■|Introduction
- Identification of differences among organisms and placing them into groups that reflect their most significant features and relationship is called biological classification.
- Since the dawn of civilisation, there have been many attempts to classify living organisms.
- Biological classification was first proposed by Aristotle who used simple morphological characteristics to classify plants into trees, shrubs and herbs. He also classified animals into two groups, those which had red blood and those that did not.
🔑 NOTE
□The purpose of biological classification is to organise the vast number of known plants into categories that could be studied easily.
■|Systems of Biological Classification
- Systems of classification are categorised into three types:
□1. Artificial systems of classification
□2. Natural systems of Classification
□3. Phylogenetic Systems of Classification
●1. Artificial system of classification
- Based on few morphologicial characters for grouping of organisms.
- Theophrastus was the first who proposed artificial system of classification on the basis of habit. He classified plants into four groups namely herbs, under shrubs, shrubs and trees.
- Carolus Linnaeus (father of taxonomy) proposed artificial system of classification on the basis of few sexual characters like number of stamens and carpel.
□Drawbacks of Artificial Classification System:
(i) Natural or phylogenetic relationships were not
considered.
(ii) Only few characters are used in this system, therefore, diverse animals and plants were placed into limited number of groups.
●2. Natural systems of Classification
- This system is based on natural affinities among organisms. In this, organisms are classified on the basis of mainly morphological, reproductive, anatomical, cytological, physiological, embryological features, etc.
- John Ray was the first to propose a natural system of classification.
- George Bentham and J.D. Hooker proposed a natural system of classification of angiosperms that was published in ‘Genera Plantarum’.
- They classified plant kingdom into two subkingdoms: Cryptogamia and Phanerogamia. The phanerogamia classified into three classes–dicotyledons, gymnosperms and monocotyledons.
□Demerits of Natural Classification System
(i) They did not use phylogenetic trends in their
classification.
(ii) Gymnosperms placed between dicots and monocots that is not acceptable.
3. Phylogenetic Systems of Classification :
- This system is based on phylogenetic, i.e., evolutionary relationships of organisms. The graphic representation of evolutionary relationships is called family tree or cladogram.
■|Kingdom Classification Of Living Organisms
1. Two kingdom classification system
- Linnaeus introduced two kingdom classification. He classified all organisms into two kingdoms –Kingdom plantae and kingdom Animalia.
- Kingdom Plantae involves autotrophic, fixed organisms while kingdom Animalia includes motile, heterotrophic organisms.
□Limitations of Two-Kingdom System
(i) Unicellular and multicellular forms have been placed in both the kingdoms though they have different organisation.
(ii) Bacteria and cyanobacteria are included under plants but they are prokaryotes.
(iii) Lichens are also included in plants but they are symbiotic organisms.
(iv) Fungi are included under plantae but they are non-
photosynthetic and heterotrophic.
2. Three kingdom classification system
- It includes three kingdoms namely Protista, Plantae and Animalia.
- Ernst Haeckel proposed and classified all one-celled eukaryotes into separate kingdom protista. Kingdom
protista includes algae, slime moulds, protozoans, fungi and bacteria.
□Limitations of Three-kingdom system
(i) Bacteria and fungi were placed with algae and protozoan.
(ii) In this kingdom, acellular and multicellular organisms were kept together.
■3. Four kingdom classification system
- It was proposed by Copeland (1956). He established a new kingdom Monera for all acellular prokaryotes containing incipient nucleus like bacteria, blue green algae.
- The four kingdoms were monera, protista, plantae (metaphyta) and Animalia
Fig.: Representation of four kingdom classification
□Limitatiion of four Kingdom System:
Protozoans, red and brown algae and fungi were placed in protista that is not acceptable.
■4. Five kingdom classification system
- It was proposed by R.H. Whittaker (1969).
The five kingdoms defined by him were:
(i) Monera
(ii) Protista
(iii) Fungi
(iv) Plantae
(v) Animalia
The main criteria for classification used by him includes:
(i) Cell structure
(ii) Thallus organisation
(iii) Mode of nutrition
(iv) Reproduction
(v) Phylogenetic relationships
□Merits of five kingdom classification
(i) It exhibits phylogenetic relationship between diverse groups of organisms.
(ii) Prokaryotes are classified in a separate kingdom
according to their genetic, cellular, reproductive and
physiological differences.
(iii) Fungi was grouped under a separate kingdom and protozoa was excluded from kingdom animalia.
□Limitations of five kingdom classification
(i) Monera and Protista include diverse life forms because both include photosynthetic and heterotrophic organisms and also have cells with and without cell wall.
(ii) Viruses and viroids have not given proper place in this classification system.
■|Kingdom Monera
- All prokaryotic organisms were grouped together under Kingdom Monera.
- Bacteria are the sole members of the Kingdom Monera. They are the most abundant micro-organisms.
●General characteristics
- Cell wall is made up of peptidoglycan except archaebacteria and mycoplasma.
- Genetic material is naked DNA known as nucleoid.
Histones, nuclear envelope, nucleoplasm, nucleolus and chromatin are absent.
- All membrane bound organelles are absent.
- Contains 70S type of ribosomes.
- Since mitochondria is absent, therefore respiratory enzymes are associated with plasma membrane.
Table: Characteristics of the five kingdoms
-Nutrition is autotrophic (chemosynthetic and photosynthetic) and heterotrophic (saprophytic/parasitic).
- Reproduction takes place by asexual methods only.
Habitat.
- They show cosmopolitan distribution.
- They found everywhere, i.e., in air, water, soil and in plants and animals.
- 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.
□Types of bacteria
◇1. On the bassis of their shape:
(i) Coccus (pl: Cocci)
– Spherical shaped, e.g., Streptococcus lactis
(ii) Bacillus (pl: Bacilli)
– Rod shaped, e.g., E. coli, Lactobacillus
(iii) Spirillum (pl: Spirilla)
– Spiral shaped bacteria, e.g., Spirillum volutans, Spirochete.
(iv) Vibrio (pl: vibruim)
– Comma shaped, e.g., Vibrio cholerae
(v) Pleomorphic bacteria
– Variable in shape, e.g., Rhizobium.
Fig.: Bacteria of different shapes
◇2. On the basis of nutrition:
i. Autotrophs
These bacteria use light or chemical energy for their own food synthesis. On the basis of source of energy autotrophs are of following two types:
(A) Photosynthetic autotrophs
●They use light energy for food synthesis
(B) Chemosynthetic autotrophs
● They use chemical energy instead of light energy for food
synthesis.
● Chemical energy is obtained from oxidation of inorganic
or organic compounds.
Fig.: Classification of bacteria on the basis of nutrition
ii. Heterotrophs
● Most of the bacteria are heterotrophic i.e., they can not manufacture their own food.
● They receive their own food from dead organic matter or living organism.
These are of following types:
◇(A) Saprotrophic bacteria: These bacteria obtain food from dead and decaying organic matter.
These are of two types:-
(a) Obligate saprotrophic: These bacteria obtain food
only from dead organic matter. These are completely
saprotrophs, e.g., Bacillus vulgaris, Clostridium
botulinum.
(b) Facultative parasite: These are normally
saprophytic in nature, but in the absence of dead
organic matter they can become parasitic. e.g.,
Pseudomonas, Staphylococcus.
◇(B) Parasitic bacteria: They obtain their food from living organisms (host).
These are of two types:
(a) Obligate parasite: They always remain parasitic.
e.g., Mycobacterium leprae.
(b) Facultative Saprotrophic: They are normally
parasitic in nature but in the absence of living host,
they may become saprotrophs, e.g., Mycobacterium
tuberculosis.
(C) Symbiotic bacteria
These bacteria lives in mutual relationship with other living organisms & convert atmospheric nitrogen into nitrogenous compounds like amino acid, NO3
or salts of ammonia. E.g., Rhizobium.
■Domains Of Kingdom Monera
(1) Archaebacteria
- Most primitive form of life.
- Found in most extreme environmental conditions like high salt concentration, high temperature, etc. .
- Cell wall is composed of non-cellulosic polysaccharides (also known as pseudomurein). Plasma membrane has long chain branched lipids (phytanols) which decrease membrane fluidity and help to increase tolerance against extremes of
heat, low pH.
□Types of Archaebacteria
(i) Methanogens
- Obligate anaerobes, found in marshy habitats, swamps, ruminants, sewage treatment plants.
- They are present in the gut of several ruminant animals such as cows and buffaloes and thus are responsible for the production of methane (biogas) from the dung of these animals. E.g., Methanogenium, Methanosarcina, Methanobacterium, etc.
(ii) Halophiles
- Facultative anaerobes, found in salt lakes, dead sea and industrial plants.
- Consists of a reddish pigment bacteriorhodopsin in their membrane to trap sunlight and form ATP directly.
- They survive in salty water due to presence of branched chain lipids in their cell membrane. E.g., Halobacterium, Halococcus.
(iii) Thermoacidophiles
- Facultative anaerobe, found in hot water springs at high temperature and low pH.
- They are able to tolerate high temperature due to homopolar bonds in their proteins.E.g.,Thermoplasma, Sulfolobus.
(2) Eubacteria
- There are thousands of different eubacteria or ‘true bacteria’.
- They are characterised by the presence of a rigid cell wall, and if motile, a flagellum.
(3) Cyanobacteria
- The cyanobacteria (also referred to as blue-green algae) have chlorophyll a similar to green plants and are photosynthetic autotrophs.
- The cyanobacteria are unicellular, colonial or filamentous, fresh water/marine or terrestrial algae. The colonies are generally surrounded by gelatinous sheath.
- They often form blooms in polluted water bodies. Some of these organisms can fix atmospheric nitrogen in specialised cells called heterocysts, e.g., Nostoc and Anabaena.
- Reserve food material is mainly cyanophycean starch.
- They were first to evolve O2 in photosynthesis, so they are called as ‘oxygenic photosynthetic’ organisms.
Fig.: A filamentous blue-green algae – Nostoc
□Economic importance of cyanobacteria
- They provide fertility to soil by nitrogen fixation, e.g., Nostoc, Anabaena.
- They are the source of proteinaceous food for animals, e.g., Spirulina.
- Blue green algae (BGA) secretes an acidic chemical which decreases the alkalinity of soil, e.g., Nostoc commune, Scytonema ocellatum, Aulosira fertilissima.
- Some BGA are used as green manure, e.g., Anabaena, Spirulina.
- Some BGA secrete toxin, which inhibits the growth of
mosquito larva in water, eg., Oscillatoria, Anabaena,
Aulosira.
🔑 NOTE
●Water bloom (excessive growth of algae in water): BGA grow rapidly in water and secrete toxic substances. These toxic substances is the main cause of death in aquatic animals. It also gives toxicity and bad odour to the water.
E.g., Oscillatoria.
4. Mycoplasma
- Mycoplasma are organisms that completely lack a cell wall.
- They are the smallest living cells known and can survive without oxygen.
- Many mycoplasma are pathogenic in animals and plants.Gram Staining Technique
- Hans Christian Gram (1884) developed this technique to differentiate between Gram positive and Gram negative bacteria.
◇Steps:
(i) Bacteria are stained by weak alkaline solution of crystal violet (Gram stain) resulting the former taken up blue colour.
(ii) They are then treated with 0.5% iodine solution.
(iii) After staining, bacteria are washed with acetone or ethyl alcohol. After washing, some bacteria retain the stain and some bacteria are decolourised.
(iv) Bacteria which retain stain (violet or purple) are called Gram positive bacteria, e.g., Bacillus subtilis and bacteria which decolourises are known as Gram negative, e.g., E.coli. Gram negative bacteria are counter stained by safranin.
Table: Difference between Gram positive and Gram negative bacteria
■Reproduction
Bacteria perform reproduction by the following methods:
(i) By Binary Fission:
- It takes place during favourable conditions.
Fig.: Dividing Bacterium
(ii) By endospore:
- It takes place in adverse conditions. Endospore is thick walled, highly resistant and surrounded by four layers.
(a) Exosporium: Outermost lipoproteinaceous layer.
(b) Spore coat: Disulfide rich protein, impervious, tough and, resistant.
(c) Cortex: Very thick and composed of peptidoglycan.
(d) Core wall or spore wall : Thick delicate proteinaceous innermost layer.
🔑 NOTE
-The endospore contains anticoagulant dipicolinic acid (DPA) and calcium in the form of calcium dipicolinatewhich provides high temperature resistant property.
◇Economic Importance Of Bacteria
- Saprophytic bacteria: These are major decomposers or mineralisers of earth for regulating biogeochemical cycles.
- Ammonifying bacteria: They convert nitrogeneous
compounds / proteins of dead plants and animals or their excretory products into ammonia. E.g., Bacillus ramosus,B. mycoides, B. vulgaricus.
- Nitrifying bacteria: They convert ammonia into nitrates.
- Symbiotic nitrogen fixers: E.g., Rhizobium in root nodules of leguminous plants; Frankia in root nodules of Casuarina, Alnus; Nostoc in Trifolium alexandrium;etc.
- Retting of fibres: Some bacteria dissolve pectin of middle lamella to separate individual fibres of flax, jute, hemp. E.g., Clostridium perfringens, Pseudomonas fluorescence.
- Production of vinegar: E.g., Acetobacter aceti.
- Single cell protein (SCP): Dried biomass of microorganism obtained after culturing, harvesting and drying is called SCP. The latter is used as protein source in animal feeds and food supplement of man.
E.g., Methylophilus methylotropus, Rhodopseudomonas capsulata.
- Production of Lactic acid: It is performed by the activity of various lactic acid bacteria (LAB)
- Antibiotics: Many types of antibiotics are obtained by eubacteria and actinomycetes.
- Vitamins: E.g., Clostridium butylicum is used to prepare riboflavin.
- Enzymes: Many enzymes are extracted commercially by bacterial activities. E.g., streptokinase from Streptococcus pyrogens, protease from Bacillus subtilis, pectinase from Clostridium perfringens.
◇Harmful Activities
- Food poisoning : It occurs due to toxins produced by some bacteria in food. The eating of such toxic food may cause even death. E.g., Clostridium botulinum, Streptococcus.
- Spoilage of food: Spoilage of curd by Clostridium; spoilage of milk or milk products by Lactobacillus; spoilage of protein rich food by Pseudomonas; meat by Salmonella and Lactobacillus; rotting of egg by Proteus, Pseudomonas, Leuconostoc; souring of wine by Acetobacter aceti.
- Denitrification : Some bacteria convert nitrates and ammonia into nitrogen. E.g., Thiobacillus denitrificans, Pseudomonas denitrificans.
- Disease: Many bacteria can cause disease.
■|Kingdom Protista
- Ernst Heackel coined the term protista.
●General Characters
- All the organism included in Protista are unicellular
eukaryotes.
- They bear well-defined nucleus. Protists can be uninucleate, binucleate or multinucleate.
- Locomotion takes place by flagella or cilia, pseudopodia, contractions and mucilage extrusion.
- Mode of nutrition is of various types – holophytic
(photosynthetic), ingestive (holozoic), absorptive (parasitic, saprophytic).
- The common mode of reproduction is asexual reproduction. It involves binary fission, budding, plasmotomy, sporulation, cyst formation, etc.
- Sexual reproduction involves meiosis and karyogamy. Meiosis is zygotic in some forms and gametic in others.
◇Domains of Kingdom Protista:
●Kingdom protista include Chrysophytes, Dinoflagellates, Euglenoids, Slime moulds and Protozoans.
(1) Chrysophytes
- 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.
- The cell walls of diatoms are made up of silica embedded at many places due to which cell walls are indestructible. The cell wall form two thin overlapping shells, which fit together as in a soap box.
🔑 NOTE
● Due to a transparent siliceous shell (silica deposited in cellulosic cell wall), the cell wall of diatoms is hard. It does not get destroyed after their death and deposited at the bottom of sea. This accumulation occurs over billions of years and is referred to as diatomaceous earth or diatomite.
● The reserved food is oil and chrysolaminarin or leucosin (a polysaccharide).
◇Reproduction
(i) Mainly asexual: By binary fission.
(ii) Sexual reproduction: Very rare - by gametic meiosis.
◇Economic importance :
- Diatomaceous earth or diatomite is employed as an insulation material in refrigerators, boilers and furnaces.
- Diatomite is also used as cleaning agent in tooth pastes and metal polishes.
- Diatomite is porous and chemically inert. Therefore, it is used in filtration of sugar, alcohols and antibiotics.
- Diatoms are important producer in the form of
phytoplanktons and they are major sources of food to aquatic animals.
(2) Dinoflagellates
- Mostly marine and photosynthetic organisms.
- Dinoflagellates are yellow-brown or golden brown in colour. These colour of dinoflagellates is due to the pigments present in them - chlorophyll a, chlorophyll c and xanthophylls (dinoxanthin & didinoxanthin).
- Dinoflagellates have two flagella - one is transverse and other is longitudinal. Dinoflagellates shows peculiar spinning movement, therefore they are called as “whorling whips”.
- They have starch and oil as stored food.
🔑 NOTE
◇Very often, red dinoflagellates 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. E.g., Gonyaulax.
●Reproduction
(i) Mainly Asexual – Binary fission.
(ii) Sexual reproduction is very rare in them. Dinoflagellates are haploid so they reproduce by zygotic meiosis.
Fig.: Some dinoflagellates – (a) Peridinium (b) Gymnodinium(c) Gonyaulax
(3) Euglenoids
- Previously euglenoids were placed in plant kingdom due to their photosynthetic ability. But due to the absence of cell wall and animals like nutrition some scientists placed them in animal kingdom. But now according to five kingdom classification, they are included in Protista.
- They are unicellular protists found in fresh water habitats and damp soils.
- Their cell membrane is covered with pellicle. Pellicle is made up of lipoprotein and it is elastic in nature.
- They have a contractile vacuole. These contractile vacuoles helps in osmoregulation. Though they are photosynthetic in the presence of sunlight, when deprived of sunlight they behave like heterotrophs by predating on other smaller organisms.
-On the basis of nutrition, they are known as mixotrophic. E.g., Euglena (shows both holophytic and holozoic nutrition).
- The reserve food is paramylum as paramylum granules that is stored in the cytoplasm instead of chloroplasts.
- They performs multiplication by longtiudinal binary fission in favourable conditions and by cyst formation in unfavourable conditions.
🔑 NOTE
■Euglena is considered as a connecting link between plant kingdom and animal kingdom because it shows features of both plants and animals.
Table: Plant-like and animal-like characteristics of Euglena
(4) Slime Moulds
- These organisms develop a slimy mass at the time of their vegetative phase therefore they are called 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.
- The reserve food is glycogen and oil.
- On the basis of structure, they are of two types: acellular slime mould and cellular slime mould.
◇Reproduction
Slime moulds have both asexual & sexual type of reproduction.
🔑 NOTE
-The cellular slime moulds bear features of both plants and animals.
Animal-like features: Absence of cell wall in vegetative phase and feeding like Amoeba
Plant-like feature: the reproductive phase is plant-like as the spores have a cell wall composed of cellulose.
(5) Protozoans
- 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.
(i) Amoeboid Protozoans
● Locomotory organ – Pseudopodia (one or two or many).
● Habit & Habitat – Fresh water, sea water (marine) or moist soil.
● Food capturing – Prey is captured by putting out
pseudopoida (false feet)
● Marine forms have silica shells on their surface.
E.g., Amoeba, Entamoeba (act as parasite)
(ii) Flagellated Protozoan :
● Locomotory organ – Flagella.
● Habit and Habitat – Free living or parasitic, some are aquaitic.
● The parasitic forms cause diaseases such as sleeping sickness.
E.g., Trypanosoma, Leishmania, Giardia, etc.
(iii) Cilliated Protozoan:
● Locomotory organ – Cilia and the co-ordinated movement of rows of cilia causes the water laden with food to be steered into the gullet a cavity that opens to the outside of the cell surface.
● Habit and Habitat – Aquatic, free living, parasitic and commensal.
● Food Capturing – Gullet with outer cystostome opening for food ingestion while cytopyge for egestion. Food digestion in food vacuole in free living forms.
E.g., Paramoecium, Opalina.
(iv) Sporozoans
● Locomomotory organ – absent.
● Habit and Habitat – They all are parasite, mostly
intracellular parasite that have an infectious spore like stage in their life cycle. Eg. Plasmodium, Monocystis.
🔑 NOTE
-The most notorious sporozoan is Plasmodium (malarial parasite) which causes malaria, a disease which has a staggering effect on human population.
■|Kingdom Fungi (Mycota)
- The fungi constitute a unique kingdom of heterotrophic organisms. They show a great diversity in morphology and habitat.
- Fungi are cosmopolitan & moisture allows them to grow on leather, wood, pickle and bread. Some fungi live parasitically in plants, animals and human body.
-They lack chloroplast, so fungi are heterotrophs. Fungi obtain their own food from dead organic matter or living organisms.
On the basis of source of food, fungi are of two types:
(i) Saprophytic
● They obtain their own food from dead organic matter such as bread, rottening fruit, vegetable and dung.
● Nutrition is absorptive type.
(ii) Parasitic:
● They obtain their own food from living organism such as plants, animals and human beings.
● They obtain nutrition with the help of haustoria.
● Some fungi are found symbiotically associated with algae and form lichens and with the roots of higher plants to form mycorrhiza.
● 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.
● Cell wall is made up of chitin & polysaccharides. Some quantity of proteins, lipids and cellulose also present.
🔑 NOTE
◇ Cell wall of the members of class-oomycetes is mainly made up of cellulosic compounds & glycans.
◇ In fungi the stored food remains in the form of glycogen and oil.
●Reproduction in fungi
Reproduction takes place is of three different types;
(i) Vegetative Reproduction
(a) Fragmentation: If due to some reason, the fungal
filament, i.e., mycelium, breaks into small pieces then these pieces form a new fungal filament.
(b) Budding: Sometimes a bud like protuberance is formed in non–mycelial fungus which gets separated from the mother fungi and functions as young fungi.
At the time of separation of bud from its mother cell or fungi, the nucleus of mother cell divide mitotically (or amitotically
- in yeast) into two parts. Out of these two nuclei, one
remains within the mother cell while the other migrates to the bud. E.g., Saccharomyces.
(c) Fission: Sometimes the fungal cell divides into two parts. The division of nucleus also occurs.
E.g., Schizosaccharomyces (Yeast).
(ii) Asexual Reproduction
Asexual reproduction takes place by the formation of different types of spores.
(a) Sporangiospores: Sporangiospores are the asexual spores that are produced in an enclosed, sac like structure, called sporangium, at the end of the sporangiophores. The formation of sporangiospores takes place endogenously.
Sporangiospores are of two types:
- Zoospore: When the sporangiospores formed in sporangia are flagellated and motile, then they are called as zoospores. In this condition the sporangia are called as zoosporangia.
- Aplanospore: When sporangiospores are non flagellated and non motile then they are called aplanospores. Each conidium forms fungal filament (mycelium) by germination.
(b) Conidia: Conidia are asexual non-motile spores formed at the tip of conidiophores. The formation of conidia takes place exogenously. Each conidium forms fungal filament (mycelium) by germination.
(iii) Sexual Reproduction
- Sexual reproduction takes place by oospores, ascospores and basidiospores.
- The various spores are produced in distinct structures called fruiting bodies.
- The structure in which gametes are formed are called gametangia.
The sexual cycle involves the following three steps:
(i) Fusion of protoplasms between two motile or non-motile gametes called plasmogamy.
(ii) Fusion of two nuclei called karyogamy.
(iii) Meiosis in zygote resulting in haploid spores.
In some fungi the fusion of two haploid cells immediately results in diploid cells (2n). -However, 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.
●Classification of fungi
Fungi is divided into following classes on the basis of the morphology of the mycelium, mode of spore formation and fruiting bodies.
(1) Phycomycetes
- Found in aquatic habitats and on decaying wood in moist and damp places or as obligate parasites on plants.
- The mycelium is multinucleate (coenocytic) and aseptate.
-Asexual reproduction takes place by zoospores (motile) or by aplanospores (non-motile). These spores are endogenously produced in sporangium. A zygospore is 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.)
🔑 NOTE
◇ Coprophilous fungi: The fungi which grow on dung
are called coprophilous.
◇ Rhizopus, known as bread mold, prefer to grow on bread. The tip of mycelium of Rhizopus is black coloured.
Fig.: Agaricus
●Economic importance
- Spoilage of food: Exposed bread and other food particles are spoiled by Rhizopus and Mucor species.
- Soft rot: Rhizopus species attack sweet potato, apple and strawberry producing soft rot or leak disease. Germinating maize grains are also attacked.
- Mucormyucosis: Mucor pusillus and M. ramosissimus may attack internal human organs, including lungs, alilmentary canal and nervous system.
- Fermented foods: Tempeh (a solid food from soyabean) and sufu (Chinese cheese) are prepared with the help of Rhizopus and Mucor respectively.
- Chemicals: Citric acid prepared by Mucor from molasses, fumaric acid and cortisone by Rhizopus stolonifer, lactic acid by R. stolonifer and R.nodosus and alcohol by R. oryzae and M. javanicus.
- Antibiotic: Ramysin is produced by Mucor ramannianus.
- Waste water treatment: Growth of Mucor arrhizus removes heavy metal contamination of water.
(2) Ascomycetes (sac fungi)
- Commonly known as sac-fungi, the ascomycetes are mostly multicellular, e.g., Penicillium, or rarely unicellular, e.g., yeast (Saccharomyces).
- They are saprophytic, decomposers, parasitic or coprophilous (growing on dung).
- The mycelium is sepatate & branched except yeast which is unicellular.
● The cell wall contains chitin
● Asexual reproduction takes place by asexual spores
conidia which are produced exogenously on the special mycelium called conidiophores.
● Sexual reproduction occurs by ascospores (sexual spores) which are produced endogenously in sac like asci. These asci are arranged in different types of fruiting bodies called ascocarps.
■Economic importance:
- Aspergillus flavus (formerly called guinea Pig of plant Kingdom) produces a carcinogenic toxin called aflatoxin.
- Aspergillus niger produces citric acid and oxalic acid.
- Claviceps: Plant diseases such as Ergot of rye is caused due to Claviceps purpurea while ergot of bajra is caused due to C. microcephalla.
- Neurospora (Pink bread mould): Neurospora crassa
(Drosophila of Plant kingdom) is extensively used in
biochemical & genetic work.
- Morels and truffles: They are edible and are considered delicacies.
- A. Fleming discovered antibiotic penicillin from Penicillium notatum.
- Ripening of Camembert and Roquefort cheese is performed by P. camemberti and P. roqueforti respectively.
- Brewing Industry: Saccharomyces cerevisiae and
Saccharomyces ellipsoidens perform alcoholic fermentation.
(3) Basidiomycetes (club fungi)
- Commonly known forms of basidiomycetes are mushrooms, bracket fungi or puffballs.
- These are most advanced fungi and best decomposers of wood. Their fruiting bodies are large and visible such as mushrooms, puff ball, toad stools.
- They grow in soil, on logs and tree stumps and in living plant bodies as parasites, e.g., rusts and smuts.
- Mycelium is septate and branched. Septa has dolipore except in rusts and smuts.
●Mycelium is of two types:
(i) Primary mycelium: Monokaryotic, short-lived, haploid and formed by basidiospores.
(ii) Secondary mycelium: Dikaryotic, long-lived and
containing binucleated cells (n + n) formed from primary mycelium by somatogamy.
■Need to know
Secondary mycelium can perennate in the soil or wood as sclerotia (rounded or ellipsoid form masses of hyphae) or rhizomorph (root-like aggregation of hyphae).
- The asexual spores are generally not found, but vegetative reproduction by fragmentation is common.
- The sex organs are absent, but 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.
■Need to know
Clamp connection: It is a tubular relationship between two neighbouring cells. With the help of this connection the nucleus of one cell can migrate to the neighbouring cell, due to which the other cell becomes dikaryotic (binucleate). Clamp connection is used to change monokaryotic mycelium to dikaryotic mycelium in basidiomycetes.
(4) Deuteromycetes (fungi imperfecti)
- Commonly known as fungi imperfecti because the sexual stage is either absent or not reported.
- It is considered as artificial group without any common relationship. These are actually sac fungi that have lost the ability to reproduce sexually.
- Mycelium is septate and branched.
- They reproduce only by asexual spores called conidia formed on conidiophores.
- 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.
■Need to know
●Terms
Irish Famine: Phytophthora infestans causes Late Blight of potato.
◇Bengal Famine: Helminthosporium oryzae is responsible for sesame or brown leaf spot of rice.
◇ Autoecious: A fungus that requires only one host to complete its life cycle is called autoecious fungus. E.g.,Albugo, Hemileia.
◇Heteroecious: A fungus that requires two hosts to complete its life cycle is called heteroecious fungus. E.g., Puccinia.
■|Kingdom Plantae
- Kingdom Plantae includes all eukaryotic chlorophyll-
containing organisms commonly called plants.
- A few members are partially heterotrophic such as the insectivorous plants or parasites. Bladderwort and Venus fly trap are examples of insectivorous plants and Cuscuta is a parasite.
- The plant cells have prominent chloroplasts and surrounded by a cell wall (mainly made of cellulose).
- Plantae includes algae, bryophytes, pteridophytes,
gymnosperms and angiosperms.
🔑 NOTE
In modern system of classification like Whittaker (1969), fungi, lichens and bacteria are excluded from this group and are placed in separate kingdoms.
- August Wilhelm Eichler (1883), a Viennese botanist, divided plant kingdom into two sub-kingdoms mainly on the basis of presence or absense of seeds.
● Cryptogamae are lower plants in which sex organs are hidden and seeds and flowers are absent. It includes thallophytes, bryophytes, pteridophytes.
● Phanerogamae are higher plants in which sex organs are evident; seeds present. It includes gymnosperms and angiosperms.
- Alternation of generation: Life cycle of plants has two distinct phases – the diploid sporophytic and the haploid gametophytic – that alternate with each other. The lengths of the haploid and diploid phases, and whether these phases are free– living or dependent on others, vary among different groups in plants.
■|Kingdom Animalia
- This kingdom is characterised by heterotrophic eukaryotic organisms that are multicellular and their cells lack cell walls.
- They directly or indirectly 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 that have a definite shape and size.
- Higher forms show elaborate sensory and neuromotor mechanism. Most of them are capable of locomotion.
🔑 NOTE
◇In five kingdom classification of Whittaker there is no mention of lichens and some acellular organisms like viruses, viroids and prions. These are briefly introduced here.
●|Viruses, Viroids And Lichens
- The term ‘virus’ has been derived from Latin, which means poison or venom or viscous fluid.
- The viruses are non-cellular organisms that are characterised 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.
- D.J. Ivanowsky (1892) recognised certain microbes as casual 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.
- In addition to proteins, viruses also contain genetic material, that could be either RNA or 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.
Fig.: Bacteriophage
- The protein coat called capsid made of small subunits called capsomeres, protects the nucleic acid. These capsomeres arranged in helical or polyhedral geometric forms possess antigenic properties.
- 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.
Fig.: Tobacco Mosaic Virus (TMV)
- Viroids : In 1971, T.O. Diener discovered a new infectious agent that 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. They cause persistent infections.
- Lichen: A lichen is structurally organised entity consisting of the permanent association of a fungus and an alga. 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.
● Lichens reproduce both by asexual and sexual methods.
● Lichens are very sensitive to SO2 and grow only in SO2free atmosphere. Therefore lichens are very good pollution indicators as they do not grow in polluted areas.
