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the fundamental principles of biology.
1. Aristotle was the
earliest to attempt a more scientific basis for classification.
2. 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.
3. In Linnaeus' time, a
Two Kingdom system of classification with Plantae and Animalia kingdoms was
developed that included all plants and animals respectively.
4. R.H. Whittaker
(1969) proposed a Five Kingdom Classification.
5. The kingdoms defined
by him were named Monera, Protista, Fungi, Plantae and Animalia.
6. The main criteria
for classification used by him include cell structure, body organisation, mode
of nutrition, reproduction and phylogenetic relationships.
7. The three-domain
system has also been proposed that divides the Kingdom Monera into two domains,
leaving the remaining eukaryotic kingdoms in the third domain and thereby a six-kingdom
classification.
8. Earlier
classification systems included bacteria, blue-green algae, fungi, mosses,
ferns, gymnosperms and angiosperms under ‘Plants’.
9. The character that
unified this whole kingdom was that all the organisms included had a cell wall
in their cells.
10. This placed together
groups which widely differed in other characteristics. It brought together the
prokaryotic bacteria and the blue-green algae ( cyanobacteria) with other
groups which were eukaryotic.
11. It also grouped
together the unicellular organisms and the multicellular ones, say, for
example, Chlamydomonas and Spirogyra were placed together under algae.
12. The classification
did not differentiate between the heterotrophic group – fungi, and the
autotrophic green plants, though they also showed a characteristic difference
in their walls composition – the fungi had chitin.
13. All prokaryotic
organisms were grouped together under Kingdom Monera and the unicellular
eukaryotic organisms were placed in Kingdom Protista.
14. Kingdom Protista has
brought together Chlamydomonas, Chlorella (earlier placed in Algae within
Plants and both having cell walls) with Paramoecium
and Amoeba (which were earlier placed in the animal kingdom which lack cell
wall).
KINGDOM MONERA
1. Bacteria are the
sole members of the Kingdom Monera.
2. They are the most
abundant micro-organisms. Bacteria occur almost everywhere.
3. They also live in
extreme habitats such as hot springs, deserts, snow and deep oceans where very
few other life forms can survive.
4. Bacteria are grouped
under four categories based on their shape: the spherical Coccus (pl.: cocci),
the rod-shaped Bacillus (pl.: bacilli), the comma-shaped Vibrium
(pl.: vibrio) and the spiral Spirillum (pl.: spirilla).
5. Though the bacterial
structure is very simple, they are very complex in behaviour.
6. Compared to many
other organisms, bacteria as a group show the most extensive metabolic
diversity.
7. Some of the bacteria
are autotrophic, i.e., they synthesise their own food from inorganic
substrates.
8. They may be
photosynthetic autotrophic or chemosynthetic autotrophic.
9. The vast majority of bacteria are heterotrophs, i.e., they depend on other organisms or on dead organic matter for food.
Archaebacteria
1. 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).
2. Archaebacteria
differ from other bacteria in having a different cell wall structure and this
feature is responsible for their survival in extreme conditions.
3. 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.
Eubacteria
1. There are thousands
of different eubacteria or ‘true bacteria’.
2. They are
characterised by the presence of a rigid cell wall, and if motile, a flagellum.
3. The cyanobacteria
(also referred to as blue-green algae) have chlorophyll a similar to green
plants and are photosynthetic autotrophs.
4. The cyanobacteria
are unicellular, colonial or filamentous, freshwater/marine or terrestrial
algae.
5. 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.
6. Chemosynthetic
autotrophic bacteria oxidise various inorganic substances such as nitrates,
nitrites and ammonia and use the released energy for their ATP production.
7. They play a great
role in recycling nutrients like nitrogen, phosphorous, iron and sulphur.
Heterotrophic bacteria are most abundant in nature. The majority are important
decomposers.
8. Many of them have a
significant impact on human affairs. They are helpful in making curd from milk,
production of antibiotics, fixing nitrogen in legume roots, etc.
9. Some are pathogens
causing damage to human beings, crops, farm animals and pets. Cholera, typhoid,
tetanus, and citrus canker are well-known diseases caused by different
bacteria.
10. Mycoplasma are
organisms that completely lack a cell wall.
11. They are the
smallest living cells known and can survive without oxygen.
KINGDOM PROTISTA
1. All single-celled
eukaryotes are placed under Protista, but the boundaries of this kingdom are
not well defined.
2. 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.
3. Some have flagella
or cilia. Protists reproduce asexually and sexually by a process involving cell
fusion and zygote formation.
Chrysophytes
1. 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).
2. Most of them are
photosynthetic. In diatoms the cell walls form two thin overlapping shells,
which fit together as in a soap box.
3. The walls are
embedded with silica and thus the walls are indestructible.
4. 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’.
5. Being gritty this
soil is used in polishing, filtration of oils and syrups.
6. Diatoms are the
chief ‘producers’ in the oceans.
Dinoflagellates
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).
6. Toxins released by
such large numbers may even kill other marine animals such as fishes.
Euglenoids
1. Majority of them are
freshwater 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, when deprived of sunlight they behave like
heterotrophs by predating on other smaller organisms.
5. Interestingly, the
pigments of euglenoids are identical to those present in higher plants.
Example: Euglena.
Slime Moulds
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 which may grow and
spread over several feet.
4. During unfavourable
conditions, the plasmodium differentiates and forms fruiting bodies bearing
spores at their tips.
5. The spores possess
true walls.
6. They are extremely
resistant and survive for many years, even under adverse conditions.
7. The spores are
dispersed by air currents.
Protozoans
1. All protozoans are
heterotrophs and live as predators or parasites.
2. They are believed to
be primitive relatives of animals.
3. There are four major
groups of protozoans.
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.
Flagellated protozoans:
1. The members of this
group are either free-living or parasitic.
2. They have flagella.
3. The parasitic forms
cause diseases such as sleeping sickness. Example: Trypanosoma.
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.
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.
KINGDOM FUNGI
1. The fungi constitute
a unique kingdom of heterotrophic organisms.
2. They show a great
diversity in morphology and habitat.
3. You must have seen fungi on a moist bread and
rotten fruits.
4. The common mushroom
you eat and toadstools are also fungi.
5. White spots seen on
mustard leaves are due to a parasitic fungus.
6. Some unicellular
fungi, e.g., yeast are used to make bread and beer.
7. Other fungi cause
diseases in plants and animals; wheat rust-causing Puccinia is an important
example.
8. Some are the source
of antibiotics, e.g., Penicillium.
9. With the exception
of yeasts which are unicellular, fungi are filamentous.
10. Their bodies consist
of long, slender thread-like structures called hyphae.
11. The network of
hyphae is known as mycelium.
12. Some hyphae are
continuous tubes filled with multinucleated cytoplasm – these are called
coenocytic hyphae.
13. Others have septae or cross walls in their hyphae.
14. The cell walls of
fungi are composed of chitin and polysaccharides.
15. Most fungi are
heterotrophic and absorb soluble organic matter from dead substrates and hence
are called saprophytes.
16. Those that depend on
living plants and animals are called parasites.
17. They can also live
as symbionts – in association with algae as lichens and with roots of higher
plants as mycorrhiza.
18. Reproduction in
fungi can take place by vegetative means fragmentation, fission and budding.
19. Asexual reproduction
is by spores called conidia or sporangiospores or zoospores, and sexual
reproduction is by oospores, ascospores and basidiospores.
20. The various spores
are produced in distinct structures called fruiting bodies.
21. 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.
22. When a fungus
reproduces sexually, two haploid hyphae of compatible mating types come
together and fuse. In some fungi the fusion of two haploid cells immediately
results in diploid cells (2n).
23. 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.
24. The morphology of
the mycelium, mode of spore formation and fruiting bodies form the basis for the division of the
kingdom into various classes.
Phycomycetes
1. Members of
phycomycetes 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.
5. A zygospore is
formed by fusion of two gametes.
6. These gametes are
similar in morphology (isogamous) or dissimilar (anisogamous or oogamous).
7. Some common examples
are Mucor (Figure 2.5a), Rhizopus (the bread mould mentioned earlier) and Albugo (the parasitic fungi on mustard).
Ascomycetes
1. Commonly known as
sac-fungi, the ascomycetes are mostly
multicellular, e.g., Penicillium, or rarely unicellular, e.g., yeast
(Saccharomyces).
2. They are
saprophytic, decomposers, parasitic or coprophilous (growing on dung).
3. Mycelium is branched
and septate.
4. The asexual spores
are conidia produced exogenously on the special mycelium called conidiophores.
5. Conidia on
germination produce mycelium.
6. Sexual spores are
called ascospores which are produced endogenously in sac like asci (singular
ascus).
7. These asci are
arranged in different types of fruiting bodies called ascocarps.
8. Some examples are
Aspergillus , Claviceps and Neurospora.
9. Neurospora is used extensively in biochemical and
genetic work.
10. Many members like
morels and truffles are edible and are considered delicacies.
Basidiomycetes
Commonly known forms of basidiomycetes are mushrooms, bracket fungi
or puffballs.
They grow in soil, on logs and tree stumps and in living plant
bodies as parasites, e.g., rusts and smuts.
The mycelium is branched and septate.
The asexual spores are generally not found, 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.
The basidiospores are exogenously produced on the basidium (pl.:
basidia).
Some common members are Agaricus
(mushroom) (Figure 2.5c), Ustilago (smut) and
Puccinia (rust fungus).
Deuteromycetes
1. Commonly known as imperfect
fungi because only the asexual or vegetative phases of these fungi are
known.
2. When the sexual
forms of these fungi were discovered they were moved into classes they rightly
belong to.
3. It is also possible
that the asexual and vegetative stage have been given one name (and placed
under deuteromycetes) and the sexual stage another (and placed under another
class).
4. Later when the
linkages were established, the fungi were correctly identified and moved out of
deuteromycetes. Once perfect (sexual) stages of members of dueteromycetes
were discovered they were often moved to ascomycetes and basidiomycetes.
5. The deuteromycetes
reproduce only by asexual spores known as conidia.
6. The mycelium is
septate and branched.
7. Some members are
saprophytes or parasites while a large number of them are decomposers of litter
and help in mineral cycling.
8. Some examples are
Alternaria, Colletotrichum and Trichoderma.
KINGDOM PLANTAE
1. Kingdom Plantae
includes all eukaryotic chlorophyll-containing organisms commonly called
plants.
2. A few members are
partially heterotrophic such as the insectivorous plants or parasites.
3. Bladderwort and
Venus fly trap are examples of insectivorous plants and Cuscuta
is a parasite.
4. The plant cells have
an eukaryotic structure with prominent chloroplasts and cell wall mainly made
of cellulose.
5. Plantae includes
algae, bryophytes, pteridophytes, gymnosperms and angiosperms.
6. Life cycle of plants
has two distinct phases – the diploid sporophytic and
the haploid gametophytic – that alternate with each other.
KINGDOM ANIMALIA
1. This kingdom is
characterised by heterotrophic eukaryotic organisms that are multicellular and
their cells lack cell walls.
2. They directly or
indirectly depend on plants for food.
3. They digest their
food in an internal cavity and store food reserves as glycogen or fat.
4. Their mode of
nutrition is holozoic – by ingestion of food.
5. They follow a
definite growth pattern and grow into adults that have a definite shape and
size.
6. Higher forms show
elaborate sensory and neuromotor mechanism. Most of them are capable of
locomotion.
7. The sexual
reproduction is by copulation of male and female followed by embryological
development.
VIRUSES, VIROIDS,
PRIONS AND LICHENS
1. In the five kingdom
classification of Whittaker there is no mention of lichens and some acellular
organisms like viruses, viroids and prions.
2. Viruses did not find
a place in classification since they are not considered truly ‘living’, if we
understand living as those organisms that have a cell structure.
3. The viruses are
non-cellular organisms that are characterised by having an inert crystalline
structure outside the living cell.
4. The name virus that
means venom or poisonous fluid was given by Dmitri Ivanowsky
(1892) recognised certain microbes as causal organism of the mosaic disease of
tobacco.
5. 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).
6. W.M. Stanley (1935)
showed that viruses could be crystallised and crystals consist largely of
proteins.
7. They are inert
outside their specific host cell.
8. Viruses are obligate
parasites.
9. In addition to
proteins, viruses also contain genetic material, that could be either RNA or
DNA.
10. No virus contains
both RNA and DNA.
11. A virus is a
nucleoprotein and the genetic material is infectious.
12. 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.
13. Bacterial viruses or
bacteriophages (viruses that infect the bacteria) are usually double stranded
DNA viruses.
14. The protein coat
called capsid made of small subunits called capsomeres, protects the nucleic
acid.
15. These capsomeres are
arranged in helical or polyhedral geometric forms.
16. Viruses cause
diseases like mumps, small pox, herpes
and influenza.
17. 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.
Viroids :
1. In 1971, T.O. Diener discovered a new infectious agent that was smaller than viruses and caused potato spindle tuber disease.
2. It was found to be a free RNA; it lacked the protein coat that is found in viruses, hence the name viroid.
3. The RNA of the viroid was of low molecular weight.
Prions :
1. In modern medicine certain infectious neurological diseases were found to be transmitted by an agent consisting of abnormally folded protein.
2. The agent was similar in size to viruses.
3. These agents were called prions.
4. The most notable diseases caused by prions are bovine spongiform encephalopathy (BSE) commonly called mad cow disease in cattle and its analogous variant Cr–Jacob disease (CJD) in humans.
Lichens :
1. Lichens are
symbiotic associations i.e. mutually useful associations, between algae and
fungi.
2. The algal component
is known as phycobiont and fungal component as
mycobiont, which are autotrophic and heterotrophic, respectively.
3. Algae prepare food
for fungi and fungi provide shelter and absorb mineral nutrients and water for
its partner.
4. So close is their
association that if one saw a lichen in nature one would never imagine that
they had two different organisms within them.
5. Lichens are very
good pollution indicators – they do not grow in polluted areas.
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