LESSION—1
Diversity
Of Microbes : Introduction
Microorganisms, or the microbes as they are commonly called, are smallest living
organisms. They are so small that we
cannot see them without magnifying them about 500 times with a microscope.
Microbes live almost everywhere on earth. Incredible as it may seem, some grow
at boiling temperatures in hot springs and at over 1100C in undersea
volcanic hydrothermal vents. Others live in ice at temperatures below freezing
point. Some live in very acidic environments (pH=1.0) while others are found in
saturated salt solutions microbes have also been found even in harsh
environment of Antarctica.
Microbes
preceded humans on earth by billions of years. Thus we have evolved in their
world and, more recently, they they in ours. For this reason, it should not
seem surprising that microbes live intimately with us on and in our bodies.
Without our permission they inhabit all of our orifices, including our mouth,
nose, ears, eyes and anal and genito-urinary tracts. However, most of them are
harmless symbionts using our bodies as their home and also protect us from
pathogenic species.
The major groups of microbes include bacteria, algae,
fungi, viruses and protozoa. Like higher plants and animals, most microbes are
alive and consist of one or more cells. Viruses, however, are acellular
entities on the borderline between the living and non-living; they behave like
living organisms when they gain entry to cells. Microbes range in size from
small viruses (20 nm in
diameter). to large protozoans (5
mm or more
in diameter). Thus largest
microbes are as 2,50,000 times the size of the smallest ones.
Position of Microbes
in the
Living World
The earliest system of classification categorized
living organisms in just two groups----
Animalia and the Plantae. Microbes
were placed in either kingdom of the basis of their ability of active movement
and photosynthesis. Ernst Haeckel in 1866 proposed a third group protista to include some relatively
simple biological forms. Some of the protists lacked nucleus (e.g., bacteria)
and some were nucleated organisms (e.g., protozoa, algae and fungi).
With the advent of electron microscopy, the
differences in the ultrastructure of cells become apparent. It led the
scientists to establish two different cell organization--- Prokaryotic and eukaryotic.
In prokaryotic cells the nuclear material is not surrounded by a nuclear
membrane; it includes bacteria and cyanobacteria (blue-green algae). The
eukaryotic cell organization is, however much more complex with each cell
organelle having its own limiting membrane (Table 1). All animals and plants (including algae
and fungi) have eukaryotic cell organization.
(1)
Complexity
of body structure; i.e., whether the
cell is prokaryotic or eukaryotic.
(2)
Complexity of
body organization; i.e., whether the
organism is unicellular and simple or multicellular and complex.
(3)
Mode of
nutrition; i.e., the methods used by organisms to obtain
nourishment. It may be autotrophic (photosynthetic) or heterotrophic (take food
by ingestion or
absorption).
Whittaker divided
organisms into following five kingdoms. He placed all prokaryotes in the
kingdom monera and most unicellular simple eukaryotes in the kingdom protista.
He considered fungi, which acquire solely by absorption, sufficiently different
from plants.
(I)
Kingdom--Monera
(Kingdom
of Prokaryotes)
Kingdom monera
includes all prokaryotic organisms---eubacteria (true bacteria), cynobacteria and archaebacteria.
All monerans are unicellular, they lack true nuclei and other membrane bound
organelles, such as mitochondria, plastid, lysosomes, etc. Their DNA is without
histones (proteins) and is found in the form of single chromosome. Monerans reproduce
chiefly by binary fission or may undergo genetic recombination. Monera are
important decomposers and mineralisers in the biosphere.
[II]
Kingdom—Protista
(Kingdom of unicellular
eukaryotes)
All protists are unicellular eukaryotic organisms.
They possess true nucleus and other membrane bound organelles, such as
mitochondria, chloroplast endoplasmic, reticulum etc. Most of the protists
possess flagella or cilia at some stage during their life cycle. Protists are
extremely complex; their cells show even more diversity than is found among the
cells in multicellular kingdoms.
They do not develop from an embryo as plants and
animals do. The kingdom includes a variety of life forms. For example among the
protists are algae which resemble plants (e.g., diatoms), the protozoa which
resemble animals (e.g., amoeba, paramecium) and euglenoids (e.g., euglena)
which have both plant and animal characteristics.
Protists
are either autotrophic
(photosynthetic) or holozoic which
feed on other protists by ingestion. A few protists live on animals an
parasites.
[iii]
Kingdom—Fungi
(kingdom
of multicellular decomposers)
This kingdom includes unicellular (e.g., yeasts),
multicellular (e.g., molds) and macroscopic (e.g., mushrooms) fungi. In
multicellular fungi the cells join together to form thin tubes called hyphae.
The cell wall of fungi is mostly made of chitin.
All fungi
lack chlorophyll. They are either
saprophytes (obtaining food from
dead and decaying
plant or animal matter) or parasites (obtaining food from living
organisms). Although fungi have some characteristics in common with plants but
their mode of nutrition and certain reproductive processes are not shared with
any other organisms.
[iv]
Kngdom-plantae (kingdom of multicellular
producer)
Kingdom planetae includes multicelluar autotrophic
plants, viz., thallophytes and trachephytes. These organisms are characterized
by the presence of a cell wall made of
cellulose. The main groups included in this kingdom are sea weeds like green,
red and brown alage. All mosses, frense, conifers and flowering plants.
Plants are characterized by the presence, of
photosynthetic pigment ___cholorophyll
and are the only organisms which have the capacity to synthesize complex organic molecules from carbon dioxide
and water utilizing the light energy trapped by the chlorophyll molecules as
the source of energy. Thus they are the primary producers on land and along sea
shores.
[v] Kingdom-Animalia (kingdom of multicellular consumers)
This kingdom included multicellular animals
(invertebrates and vertebrates). They are heterotrophic and obtain energy by
ingesting organic matter. Animal cells do not posses cell wall, plastids and
central vacuole. All animals are derived from zygote. They show organ-system
organization. Very few animals show cellular organization (e.g., sponges). Or tissue organization (e.g., Hydra). Animals show locomotion which is
made possible by well developed muscular cell.
Although Whittaker’s system is the most widely
accepted system of classification, it has certain anomalies. For example, the
kingdoms Monera and Protista are still heterogeneous. Both these kingdoms include
walled and wall less organisms, photosynthetic and non- photosynthetic organisms,
and unicellular and filamentous or mycelia forms. Besides this, in Whittaker’s
system algae have been separated in three kingdoms__ blue-green algae in
kingdom Monera, unicellular algae in kingdom Protista and multicellular algae
in kingdomPlantae.
A detailed study of the nucleotide sequence of tRNA in
the ribosomes of different types of cells revealed that there are distinctly
two different types of cells on prokaryotes. Thus now we know three kinds of
cells organization in the organisms _
Two prokaryotic and one eukaryotic. Accordingly Carl
Woese (1978) proposed a three Domain
system for the classification of organisms.
The rank of the Domain is above kingdom. The three Domains Woese recognized are
__ Eubacteria, Archaea (Archaebacteria)
and Eucarya. The Eubacteria includes
prokaryotes which contain peptidoglycan in their cell wall (e.g., gram positive
and gram negative bacteria, mycoplasmas).
The domain Archaea includes three kingdoms___the methanogens. extreme halophiles and
thermacidophiles. The methanogens are
strict anaerobes and have been isolate from divergent anaerobic environments
such as waterlogged soil-like sediments, marshes, marine sediments and
gastrointestinal tracts of animals including human. They produce methane from
carbon dioxide and hydrogen. The extreme
halophiles occure in highly saline
environments such as salt lakes, salt evaporation ponds and surface of salt
preserved food. These are obligate anaerobes. Thermoacidophiles grow in hot
acidic environments such as hot water springs, geothermally heated marine
sediments and submarine hydrothermal vents. They are obligate aerobes, facultative
aerobes or obligate anaerobes.
A comparison of the three cell types as proposed by
Carl Woese is given in Table 2.
Comments
Post a Comment