Class 11 Biology: Chapter 2- Biological Classification

 

Chapter 2

BIOLOGICAL CLASSIFICATION

Syllabus: Five kingdom classification; Salient features and classification of Monera, Protista and Fungi into major groups; Lichens, Viruses and Viroids.

 

2.1 Kingdom Monera

Bacteria are the sole members of the Kingdom Monera. They are the most abundant micro-organisms. Bacteria occur almost everywhere.

Bacteria are grouped under four categories based on their shape:

1. the spherical Coccus (pl.: cocci),

2. the rod-shaped Bacillus (pl.: bacilli),

3. the comma-shaped Vibrium (pl.: vibrio) and the

4. spiral Spirillum (pl.: spirilla)

 

Here are some of the main characteristics of Kingdom Monera:

1.     Cell type: Prokaryotic Cells  (lack a true nucleus and membrane-bound organelles)

2.     Body Organisation: Unicellular.

3.     Mode of Reproduction: Asexual Reproduction.

4.     Wide Variety of Shapes: Monerans can have various shapes, including spherical (cocci), rod-shaped (bacilli), and spiral (spirilla).

5.     Cell Walls: Most Monerans have cell walls made of peptidoglycan.

6.    Mode of nutrition: Heterotrophic (Majorly) and Autotrophic (Photosynthetic & Chemosynthetic).

7.    Importance in Ecosystems: Monerans play essential roles in many ecosystems. They can be decomposers, producers, or consumers, and can even be found living in symbiotic relationships with other organisms.

 

Monera can be divided into two categories:-

 

 

          

2.1.1 Archaebacteria (Ancient or oldest bacteria)

1. These bacteria contain a cell wall composed of various polysaccharides and glycoconjugates. Archaebacteria lack peptidoglycan.

Glycoconjugates are complex molecules composed of carbohydrates (sugars) covalently linked to other biomolecules such as proteins (forming glycoproteins) or lipids (forming glycolipids).

 

2. These bacteria are special since they live in some of the most harsh habitats such as

i)          extreme salty areas (halophiles)

ii)         hot springs (thermoacidophiles, hot and acidic environment)

iii)       marshy areas (methanogens as they produce methane gas).

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.

 

 

2.1.2 Eubacteria ( True bacteria)

1. They are characterised by the presence of a rigid cell wall made of peptidoglycan, and if motile, a flagellum.

These can be divided into three forms:-

 

 

        

a. Photosynthetic autotrophs: 

i) The cyanobacteria (also referred to as blue-green algae but actually these are not algae) have chlorophyll a (a has methyl group and b has aldehye group) similar to green plants and are photosynthetic autotrophs.            

 

                        

ii) The cyanobacteria are unicellular, colonial or filamentous, freshwater/marine or terrestrial algae.

iii)  The colonies are generally surrounded by gelatinous sheath.

iv) Some of these organisms can fix atmospheric nitrogen in specialised cells called heterocysts, e.g., Nostoc and Anabaena.

iv)  They often form blooms in polluted water bodies.

b. Chemosynthetic autotrophs:

i) Chemosynthetic autotrophic bacteria oxidise various inorganic substances such as nitrates, nitrites and ammonia and use the released energy for their ATP production. They play a great role in recycling nutrients like nitrogen, phosphorous, iron and sulphur.

                         

                          

 

 

Examples:

1.     Nitrosomonas: These bacteria oxidize ammonia to nitrite and derive energy from the process.

2.     Nitrobacter: These bacteria oxidize nitrite to nitrate and derive energy from the process.

 

c. Heterotrophic bacteria:

i) Heterotrophic bacteria are most abundant in nature. The majority are important decomposers.

ii) Useful: They are helpful in making curd from milk, production of antibiotics, fixing nitrogen in legume roots, etc. Ex. Lactobacillus bulgaricus

iii) Harmful (Pathogens): Some are pathogens causing damage to human beings, crops, farm animals and pets. Cholera, typhoid, tetanus, citrus canker are well known diseases caused by different bacteria.

 

Disease	Symptoms	Caused by
Cholera	Severe diarrhea (often described as "rice water" stools) - Vomiting - Rapid heart rate - Muscle cramps - Dehydration - Sunken eyes - Dry mouth and tongue - Low blood pressure - Shock	Vibrio cholerae
Typhoid	High fever - Abdominal pain - Headache - Weakness and fatigue - Loss of appetite - Diarrhea or constipation - Rose-colored spots on the trunk of the body - Confusion or delirium	Salmonella typhi
Tetanus	- Stiffness in the jaw and neck - Muscle spasms and stiffness in other parts of the body - Difficulty swallowing - Sore throat - Sweating - Elevated blood pressure - Rapid heart rate - Fever	Clostridiium tetani
Citrus canker	- Lesions on the leaves, stems, and fruit of citrus trees - Premature fruit drop - Defoliation (loss of leaves) - Stunted growth of the tree	Xanthomonas citri

 

iv) Reproduction in Bacteria:

a) Bacteria reproduce mainly by fission (Figure 2.3).

b) Sometimes, under unfavourable conditions, they produce spores.

c) They also reproduce by a sort of sexual reproduction by adopting a primitive type of DNA transfer from one bacterium to the other.

                              

 

v) Mycoplasma

1. These are organisms that completely lack a cell wall.

2. They are the smallest living cells known and can survive without oxygen.

3. Many mycoplasma are pathogenic in animals and plants.

4. Mycoplasma pneumonia: This is a respiratory infection that can cause symptoms such as coughing, chest pain, fever, and fatigue.

2.2 KINGDOM PROTISTA

1. All single-celled eukaryotes are placed under Protista.

2.  Members of Protista are primarily aquatic.

3. This kingdom forms a link with the others dealing with plants, animals and fungi.

4. Being eukaryotes, the protistan cell body contains a well defined nucleus and other membrane-bound organelles.

5. Some have flagella or cilia.

6. Protists reproduce asexually and sexually by a process involving cell fusion and zygote formation.

 

 

A. Chrysophytes (golden plant)

1. This group includes diatoms and golden algae (desmids).

2. They are found in fresh water as well as in marine environments.

3. They are microscopic and float passively in water currents (plankton).

4. Most of them are photosynthetic.

5. In diatoms the cell walls form two thin overlapping shells, which fit together as in a soap box.

6. The walls are embedded with silica and thus the walls are indestructible.

7. 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’.

8. Being gritty (that are rough to the touch) this soil is used in polishing, filtration of oils and syrups. Diatoms are the chief ‘producers’ in the oceans.

 

B. Dinoflagellates (The name "dinoflagellates" comes from the Greek words "dinos" meaning "whirling" or "spinning")

1. These organisms are mostly marine and photosynthetic.

2. They appear yellow, green, brown, blue or red depending on the main pigments present in their cells.

3. The cell wall has stiff cellulose plates on the outer surface.

4. Most of them have two flagella; one lies longitudinally and the other transversely in a furrow between the wall plates.

5. 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.

Red tides due to Gonyaulax

 

C. Euglenoids (A link between autotroph and heterotroph)

1. Majority of them are fresh water organisms found in stagnant water.

2. Instead of a cell wall, they have a protein rich layer called pellicle which makes their body flexible.

3. They have two flagella, a short and a long one.

4. Though they are photosynthetic in the presence of sunlight (due to the presence of chlorophyll in some of them), 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

 

D. Slime Moulds (Also known as false fungi as behaves like fungi )

1. Slime moulds are saprophytic protists.

2. The body moves along decaying twigs and leaves engulfing organic material.

3. Under suitable conditions, they form an aggregation called plasmodium (the structure formed by the fusing of many slime mould cells and acts as a single unit) which may grow and spread over several feet. During unfavourable conditions, the plasmodium differentiates and forms fruiting bodies bearing spores at their tips.

4. 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.

	Chrysophytes	Dinoflagellates	Euglenoids	Slime Moulds
Habitat	Fresh water and marine	mostly marine	Majority of them are fresh water	Diverse range of habitats
Cell wall	Made of silica so indestructible	cell wall has stiff cellulose plates	Absent	Present
Mode of Nutrition	Mostly Photosynthetic	Mostly Photosynthetic	Both Photosynthetic and heterotrophic	Heterotrophic (saprotrophic)
Flagella	Absent	Two	two flagella, a short and a long one.	Absent
Main Characteristics	cell wall deposits accumulation over billions of years is referred to as ‘diatomaceous earth’	rapid multiplication of red dinoflagellates make the sea appear red	Instead of a cell wall, they have a protein rich layer called pellicle	Under suitable conditions multiple uninucleate cells form aggregation called plasmodium. During unfavourable conditions forms fruiting bodies bearing spores at their tips which germinate again single cells slime moulds.

 

E. Protozoans

1. All protozoans are heterotrophs and live as predators or parasites.

2. They are believed to be primitive relatives of animals as most of them lack cell wall.

3. There are four major groups of protozoans.

i) Amoeboid protozoans                         ii) Flagellated protozoans   

iii) Ciliated protozoans                            iv) Sporozoans

 

i)  Amoeboid protozoans

1. These organisms live in fresh water, sea water or moist soil.

2. They move and capture their prey by putting out pseudopodia (false feet) as in Amoeba.                  

                                

        

3. Marine forms have silica shells on their surface.

4. Some of them such as Entamoeba are parasites.

(The most common disease caused by Entamoeba is amoebiasis, also known as amebiasis or amoebic dysentery.

Amoebiasis is caused by the species Entamoeba histolytica and is typically transmitted through ingestion of contaminated food or water. The symptoms of amoebiasis can range from mild diarrhea to severe dysentery with bloody stools.)

ii) Flagellated protozoans   

1. The members of this group are either free-living or parasitic.

2. They have flagella.

3. The parasitic forms cause diaseases such as sleeping sickness.

Example: Trypanosoma.

                                                  

    

iii) Ciliated protozoans

1. These are aquatic, actively moving organisms because of the presence of thousands of cilia.

2. They have a cavity (gullet) that opens to the outside of the cell surface.

3. The coordinated movement of rows of cilia causes the water laden with food to be steered into the gullet. Example: Paramoecium.

                      

 

 

iv) Sporozoans

1. This includes diverse organisms that have an infectious spore-like stage in their life cycle.

2. The most notorious is Plasmodium (malarial parasite) which causes malaria, a disease which has a staggering effect on human population.

Amoeboid protozoans	Flagellated protozoans	Ciliated protozoans	Sporozoans
fresh water, sea water or moist soil	either free-living or parasitic	These are aquatic	Have an infectious spore-like stage in their life cycle.
pseudopodia (false feet) as in Amoeba.	They have flagella	Actively moving organisms because of the presence of thousands of cilia	Example: Plasmodium
Marine forms have silica shells	parasitic forms cause diaseases such as sleeping sickness	They have a cavity (gullet) that opens to the outside
Some of them such as Entamoeba are parasites.	Example: Trypanosoma	Example: Paramoecium

 

KINGDOM FUNGI

1. Mode of Nutrition

The fungi constitute a unique kingdom of heterotrophic organisms which can be further divided into three categories:

i) Saprophytic     (fungus on moist bread and rotten fruits)

ii) Parasitic  (White spots seen on mustard leaves and wheat rust-causing Puccinia)

 

iii) Symbiotic

 a) Fungi (mycobiont)   +   algae as lichens (phycobiont)

b) with roots of higher plants as mycorrhiza (myco means fungi and rhiza means roots).

 

2. Harmful effects of fungi

i) White spots seen on mustard leaves are due to a parasitic fungus.

ii) Fungi cause diseases in plants and animals; wheat rust-causing Puccinia is an important example.

3. Uses of fungi

i) Yeast are used to make bread and beer.

ii) As antibiotics, e.g., Penicillium.

4. Habitat

i) Fungi are cosmopolitan (means found everywhere) and occur in air, water, soil and on animals and plants.

ii) They prefer to grow in warm and humid places.

5. Shape and structure

i) With the exception of yeasts which are unicellular, fungi are filamentous (thread like structure).

ii) Their bodies consist of long, slender thread-like structures called hyphae.

iii) The network of hyphae is known as mycelium.

iv) Some hyphae are continuous tubes filled with multinucleated cytoplasm – these are called coenocytic hyphae.

v) Others have septae or cross walls in their hyphae.

vi) The cell walls of fungi are composed of chitin and polysaccharides.

6. Reproduction

Reproduction in fungi can take place by

i) Vegetative                                    ii) Sexual                                   iii) Asexual

a) Vegetative Reproduction

Fragmentation	Fission	Budding

 

Asexual	Sexual
Asexual reproduction is by spores called conidia or sporangiospores or zoospores	Sexual reproduction is by oospores, ascospores and basidiospores.
Fruiting bodies
i)                  Cycle in Asexual Reproduction

 ii) Cycle in Sexual Reproduction

                            

Note: Main body is haploid.

 

                                                                                       

 7. Classification of Fungi

The morphology of the mycelium, mode of spore formation and fruiting bodies form the basis for the division of the kingdom into various classes.

 

A. Phycomycetes

1. These are found in aquatic habitats and on decaying wood in moist and damp places or as obligate parasites on plants.

2. The mycelium is aseptate and coenocytic.

3. Asexual reproduction takes place by zoospores (motile) or by aplanospores (non-motile).

4. These spores are endogenously produced in sporangium.

 Sexual reproduction

i) A zygospore (in zygomycota fungi) is formed by fusion of two gametes.

 

6. These gametes are

i) similar in morphology (isogamous) or      

                      

ii) dissimilar (anisogamous) or

iii) oogamous ( when one is motile and other is non motile)

6. Some common examples are Mucor, Rhizopus (the bread mould mentioned earlier) and Albugo (the parasitic fungi on mustard).

B. Ascomycetes

1. Commonly known as sac-fungi.

2. Mostly multicellular, e.g., Penicillium, or rarely unicellular, e.g., yeast (Saccharomyces). 3. They are saprophytic, decomposers, parasitic or coprophilous (growing on dung).

4. Mycelium is branched and septate.

5. Reproduction:

a) Asexual Reproduction

i) The asexual spores are conidia produced exogenously on the special mycelium called conidiophores.

ii) Conidia on germination produce mycelium.

b) Sexual Reproduction

i) Sexual spores are called ascospores which are produced endogenously in sac like asci (singular ascus).

ii) These asci are arranged in different types of fruiting bodies called ascocarps.

 

 

9. Examples are Aspergillus, Claviceps and Neurospora. Neurospora is used extensively in biochemical and genetic work (Researchers have identified numerous genes and regulatory elements that control the expression of genes in Neurospora). Many members like morels and truffles are edible and are considered delicacies.

C. Basidiomycetes

1. Commonly known forms of basidiomycetes are mushrooms, bracket fungi or puffballs. 2. They grow in soil, on logs and tree stumps and in living plant bodies as parasites, e.g., rusts and smuts.

3. The mycelium is branched and septate.

4. Reproduction :

i) Vegetative reproduction by fragmentation.

ii) Asexual reproduction is absent.

iii) The sex organs are absent, but plasmogamy is brought about by fusion of two vegetative or somatic cells of different strains (a strain refers to a group of organisms within a species that share specific genetic or phenotypic characteristics that distinguish them from other groups within the same species. )or genotypes.

6. The resultant structure is dikaryotic which ultimately gives rise to basidium.

7. Karyogamy and meiosis take place in the basidium producing four basidiospores.

8. The basidiospores are exogenously produced on the basidium (pl.: basidia).

9. The basidia are arranged in fruiting bodies called basidiocarps.

10. Some common members are Agaricus (mushroom), Ustilago (smut) and Puccinia (rust fungus).

 

D. Deuteromycetes

1. Commonly known as imperfect fungi because only the asexual or vegetative phases of these fungi are known.

Reproduction:

i) Asexual reproduction: The deuteromycetes reproduce only by asexual spores known as conidia.

ii) Sexual reproduction is absent.

3. The mycelium is septate and branched.

4. Some members are saprophytes or parasites while a large number of them are decomposers of litter (litter refers to dead organic material, such as leaves, twigs, branches, and other plant debris) and help in mineral cycling.

5. Some examples are Alternaria, Colletotrichum and Trichoderma.

Reproduction	Phycomycetes	Ascomycetes	Basidiomycetes	Deuteromycetes
Vegetative	Absent	Absent	fragmentation	Absent
Asexual	Zoospores and aplanospores produced in sporangiophore	Conidiophores	Absent	Conidiophores
Sexual	zygospores	Ascospores in ascocarp	Basidiospores in Basidiocarp	Absent
examples	RAM Rhizopus, mucor, albugo	CAN Claviceps Aspergillus,  and Neurospora	Mushrooms and Bracket fungi	CAT Colletotrichum Alternaria and Trichoderma.

 

VIRUSES, VIROIDS, PRIONS AND LICHENS

1. Viruses

1. The viruses are non-cellular organisms that are characterised by having an inert crystalline structure outside the living cell. So, not truly living.

2. These are parasitic. Once they infect a cell they take over the machinery of the host cell to replicate themselves, killing the host.

3. Virus means venom or poisonous fluid.

Discovery

1. Dmitri Ivanowsky (1892) recognised certain microbes as causal organism of the mosaic disease of tobacco.

2. These were found to be smaller than bacteria because they passed through bacteria-proof filters.

3. M.W. Beijerinek (1898) demonstrated that the extract of the infected plants of tobacco could cause infection in healthy plants and named the new pathogen “virus” and called the fluid as Contagium vivum fluidum (infectious living fluid).

4. W.M. Stanley (1935) showed that viruses could be crystallised and crystals consist largely of proteins.

(Note: No living cell can be crystallized like the crystals of salts and sugars, but viruses could be crystallized, so a link between living and non living.)

 

Structure

1. They are inert (inactive) outside their specific host cell.

2. In addition to proteins, viruses also contain genetic material, that could be either RNA or DNA. No virus contains both RNA and DNA.

3. A virus is a nucleoprotein (nucleic acid + protein) and the genetic material is infectious.

4. In general, viruses that infect plants have single stranded RNA.

5. Viruses that infect animals have either single or double stranded RNA or double stranded DNA.

6. Bacterial viruses or bacteriophages (viruses that infect the bacteria) are usually double stranded DNA viruses.

7. The protein coat called capsid made of small subunits called capsomeres, protects the nucleic acid.

8. These capsomeres are arranged in helical or polyhedral geometric forms.

 

Diseases caused by viruses

1. Viruses cause diseases like mumps, small pox, herpes and influenza.

2. AIDS in humans is also caused by a virus.

3. In plants, the symptoms can be mosaic (black spots) formation, leaf rolling and curling, yellowing and vein clearing, dwarfing and stunted growth.

 

 

 

Viroids

Discovery

1. In 1971, T.O. Diener discovered a new infectious agent that was smaller than viruses and caused potato spindle tuber (like the shape of tauri) disease.

Structure

1. It was found to be a free RNA; it lacked the protein coat that is found in viruses, hence the name viroid.

 

 Prions

1. Some diseases were found to be transmitted by an agent consisting of abnormally folded protein. The agent was similar in size to viruses. These agents were called prions.
2. Prions, on the other hand, are infectious proteins that do not contain genetic material.
3. They are misfolded versions of normal cellular proteins found in the body.
4. ivWhen a prion enters a healthy organism, it can induce the normal proteins to adopt the misfolded conformation, thereby converting them into infectious prions.
5. The misfolded prions then accumulate and cause damage to the nervous system, leading to neurodegenerative diseases like Creutzfeldt-Jakob disease (CJD), variant CJD, or mad cow disease (bovine spongiform encephalopathy, BSE).
6. Unlike viruses, prions do not rely on nucleic acids for their replication and transmission.

 

Diseases

1. The most notable diseases caused by prions are bovine spongiform encephalopathy (BSE) commonly called mad cow disease (which leads to the degeneration of brain tissue and the development of characteristic spongy holes in the brain. ) in cattle and its analogous (comparable) variant Cr–Jacob disease (CJD) (Creutzfeldt-Jakob disease )in humans.

 

Lichens

1. Lichens are symbiotic associations i.e. mutually useful associations, between algae and fungi. The algal component is known as phycobiont and fungal component as mycobiont, which are autotrophic and heterotrophic, respectively.

2. Algae prepare food for fungi and fungi provide shelter and absorb mineral nutrients and water for its partner.

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

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