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Biological classification refers to the scientific procedure in which living organisms are classified and arranged into groups ndfelement and sub-groups in a hierarchial manner on the basis of their similarities and dissimilarities.
⚫ Aristotle was the earliest to attempt a scientific basis for classification. Later Linnaeus gave the two kingdom system of classification with Plantae and Animalia kingdoms.
This system though used till very recently, but was unable to distinguish between the eukaryotes and prokaryotes, unicellular
and multicellular organisms and photosynthetic (plants) and non-photosynthetic organisms (fungi). In 1969, RH Whittaker proposed a five kingdom system of classification. He divided all living organisms into Monera,
Protista, Fungi, Plantae and Animalia as summarised in table below.
Earlier classification systems considered bacteria, BGA (Blue-Green Algae), fungi, mosses, fems, gymnosperms and angiosperms as plants due to the presence of cell wall in them. This classification system placed prokaryotic bacteria and BGA with other eukaryotic groups.
. It also grouped unicellular and multicellular, organisms together, eg. Chlamydomonas and Spirogyra (in algae). This system did not consider the differences in mode of nutrition and cell wall composition, so grouped fungi (heterotroph,chitinous cell wall) with plants (autotroph, cellulosic cell wall).
• Five kingdom classification considered such characteristics and segregated prokaryotic organism under Monera, uricellular eukaryotes in Protista (this united Chlamydomonas and Chlorella with Paramecium and Amoeba earlier placed in plants and animals, respectively)
Kingdom-Monera
⚫ Bacteria are the sole members of this kingdom.
⚫ Bacteria are the most abundant microorganisms occurring in air, water, soil as well as in extreme habitats like deserts, snow, hot springs, etc.
⚫ Bacteria have been grouped under four categories based on
their shape
• Coccus (cocci). - Spherical
Bacillus (bacilli). - Rod-shaped
Vibrium (vibrio) - Comma-shaped
Spirillum (spirilla). - Spiral-shaped.
Bacteria show a wide range of mode of nutrition. They may be autotrophic (synthesise their own food from inorganic substrates), chemotrophic (photosynthetic autotrophic), saprophytic or heterotrophic (depend on other organisms for food).
⚫ Bacteria are further divided into Archaebacteria and Eubacteria.
Archaebacteria
• Archaebacteria live in extreme environmental conditions.
• These include
• Halophiles Bacteria residing in salty areas.
⚫ Thermoacidophiles Bacteria residing in hot springs.
⚫ Methanogens Bacteria which survive in marshy area (these are present in gut of many ruminant animals like cows and buffaloes).
Archaebacteria differ from other bacteria in having different cell wall structure. Their cell wall is made up of murein and contains high amount of unsaturated fatty acids, which is responsible for ensuring their survival in extreme conditions.
Eubacteria
• Another class-Eubacteria is also known as 'true bacteria". ⚫ These have rigid cell wall made up of peptidoglycan.
They could be photosynthetic autotrophs, chemosynthetic, autotrophs and heterotrophic bacteria.
Photosynthetic autotrophs include blue-green algae, which have chlorophyll-a similar to green plants. Also known as cyanobacteria.
⚫ These could be unicellular, colonial or filamentous, freshwater/marine or terrestrial algae.
. Some bacteria can fix atmospheric nitrogen in specialised cells known as heterocyst, e.g. in Nostoc and Anabaena.
⚫ Some bacteria utilise inorganic substances like nitrate, nitrite, ammonia, etc., for oxidation and release of energy for ATP production. These are known as chemosynthetic autotrophic bacteria.
• Heterotrophic bacteria (most abundant in nature) are dependent on other organisms for nutrition. These include N₂-fixing bacteria, pathogens, etc.
⚫ These reproduce asexually by binary fission. . During unfavourable conditions, these form spores.
⚫ These also show conjugation, a type of sexual reproduction in which DNA is transferred from one bacteria to another through a conjugal tube.
Pleomorphic bacteria, which lack cell wall is known as mycoplasma. They are pathogenic and the smallest microorganism known.
Kingdom-Protista
All single-celled eukaryotes are placed under Protista.
• Members of kingdom-Protista are the connecting link
between prokaryotic monerans and complex multicellular kingdoms-Fungi, Plantae and Animalia. These include chrysophytes, dinoflagellates, euglenoids,
slime moulds and protozoans.
⚫ These show a well-defined nucleus and membrane bound organelles.
⚫ They reproduce asexually and sexually by a process involving cell fusion and zygote formation.
⚫ Kingdom-Protista has been further divided into the
following groups
• Chrysophytes include diatoms and golden algae known as desmids. They are found in marine environment.
The cell wall of diatoms is embedded with silica and forms two thin overlapping sheath as in soap box.
■ Diatomaceous earth is the large amount of cell wall deposits of diatoms in their habitat. These are used in polishing, filtration of oils and syrups.
⚫ Dinoflagellates are marine and photosynthetic microorganisms.
⚫ Due to the presence of different pigments, they appear yellow, green, brown and red. As the name suggest they have two flagella one lies longitudinally and other transversely in furrow between wall plates.
• Gonyaulax is a red dinoflagellate, which undergoes rapid multiplication and forms red tides. Toxins released by these microorganisms when present in such large numbers may even kill other marine animals such as fishes.
• Euglenoids are freshwater organism found in stagnant water.
⚫ Cell wall is absent, a protein rich layer called pellicle is present over the surface. In the presence of sunlight, they behave as autotrophs, while in its absence they behave as heterotrophs, e.g. Euglena.
⚫ Slime moulds are saprophyte, which are dependent on dead and decaying organic matter.
They form an aggregation called plasmodium. . During unfavourable conditions, they form spores,
which are highly resistant.
• Protozoans are heterotrophs and live as parasites or predators. These are grouped into
Amoeboid protozoans found in fresh or marine water or moist soil. They have pseudopodia (false feet) to capture prey as in Amoeba.
. Flagellated protozoans either free-living or parasitic having flagella. The parasitic forms cause diseases, e.g.
sleeping sickness by Trypanosoma. . Ciliated protozoans are aquatic, actively moving organisms due to thousands of cilia present on them. The coordinated ciliary movement drives food into cavity called gullet, e.g. Paramecium.
⚫ Sporozoans are non-motile forms with an infectious spore like stage in their life cycle, e.g. malaria causing parasite
Plasmodium.
Kingdom-Fungi
⚫ These are heterotrophic organisms with their cell wall made up of chitin.
⚫ These have cosmopolitan distribution and are found in warm and humid places.
⚫ Fungal body consists of long, thread-like structures called hyphae, which together form a network called mycelium. In certain organisms, hyphae are continuous tube with multinucleated cytoplasm (coenocytic), while others have septae or cross walls.
⚫ Their mode of nutrition is saprophytic and parasitic. They can also live as symbionts in association with algae as lichen and with roots of higher plants as mycorrhiza.
• Reproduction in fungi occurring by vegetative means, includes fragmentation, fission and budding, asexually by zoospore production conidia, etc., and sexually by oospores, ascospores and basidiospores.
• Sexual cycle involves plasmogamy (fusion of two protoplasts), karyogamy (fusion of two haploid nuclei) and meiosis.
. In some fungi, two haploid cells result in diploid cells. In some cases, dikaryon stage occurs in which two nuclei are present within a cell. This phase is known as dikaryophase of fungus.
⚫ Production of dikaryon (n+n, i.e. two nuclei per cell) is a characteristic of the classes-Ascomycetes and Basidiomycetes.
⚫ Fungi are classified into classes-Phycomycetes, Ascomycetes, Basidiomycetes and Deuteromycetes.
• Phycomycetes are lower fungi or algal fungi, their mycelium is aseptate and coenocytic, reproduce asexually through zoospores or aplanospores and sexually through isogamy or anisogamy, e.g. Rhizopus, Mucor, etc.
• Ascomycetes are sac fungi, their mycelium is branched and septate, asexual spores are conidia and sexual spores are ascospores, e.g. Aspergillus, Neurospora, etc.
• Basidiomycetes are club fungi, their mycelium is branched and septate, reproduce asexually by fragmentation and their vegetative cells fuse to form dikaryotic structure (plasmogamy), e.g. mushrooms, bracket fungi, etc.
⚫ Deuteromycetes are imperfect fungi, reproduce asexually by conidia and sexual forms absent in these e.g. Alternaria, Trichoderma, etc.
⚫ Heterothallism is the condition in fungal organisms where different thalli exist within a single genus of fungus. Kingdom-Plantae
• These include chlorophyllous organisms with cellulosic cell wall.
⚫ Life cycle consists of a dominant sporophyte and a highly reduced gametophyte showing alternation of generations.
. Few members are insectivorous plants, e.g. Cuscuta, bladderwort, etc.
• Plants are classified into algae, bryophytes, pteridophytes, gymnosperms and angiosperms.
Kingdom-Animalia
Animals are motile organisms showing holozoic nutrition. They follow a definite growth pattern.
⚫ They directly or indirectly depend on plants for food.
• Sexual reproduction is by copulation of male and female gametes followed by embryo development. Viruses, Viroids, Prions and Lichens.
Viruses and viroids are the non-cellular organisms, which are not characterised in the system of classification given by Whittaker.
⚫ They have both living and non-living characteristics.
. They form inert crystalline structure outside the living cell, but inside the host cell they can multiply easily.
They take over the host machinery and replicate themselves.
⚫ Pasteur and DJ Ivanowsky gave the name virus, which means venom or poisonous fluid.
⚫MW Beijerinck in 1898, called fluid obtained from infected tobacco plant as Contagium vivum fluidum (infectious living fluid).
• Viruses are obligate parasite. These are inert outside specific host cell and exist in crystalline form as demonstrated by WM Stanley.
Genetic material of viruses could be DNA or RNA.
⚫ Virus contain a protein coat called capsid, which is made up of capsomeres. Capsomeres are arranged in a helical or polyhedral geometric form.
Bacteriophage virus
• Viruses which infect plants have ssRNA, while which infect animals are either ssDNA/RNA or dsDNA/RNA.
• Viruses which infect bacteria are known as bacteriophage.
These are usually dsDNA viruses.
⚫ Viroids, discovered by TO Diener are the smallest known agents of infectious disease. These are only naked nucleic acid without a protein coat.
• Prions are abnormally folded proteins with cause infectious
neurological diseases, e.g. mad cow disease in cattle.
⚫ Lichens are the symbiotic association of algae (phycobiont) and fungi (mycobiont). They are also not included in five kingdom system.
Chapter 2
Biological classification
Question 1:
Discuss how classification systems have undergone several changes over a period of time?
Answer:
The classification systems have undergone several changes with time. The first attempt of classification was made by Aristotle. He classified plants as herbs, shrubs, and trees. Animals, on the other hand, were classified on the basis of presence or absence of red blood cells. This system of classification failed to classify all the known organisms.
Therefore, Linnaeus gave a two kingdom system of classification. It consists of kingdom Plantae and kingdom Animalia. However, this system did not differentiate between unicellular and multicellular organisms and between eukaryotes and prokaryotes. Therefore, there were large numbers of organisms that could not be classified under the two kingdoms.
To solve these problems, a five kingdom system of classification was proposed by R.H Whittaker in 1969. On the basis of characteristics, such as cell structure, mode of nutrition, presence of cell wall, etc., five kingdoms, Monera, Protista, Fungi, Plantae, and Animalia were formed.
Question 2:
State two economically important uses of:
(a) Heterotrophic bacteria
(b) Archaebacteria
Answer:
(a) Heterotrophic bacteria
(1) They act as decomposers and help in the formation of humus.
(2) They help in the production of curd from milk.
(3) Many antibiotics are obtained from some species of bacteria.
(4) Many soil bacteria help in fixation of atmospheric nitrogen.
(b) Archaebacteria
(1) Methane gas is produced from the dung of ruminants by the methanogens.
(2) Methanogens are also involved in the formation of biogas and sewage treatment.
Question 3:
What is the nature of cell-walls in diatoms?
Answer:
The cell walls of diatoms are made of silica. Their cell wall construction is known as frustule. It consists of two thin overlapping shells that fit into each other such as a soap box. When the diatoms die, the silica in their cell walls gets deposited in the form of diatomaceous earth. This diatomaceous earth is very soft and quite inert. It is used in filtration of oils, sugars, and for other industrial purposes.
Question 4:
Find out what do the terms ‘algal bloom’ and ‘red-tides’ signify.
Answer:
Algal bloom
Algal bloom refers to an increase in the population of algae or blue-green algae in water, resulting in discoloration of the water body. This causes an increase in the biological oxygen demand (BOD), resulting in the death of fishes and other aquatic animals.
Red-tides
Red tides are caused by red dinoflagellates (Gonyaulax) that multiply rapidly. Due to their large numbers, the sea appears red in colour. They release large amounts of toxins in water that can cause death of a large number of fishes.
Question 5:
How are viroids different from viruses?
Answer:
Viroids were discovered in 1917 by T.O. Denier. They cause potato spindle tuber disease. They are smaller in size than viruses. They also lack the protein coat and contain free RNA of low molecular weight.
Question 6:
Describe briefly the four major groups of Protozoa.
Answer:
Protozoa are microscopic unicellular protists with heterotrophic mode of nutrition. They may be holozoic, saprobic, or parasitic. These are divided into four major groups.
(1) Amoeboid protozoa or sarcodines
They are unicellular, jelly-like protozoa found in fresh or sea water and in moist soil. Their body lacks a periplast. Therefore, they may be naked or covered by a calcareous shell. They usually lack flagella and have temporary protoplasmic outgrowths called pseudopodia. These pseudopodia or false feet help in movement and capturing prey. They include free living forms such as Amoeba or parasitic forms such as Entamoeba.
(2) Flagellated protozoa or zooflagellates
They are free living, non-photosynthetic flagellates without a cell wall. They possess flagella for locomotion and capturing prey. They include parasitic forms such as Trypanosoma, which causes sleeping sickness in human beings.
(3) Ciliated protozoa or ciliates
They are aquatic individuals that form a large group of protozoa. Their characteristic features are the presence of numerous cilia on the entire body surface and the presence of two types of nuclei. All the cilia beat in the same direction to move the water laden food inside a cavity called gullet. They include organisms such as Paramoecium, Vorticella,etc.
(4) Sporozoans
They include disease causing endoparasites and other pathogens. They are uninucleate and their body is covered by a pellicle. They do not possess cilia or flagella. They include the malaria causing parasite Plasmodium.
Question 7:
Plants are autotrophic. Can you think of some plants that are partially heterotrophic?
Answer:
Plants have autotrophic mode of nutrition as they contain chlorophyll pigment. Thus, they have the ability to prepare their own food by the process of photosynthesis. However, some insectivorous plants are partially heterotrophic. They have various means of capturing insects so as to supplement their diet with required nutrients derived from insects, causing proliferation of growth. The examples include pitcher plant (Nepenthes), Venus fly trap, bladderwort, and sundew plant.
Question 8:
What do the terms phycobiont and mycobiont signify?
Answer:
Phycobiont refers to the algal component of the lichens and mycobiont refers to the fungal component. Algae contain chlorophyll and prepare food for fungi whereas the fungus provides shelter to algae and absorbs water and nutrients from the soil. This type of relationship is referred to as symbiotic.
Question 9:
Give a comparative account of the classes of Kingdom Fungi under the following:
(i) Mode of nutrition
(ii) Mode of reproduction
Answer:
(A) Phycomycetes– This group of fungi includes members such as Rhizopus, Albugo, etc.
(i) Mode of nutrition
They are obligate parasites on plants or are found on decaying matter such as wood.
(ii) Mode of reproduction
Asexual reproduction takes place through motile zoospores or non-motile aplanospores that are produced endogenously in sporangium.
Sexual reproduction may be of isogamous, anisogamous, or oogamous type. It results in the formation of thick-walled zygospore.
(B) Ascomycetes- This group of fungi includes members such as Penicillium, Aspergillus, Claviceps, and Neurospora.
(i) Mode of nutrition
They are sporophytic, decomposers, parasitic or coprophilous (growing on dung).
(ii) Mode of reproduction
Asexual reproduction occurs through asexual spores produced exogenously, such as conidia produced on conidiophores.
Sexual reproduction takes place through ascospores produced endogenously in sac-like asci and arranged inside ascocarps.
(C) Basidiomycetes– This group of fungi includes members such as Ustilago, Agaricus and Puccinia.
(i) Mode of nutrition
They grow as decomposers in soil or on logs and tree stumps. They also occur as parasites in plants causing diseases such as rusts and smuts.
(ii) Mode of reproduction
Asexual reproduction takes place commonly through fragmentation. Asexual spores are absent.
Sex organs are absent but sexual reproduction takes place through plasmogamy. It involves fusion of two different strains of hyphae. The resulting dikaryon gives rise to a basidium. Four basidiospores are produced inside a basidium.
(D) Deuteromycetes – This group of fungi includes members such as Alternaria, Trichoderma, and Colletotrichum.
(i) Mode of nutrition
Some members are saprophytes while others are parasites. However, a large number act as decomposers of leaf litter.
(ii) Mode of reproduction
Asexual reproduction is the only way of reproduction in deuteromycetes. It occurs through asexual spores called conidia.
Sexual reproduction is absent in deuteromycetes.
Question 10:
What are the characteristic features of Euglenoids?
Answer:
Some characteristic features of Euglenoids are as follows.
🔵 Euglenoids (such as Euglena) are unicellular protists commonly found in fresh water.
🔵 Instead of cell wall, a protein-rich cell membrane known as pellicle is present.
🔵 They bear two flagella on the anterior end of the body.
🔵 A small light sensitive eye spot is present.
🔵 They contain photosynthetic pigments such as chlorophyll and can thus prepare their own food. However, in absence of light, they behave similar to heterotrophs by capturing other small aquatic organisms.
🔵 They have both plant and animal-like features, which makes them difficult to classify.
Question 11:
Give a brief account of viruses with respect to their structure and nature of genetic material. Also name four common viral diseases.
Answer:
Viruses are sub-microscopic infectious agents that can infect all living organisms. A virus consists of genetic material surrounded by a protein coat. The genetic material may be present in the form of DNA or RNA.
Most of the viruses, infecting plants, have single stranded RNA as genetic material. On the other hand, the viruses infecting animals have single or double stranded RNA or double stranded DNA.
Bacteriophages or viruses infecting bacteria mostly have double stranded DNA. Their protein coat called capsid is made up of capsomere subunits. These capsomeres are arranged in helical or polyhedral geometric forms.
A.I.D.S, small pox, mumps, and influenza are some common examples of viral diseases.
Question 12:
Organise a discussion in your class on the topic- Are viruses living or non-living?
Answer:
Viruses are microscopic organisms that have characteristics of both living and non-living. A virus consists of a strand of DNA or RNA covered by a protein coat. This presence of nucleic acid (DNA or RNA) suggests that viruses are alive. In addition, they can also respond to their environment (inside the host cell) in a limited manner.
However, some other characters, such as their inability to reproduce without using the host cell machinery and their acellular nature, indicate that viruses are non-living. Therefore, classifying viruses has remained a mystery for modern systematics.
