacterial photosynthesis does not release oxygen.

Chemosynthetic: These bacteria are able to synthesise organic food from inorganic raw materials with the help of energy derived from
exergonic chemical reactions. Examples Nitrifying bacteria (Nitrosomonas), iron bacteria (Ferrobacillusferroxidants), sulphuroxidising bacteria (Beggiatoa).

Heterotrophic Bacteria
These bacteria obtain food from different sources. These may be of followingtypes
Saprophytes:  These are called decomposers, detrivores or transformers. These obtain food by decomposing dead-bodies, excreta of animals, dead plants and their parts.
Parasites:  These are disease causing bacteria called as pathogens, e.g., Salmonelbtyphimurium, which causes typhoid in human.
Symbionts: These bacteria live in mutually beneficial association with other organisms, e.g., Rhizobium and Bacillus, species form nodules in root of leguminous plants.

Bacteria reproduce by asexual and sexual (parasexual) processes.
Asexual Reproduction: Asexual reproduction occur by binary fission and endospore formation.
Binary Fission:  It is a simple cell division in which bacterial cell divides in two parts. A constriction appears at the centre of the cell, deepens further and grows from margin to centre and finally two cells are produced.
Endospore Formation:  Endospores are perennial structures which help in survival even during harsh environmental conditions, e.g., in Cbstridium and Bacillus. The endospore has many wall layers. It has heat resistant chemicals called sialic acid and dipicolinic acid.

Sexual Reproduction
Sexual reproduction occurs by a parasexual process actually called genetic recombination. The three methods involved are as follows
Conjugation: The male cell (donor cells) has fertility plasmid or F-factor, which connect itself to cell wall of female cells (recipient cells).
Transformation:  The process was discovered by Griffith in 1928. It is a process where segments of DNA are transferred from one bacterial cell to another via the liquid medium.
Transduction: During this process, the segment of DNA are transferred from one bacterium to another by the viruses (bacteriophages).

Uses of Bacteria
Bacteria is useful in the following ways: Bacteria are natural scavangers. They obtain their nutrition by decomposing dead bodies, dead plants and animal excreta. These are used in fermentation process for vinegar manufacturing, yogurt making, etc. Some bacteria help in retting of jute and coconut plant fibres. The separated fibres are used in making ropes or gunny bags. The genus Streptomyces has many species used to produce different antibiotics.
Some important antibiotics using various bacteria are neomycin, omycetin, streptomycin. Bacteria play important role in different steps of nitrogen cycle. Some important bacteria in nitrogen cycle, e.g, Clostridium, Azotobacter (soil bacteria), Rhizobium leguminosarum, Bacillus radicicola (in nodules), Nitrosomonas, Nitrosobacter, Pseudomonas etc.

Harmful Effects of Bacteria
Bacteria is harmful in the following ways, Some saprophytic bacteria like Lactobacillus spoil milk and milk products. Food poisoning occurs due to the production of toxins by some bacteria like Clostridium botulinum. They cause botulism, which can kill humans by respiratory paralysis. Bacteria are responsible for various plant diseases like citrus canker in lemon leaves and fruits, soft rot in carrot plants, blight disease in rice plants, crown gall disease in apple trees and rose plants. In humans, bacteria cause diseases like cholera (Vibrio cholerae), gastric ulcer (Heliobacter pylori), tuberculosis (Mycobacterium tuberculosis), sexually transmitted diseases like gonorrhea (Neisseriagonorfhoeae), syphilis (Treponemapallidum), etc. In animals like horse, cattle and sheep, anthrax disease is caused by Anthracis.

Cyanobacteria, member of this group (blue-green algae) have many characters similar to bacteria. The examples of cyanobacteria are Nostoc, Oscillatoria, Spirulina, Rivularia, Anabaena, etc. They can survive in a wide variety of habitats, such as hot springs, sea water, polluted water, etc.
Cyanobacteria have following three forms
(i) Unicellular as in Chrococcus.
(ii) Colonial as in Microcystis and Gloeocapsa.
(iii) Filamentous as in Nostoc, Oscillatoria and Anabaena.

Cyanobacteria - Cell Structure
Cell has a definite firm and rigid cell wall surrounded by mucilaginous sheath. The cell wall is followed by plasma membrane made up of lipid and proteins. The membrane bound structures like true mitochondria, endoplasmic reticulum, Golgi bodies, etc., are absent.
Nutrition: Cyanobacteria are mosdy photoautotrophs. They contain chlorophyll-a and other photosynthetic pigments.

Cyanobacteria multiply asexually and vegetatively. Sexual reproduction does not occur. The types of multiplication are: Binary fission occurs in unicellular forms. Fragmentation occurs in colonial and filamentous forms. Conidia are asexually produced spores of fungi. Endospores and exospores are non-reproductive structures.

Uses of Cyanobacteria
Some uses of cyanobacteria are
Some cyanobacteria have the ability to fix atmospheric nitrogen. The green manuring by farmers is done on this basis to enrich the soil with nitrogenous fertilisers. Cyanobacteria like Anabaena, Tolypothrix, etc., help in prevention of soil erosion and its conservation. Spirulina is a protein rich supplement for humans. It is a fast growing cyanobacteria. It is also known as Single Cell Protein (SCP). Cyanobacteria like Anabaena and Aulosira prevent mosquito larvae to grow in surroundings.

Harmful Effects of Cyanobacteria
Some harmful effects of cyanobacteria are: Cyanobacteria discolour the walls and roofs of buildings, movements and statues. Oscillatoria causes asthma and gastrointestinal problems by releasing its toxins. Growth of Oscillatoria in water bodies shows pollution by organic matter. Excessive growth of cyanobacteria form water blooms, which decreases oxygen level in water causing death of aquatic animals.

All single-celled eukaryotes are placed under Protista, but the boundaries of this kingdom are not well defined. What may be ‘a photosynthetic protistan’ to one biologist may be ‘a plant’ to another. In this book we include Chrysophytes, Dinoflagellates, Euglenoids, Slime moulds and Protozoans under Protista. Members of Protista are primarily aquatic. This kingdom forms a link with the others dealing with plants, animals and fungi. 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. It was first proposed by Ernst Haeckel (1866). Physiologically kingdom-Protista acts as a connecting link between the kingdom-Monera and the complex multicellular kingdom-Fungi, Plantae and Animalia. Kingdom-protista includes the following categories such as dinoflagellates, chrysophytes, euglenoids, slime moulds and protozoans.

The general Characteristic Features Of Kingdom-Protista are given below: These are mostly aquatic organisms. Some protists also live in the bodies of animals as parasites. The cells are eukaryotic. These contain membrane bound cell organelles like mitochondria, Golgi complex, endoplasmic reticulum, 80S ribosomes, etc. Locomotion may either occur by Pseudopodia (Amoeba, Euglypha), Cilia (Parameciuni), Wriggling (sporozoans, non-flagellates) and Mucilage propulsion (some protists like diatoms). Diatoms do not have any organelles for locomotion. Protists shows various modes of nutrition such as: Photosynthetic (holophytic) Dinoflagella- tes, diatoms and euglenoids. Halozoic (zootrophic) Protozoans like Amoeba and Paramecium. Saprobic (saprotrophic) In slime moulds. Parasitic Trypanosoma, Giardia, Plasmodium, Entamoeba.
Symbiotic In zooflagellates like Trichonympha and Lophomonas.
Pinocytosis In Amoeba to absorb soluble organic substances. Most of the protists are aerobic. However, some protists that live at the bottom of aquatic habitats can respire anaerobically.
Protists reproduce asexually and sexually by a process involving cell fusion and zygote formation.

This group includes diatoms and golden algae (desmids).This group includes diatoms and golden algae (desmids). They are found in fresh water as well as in marine environments. They are microscopic and float passively in water currents (plankton). 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.

Diatoms occur in all aquatic and moist terrestrial habitats and are also known as chief producer in the ocean. They pile up at the bottom of water reservoirs and form big heaps called diatomaceous earth. They are microscopic unicellular organisms of different shapes, such as circles, semicircles, triangular, spindle-shaped, boat-shaped, etc. The body wall of the diatoms is made up of cellulose impregnated with glass like silica. The cell wall has two overlapping halves like a sapbox called shell or ffustule, i.e., a lid and a lower half fitted together. Diatoms are variously coloured, do not passess flagella except in the reproductive state. Each cell has a large central vacuole. Chloroplast are yellowish brown to greenish brown. They contain chlorophyll and c. They contain fucoxanthin that provides brownish ting. Food is reserved in the form of oils and leucosin (polysaccharide). The diatoms mostly reproduce asexually by binary fission. Sexual reproduction varies from isogamy to oogamy. Examples Navicula, Amphipleura ,Triceratium and Cymbella.

Economic Importance of Diatoms
Diatoms are economically important in the following ways- Diatoms are very important photosynthesizers. Diatomite deposits are often accompanied by petroleum fields. These are used as a cleaning agent in tooth pastes and metal polishes and are used in filtration of oil and syrups. Diatoms are used as insulation material in refrigerators boilers and furnaces. These are also used to make sound-proof rooms. Diatoms are also very good pollution indicators.

Golden Algae (Desmids)
These are unicellular green algae. Their cell walls have distinct halves. Sexual reproduction occurs by ‘conjugation’ (similar to Spirogyra). They are usually found in freshwater and acts as an indicators of polluted water.

These are mainly marine and photosynthetic organism. There are about 1,000 species of photosynthetic protists. These organisms 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. Very often, red dinoflagellates (Example: 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.

ave histone and RNA. The cells possess an osmoregulatory organelle called pusule, which superficially looks like contractile vacuole. Dinoflagellates reproduce asexually through cell division or by the formation of zoospores and cysts. Varieties of eye spots’ occur in dinoflagellates. Some of them are like ocelli. Reserve food is stored in the form of starch and oils, e.g., Gonyaulax, Ceratium, NoctilThe general characteristic features of dinoflagellates are: These are important phytoplanktons. Most of them are marine but some occur in freshwater. They appear yellow, green, brown, blue or red depending on the main pigments present in their cells. The cell wall in dinoflagellates, if present is composed of number of plates made up of cellulose. Some dinoflagellates like Gonyautax and Gymnodinium grow in large number in sea and make the water look red and form ‘red tide’. Toxins released by such large numbers may even kill other aquatic animals. The cells usually possess two flagella which are of different types (heterokont). One flagellum is transverse arising from the anterior part. The other flagellum arises in the vertical furrow. Both these flagella beat in different directions. The nucleus is bigger in size, named as mesokaryon. Chromosomes do not huca, Peridinium and Gymnodinium, etc.

Euglenoids live in fresh aquatic habitats and damp soils. Majority 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. Interestingly, the pigments of euglenoids are identical to those present in higher plants. Example: Euglena.

The characteristic features of euglenoids are described below
They are unicellular flagellate protists.
Body is covered by thin and flexible pellicle. It lacks ceflulosic cell wall. Euglenoids have two flagella, usually one long and one short.
They perform creeping movements by expansion and contraction of their body. This phenomenon is called metaboly.
Nutrition is holophytic, saprobic or holozoic. This mode of nutrition is called mixotrophic.
The photosynthetic pigments include chlorophylla and b. 
Reserve food is carbohydrate in the form of paramylon or paramylum bodies.
Euglenoids reproduce by longitudinal binary fission under favourable conditions. The palmella stage is found during unfavourable conditions. Examples Euglena, Perenema, Eutreptia, Phacus, etc.

Euglena is considered as plant as well as animal. It is also called as plant animal. Plant and animal features are Plant Features Chloroplasts and chlorophyll are present has holophytic nutrition. Animal Features Presence of pellicle which is not made of cellulose. Contractile vacuole is present. Longitudinal binary fission. Euglenozoa is a diverse clade that includes predatory heterotrophs, photosynthetic autotrophs and pathogenic parasites.The main feature that distinguishes protists in this clade is the presence of a spiral or crystalline rod of unknown function inside the flagella. Consumer-Decomposer Protists (Slime Moulds) They possess the characters of both animals and fungi.

Slime Moulds
Slime moulds are saprophytic protists. Anton De Bary (1887) related them to animals and called them as Mycetozoa. These are also named as fungus animals because they share the common characters of both animals and are known as protistianjungi, and due to their protistian nature.The generalfeatures of slime moulds are discussed here (t) Slime moulds are acellular and cellular types, about 600 species of slime moulds.

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. reported by biologists out of which 27 species are known from India.

They are found in moist terrestrial places rich in decaying organic food.
The body of slime moulds is covered with mucilage having gelatinous consistency, they do not have chlorophyll.
They are surrounded by plasma membrane. However, the spores have the ceflulosic cell walls.
They show phagotropic or saprotrophic nutrition.
Both sexual and asexual modes of reproduction occur.
They are like Protozoa in their amoeboid plasmodial stage and similar to true fungi in spore formation.
Acellular slime moulds (plasmodial slime moulds) are commonly found on dead and decaying plant matter. The cellular slime moulds occur in all humus-containing upper layer of damp soil. When the food supply is shorter or conditions are not favourable, the amoeboid cells form aggregate without any fusion.
This aggregated mass is called pseudoplasmodium. The examples of cellular slime moulds are dictyostelium and polysphondylium.Plasmodium is the free-living thalloid body of the acellular slime moulds. It is wall-less mass of multinucleate protoplasm covered by slime layer. During unfavourable conditions, the Plasmodium differentiates and forms fruiting bodies bearing spores at their tips. WTiile during favourable conditions, Plasmodium can spread over several feet.
Slime moulds are beneficial as they cause the decomposition of organic matter in the soil.

All protozoans are heterotrophs and live as predators or parasites. They are believed to be primitive relatives of animals. There are four major groups of protozoans. Amoeboid protozoans: These organisms live in fresh water, sea water or moist soil. They move and capturetheir prey by putting out pseudopodia (false feet) as in Amoeba. Marine forms have silica shells on their surface. Some of them such as Entamoeba are parasites. Include unicellular protists with animal like behaviour.They were first studied by Leeuwenhoek (1677).Protozoan protists may be aquatic, terrestrial or parasites.They can cause several diseases in humans and animals.

General characteristics of protozoans are described below:
They are microscopic small unicellular and colourless organism with different shapes.
Locomotion occurs with the help of finger-like pseudopodia, flagella or hairy cilia.
All protozoans are heterotrophs and live as predators or parasites.
Respiration occurs through the general surface of the body.
Reproduction occurs by binary fission, multiple fission or budding. Sexual reproduction occurs by syngamy and conjugation.

There are four major groups of protozoans
Amoeboid Protozoans
These organisms live in freshwater, seawater or moist soil. Examples Amoeba, Entamoeba, Radiolarians, Pelomyxa, Foraminiferans and Heliozoans. 
General features of this group are following:
They move and capture their prey by putting out pseudopodia (false feet) as in Amoeba (as mouth is absent). The body is without periplast. It may be naked or have a calcareous shell. Flagella are present in some developmental stages. They also develop when food become scarce. Nutrition is holozoic. Asexual reproduction occurs by binary fission, multiple fission, spores and budding and sexual reproduction occurs by syngamy.

Flagellated protozoans: 
The members of this group are either free-living or parasitic. They have flagella. The parasitic forms cause diaseases such as sleeping sickness. Example: Trypanosoma.
General features of this group are following:
They have flagella for locomotion as their name suggests.
They may be aquatic, free-living, parasitic, commensals or symbiotic.
The body is enclosed by a firm pellicle.
Nutrition is holozoic, saprobic and parasitic.
Asexual reproduction is by binary fission.
Sexual reproduction is observed in some forms only. Various species of these protozoans causes diseases in humans. For examples, Trypanosoma (sleeping sickness) Leishmania (kala-azar, dum-dum fever), Giardia (giardiasis) & Trichomonas (leucorrhoea).

Ciliated protozoans
These are aquatic, actively moving organisms because of the presence of 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. Example: Paramoecium
Examples Paramecium, Opalina, Vorticella, Podophyra, Balantidium, etc.
Generalfeatures of this group are following: 
Many ciliates live as free-living individual in fresh % and marine water (Paramecium).
A large number of cilia present on whole body surface.Cilia are used to capture food and for locomotion.
Nutrition is holozoic except in some parasitic forms. The body is covered with flexible pellicle. There are definite regions for ingestion and egestion.
Ciliates have a larger macronucleus and smaller micronudeus.
They have small ejectabletrichocysts for defense.
Osmoregulation occurs by contractile vacuoles.
Asexual reproduction occurs by transverse binary fission or budding. Cyst formation also occurs during unfavourable condition.
Sexual reproduction by means of conjugation.

This includes diverse organisms that have an infectious spore-like stage in their life cycle. The most notorious is Plasmodium (malarial parasite) which causes malaria, a disease which has a staggering effect on human population.
General features of this group are following:
All sporozoans are endoparasites and pathogenic. Locomotory organs are absent. Nutrition is parasitic (absorptive). Body is covered with an elastic pellicle or cuticle and contractile vacuoles are absent. A sexual reproduction occurs through multiple fission and sexual reproduction by syngamy. Life cycle may include two different hosts, e.g., Plasmodium requires two hosts (digenetic), female Anopheles mosquito and human beings. It is responsible for causing malaria, in humans.

The fungi constitute a unique kingdom of heterotrophic organisms. They show a great diversity in morphology and habitat. You must have seen fungi on a moist bread and rotten fruits. The common mushroom you eat and toadstools are also fungi. White spots seen on mustard leaves are due to a parasitic fungus. Some unicellular fungi, e.g., yeast are used to make bread and beer. The science dealing with the study of fungi is called as mycology. The knowledge of fungi to mankind dates back to prehistoric times. Clausius, 1601 may be regarded as one of the earliest writers to describe fungi. Bauhin (1623) also included the account of known fungal forms in his book Pinax Theatric Botanica.

Other fungi cause diseases in plants and animals; wheat rust-causing Puccinia is an important example. Some are the source of antibiotics, e.g., Penicillium. Fungi are cosmopolitan and occur in air, water, soil and on animals and plants. They prefer to grow in warm and humid places. Have you ever wondered why we keep food in the refrigerator ? Yes, it is to prevent food from going bad due to bacterial or fungal infections. With the exception of yeasts which are unicellular, fungi are filamentous. Their bodies consist of long, slender thread-like structures called hyphae. The network of hyphae is known as mycelium. 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.

General Characteristics of Fungi:
Eukaryotic. Decomposers – the best recyclers around. No chlorophyll – non-photosynthetic. Most multicellular (hyphae) – some unicellular (yeast). Non-motile. Cell walls made of chitin (kite-in) instead of cellulose like that of a plant. Are more related to animals than the plant kingdom. Lack true roots, leaves, and stems. Absorptive heterotrophs. Digest food externally and then absorb it. Lack of chlorophyll affects the lifestyle of fungi. Not dependent on light. Can occupy dark habitats. Can grow in any direction. Can invade the interior of a substrate with absorptive filaments.

Body of fungus made of tiny filaments or tubes called hyphae. Cytoplasm and nuclei (more than 1). Each hyphae is one continuous cell. Cell wall made of chitin. A tangled mess of hyphae is called mycelium. Rhizoids are root-like parts of fungi that anchor them to the substrate. Mycelium increases the surface area of the fungi to absorb more nutrients.

Thallus Organization
The plant body of true fungi (Eumycota), the plant body is a thallus. It may be non-mycelial or mycelial. The non-mycelial forms are unicellular; however, they may form a pseudomycelium by budding. In mycelial forms, the plant body is made up of thread like structures called hyphae (sing. hypha). The mycelium may be aseptate (non-septate) or septate. When non-septate and multinucleate, the mycelium is described as coenocytic. In lower fungi the mycelium is non-septate e.g., Phycomycetae. In higher forms it is septate e.g., Ascomycotina, Basidiomycotina and Deuteromycotina. In some forms the plant body is unicelled at one stage and mycelial at the other. Their organization is sometimes described as dimorphic.

Specialised Formation
In higher forms the mycelium gets organised into loosely or compactly woven structure which looks like a tissue called plectenchyma. It is of two types:
Prosenchyma: It comprises loosely woven hyphae lying almost parallel to each other.
Pseudoparenchyma: If the hyphae are closely interwoven, looking like parenchyma in a cross-section, it is called as pseudoparenchyma. In addition to above, the fungal mycelium may form some specialized structures as under:
It’s a 'root-like' or 'string-like' elongated structure of closely packed and interwoven hyphae. The rhizomorphs may have a compact growing point.
Here the hyphae gets interwoven forming pseudoparenchyma with external hyphae becoming thickened to save the inner ones from desiccation. They persist for several years.
It is thick mattress of compact hyphae associated with the fruiting bodies.

Cell Organization
The cell wall of fungi is mainly made up of chitin and cellulose. While chitin is a polymer of N-acetyl glucosamine, the celulose is polymer of d-glucose.
Precisely, the cell wall may be made up of cellulose-glucan (Oomycetes), chitin chitosan (Zygomycetes) mannan-glucan (Ascomycotina), chitin-mannan (Basidiomycotina) or chitin-glucan (some Ascomycotina, Basidiomycotina and Deuteromycotina). Besides, the cell wall may be made up of cellulose-glycogen, cellulose-chitin or polygalactosamine-galactan.

The fungi are achlorophyllous organisms and hence they cannot prepare their food. They live as heterotrophs i.e., as parasites and saprophytes. Some forms live symbiotically with other green forms.
They obtain their food from a living host. A parasite may be obligate or facultative. The obligate parasites thrive on a living host throughout their life. The facultative parasites are in fact saprophytes which have secondarily become parasitic. While the above classification is based on the mode of nutrition, however, on the basis of their place of occurrence on the host, the parasites can be classified as ectoparasite, endoparasite and hemiendoparasite (or hemiectoparasite).

They derive their food from dead and decaying organic matter. The saprophytes may be obligate or facultative. An obligate saprophyte remains saprophytic throughout its life. On the other hand, a facultative saprophyte is infact a parasite which has secondarily become saprophytic.
Some fungal forms grow in symbiotic association with the green or blue-green algae and constitute the lichen. Here the algal component is photosynthetic and the fungal is reproductive. A few fungal forms grow in association with the roots of higher plants. This association is called as mycorrhiza. They are two types – Ectotrophicmycorrhiza and Endotrophicmycorrhiza e.g., (VAM).

The fungi may reproduce vegetatively, asexually as well as sexually:
Vegetative reproduction
Fragmentation: Some forms belonging to Ascomycotina and Basidiomycotina multiply by breakage of the mycelium.
Budding: Some unicelled forms multiply by budding. A bud arises as a papilla on the parent cell and then after its enlargement separates into a completely independent entity.
Fission: A few unicelled forms like yeasts and slime molds multiply by this process.
Oidia: In some mycelial forms the thallus breaks into its component cells. Each cell then rounds up into a structure called oidium (pl. oidia). They may germinate immediately to form the new mycelium.
Chlamydospores: Some fungi produce chlamydospores which are thick walled cells. They are intercalary in position. They are capable of forming a new plant on approach of favourable conditions.

Asexual reproduction
Sporangiospores: These are thin-walled, non-motile spores formed in a sporangium. They may be uni-or multinucleate. On account of their structure, they are also called as aplanospores.
Zoospores: They are thin-walled, motile spores formed in a zoosporangium. Example: In Pilobulus a sticky mass containing many spores is discharged as a single unit.
Conidia: In some fungi the spores are not formed inside a sporangium. They are born freely on the tips of special branches called conidiophores. The spores thus formed are called as conidia. On the basis of development, two types of conidia are recognised namely thallospores and blastospores or true conidia.

Sexual reproduction:
With the exception of Deuteromycotina (Fungi imperfecti), the sexual reproduction is found in all groups of fungi. During sexual reproduction the compatible nuclei show a specific behaviour which is responsible for the onset of three distinct mycelial phases. The three phases of nuclear behaviour are as under:
Plasmogamy : Fusion of two protoplasts.
Karyogamy : Fusion of two nuclei.
Meiosis : The reduction division. These three events are responsible for the arrival of the following three mycelial phases:
Haplophase : As a result of meiosis the haploid (n) or haplophase mycelium is formed.

Classification of Fungi
Members of phycomycetes are 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). Examples: Mucor, Rhizopus and Albugo (the parasitic fungi on mustard).

They are cosmopolitan and saprophytic fungus, living on dead organic matter. Rhizopusstolnifer occur very frequently on moist bread, hence commonly called black bread mold. Mucor is called dung mold. Both are called black mold or pin mold because of black coloured pin head like sporangia. Besides, it appears in the form of white cottony growth on moist fresh, organic matter, jams, jellies, cheese, pickles, etc.

Yeast was first described by Antony Von Leeuwenhoek in 1680. Yeast are nonmycelial or unicellular, which is very small and either spherical or oval in shape. However, under favourable conditions they grow rapidly and form false mycelium or pseudomycelium. Individual cells are colourless but the colonies may appear white, red, brown, creamy or yellow: The single cell is about 10mm in diameter. It is enclosed in a delicate membrane which is not made up of fungal cellulose but is a mixture of two polysaccharides known as mannan and glycogen.

Reproduction: Yeast reproduces by vegetative or asexual and sexual methods.
Vegetative reproduction:
'Yeast reproduce vegetatively either by budding or by fission.
Sexual reproduction
Sexual reproduction in yeasts takes place during unfavourable conditions, particularly when there is less amount of food.The sex organs are not formed in yeasts 'and the sexual fusion occurs between the two haploid vegetative cells or two ascospores which behave as gametes. The two fusing gametes are haploid and may be isogamous or anisogamous. Such kind of sexual reproduction is called gametic copulation. It is the best example of hologamy i.e., the entire vegetative thallus is transformed into reproductive body. The sexual fusion leads to the formation of diploid zygote. The zygote behaves as an ascus and forms 4 - 8 haploid ascospores. These liberate and function as vegetative cells.