Biological Classification
The biological classification of plants and animals was first proposed by Aristotle on
the basis of simple morphological characters. Linnaeus later classified all living
organisms into two kingdoms – Plantae and Animalia. Whittaker proposed an
elaborate five-kingdom classification – Monera, Protista, Fungi, Plantae, and
Animalia. The main criteria of the five kingdom classifications were cell structure,
body organization, mode of nutrition and reproduction, and phylogenetic
relationships.
In the five kingdom classifications, bacteria are included in Kingdom Monera.
Bacteria are cosmopolitan in distribution. These organisms show the most extensive
metabolic diversity. Bacteria may be autotrophic or heterotrophic in their mode of
nutrition. Kingdom Protista includes all single-celled eukaryotes such as
Chrysophytes, Dinoflagellates, Euglenoids, Slime-moulds, and Protozoans. Protists
have defined nuclei and other membrane-bound organelles.
They reproduce
both asexually and sexually. Members of Kingdom Fungi show a great diversity
in structures and habitat. Most fungi are saprophytic in their mode of nutrition.
They show asexual and sexual reproduction. Phycomycetes, Ascomycetes,
Basidiomycetes, and Deuteromycetes are the four classes under this kingdom.
The plantae includes all eukaryotic chlorophyll-containing organisms. Algae,
bryophytes, pteridophytes, gymnosperms, and angiosperms are included in this
group.
The life cycle of plants exhibit an alternation of generations – gametophytic
and sporophytic generations. The heterotrophic eukaryotic, multicellular
organisms lacking a cell wall are included in the Kingdom Animalia. The mode of
nutrition of these organisms is the holozoic. They reproduce mostly in sexual
mode. Some acellular organisms like viruses and viroids as well as lichens are
not included in the five-kingdom system of classification.
Since the dawn of civilisation, there have been many attempts to classify
living organisms. It was done instinctively not using criteria that were
scientific but borne out of a need to use organisms for our own use – for
food, shelter, and clothing. Aristotle was the earliest to attempt a more
scientific basis for classification.
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.
In Linnaeus' time, a Two Kingdom system of classification with
Plantae and Animalia kingdoms was developed that included all plants
and animals respectively. This system was used till very recently. This
system did not distinguish between the eukaryotes and prokaryotes,
unicellular and multicellular organisms, and photosynthetic (green algae)
and non-photosynthetic (fungi) organisms.
Classification of organisms
into plants and animals was easily done and was easy to understand,
but, a large number of organisms did not fall into either category. Hence
the two-kingdom classification used for a long time was found inadequate.
A need was also felt for including, besides gross morphology, other
characteristics like cell structure, nature of the wall, mode of nutrition, habitat,
methods of reproduction, evolutionary relationships, etc.
Classification
systems for living organisms have hence, undergone several changes
over time. Though plant and animal kingdoms have been a constant
under all different systems, the understanding of what groups/organisms
be included under these kingdoms has been changing; the number and
nature of other kingdoms have also been understood differently by
different scientists over time.
KINGDOM MONERA
Spore Flagellum
Cocci Bacilli Spirilla
Vibrio Bacteria of different shapes
Bacteria are the sole members of the Kingdom Monera. They are the most
abundant microorganisms. Bacteria occur almost everywhere. Hundreds
of bacteria are present in a handful of soil. 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.
Though the bacterial structure is very simple, they are very complex
in behavior. Compared to many other organisms, bacteria as a group
show the most extensive metabolic diversity. Some of the bacteria are
autotrophic, i.e., they synthesize their own food from inorganic substrates.
They may be photosynthetic autotrophic or chemosynthetic autotrophic.
The vast majority of bacteria are heterotrophs, i.e., they do not synthesize
their own food but depend on other organisms or on dead organic matter
for food.
1. Archaebacteria
These bacteria are special since they live in some of the harshest habitats
such as extreme salty areas (halophiles), hot springs (thermoacidophiles)
and marshy areas (methanogens). Archaebacteria differ from other bacteria
in having a different cell wall structure and this feature is responsible for
their survival in extreme conditions. 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. Eubacteria
There are thousands of different eubacteria or ‘true
bacteria’. They are characterized by the presence of a
rigid cell wall, and if motile, a flagellum. The
cyanobacteria (also referred to as blue-green algae)
have chlorophyll similar to green plants and are
photosynthetic autotrophs. The
cyanobacteria are unicellular, colonial, or filamentous, and freshwater/marine or terrestrial algae. The colonies
are generally surrounded by a gelatinous sheath. They
often form blooms in polluted water bodies. Some of
these organisms can fix atmospheric nitrogen in
specialized cells called heterocysts, e.g., Nostoc and
Anabaena. Chemosynthetic autotrophic bacteria
oxidize various inorganic substances such as
nitrates, nitrites, and ammonia and use the released
energy for ATP production. They play a great role
in recycling nutrients like nitrogen, phosphorous,
iron, and sulfur.
KINGDOM PROTISTA
All single-celled eukaryotes are placed under Protista, but the boundaries
of this kingdom are not well-defined. What may be a photosynthetic
protistan’ to one biologist may be ‘a plant’ to another. In this book, we
include Chrysophytes, Dinoflagellates, Euglenoids, Slime molds, and
Protozoans under Protista. Members of Protista are primarily aquatic.
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. Some have flagella or
cilia. Protists reproduce asexually and sexually by a process involving
cell fusion and zygote formation.
1. Chrysophyte
This group includes diatoms and golden algae (desmids). They are found
in freshwater as well as in marine environments. They are microscopic
and float passively in water currents (plankton). Most of them are
photosynthetic. In diatoms, the cell walls form two thin overlapping shells,
which fit together as in a soap box. The walls are embedded with silica
and thus the walls are indestructible. Thus, diatoms have left behind a large amount of cell wall deposits in their habitat; this accumulation over
billions of years is referred to as ‘diatomaceous earth. Being gritty this
soil is used in the polishing and filtration of oils and syrups. Diatoms are the
chief ‘producers’ in the oceans.
2 Dinoflagellates
These organisms are mostly marine and photosynthetic.
They appear yellow, green, brown, blue, or red depending
on the main pigments present in their cells. The cell wall
has stiff cellulose plates on the outer surface. Most of
them have two flagella; one lies longitudinally and the
other transversely in a furrow between the wall plates.
Very often, red dinoflagellates (Example: Gonyaulax)
undergo such rapid multiplication that they make the
sea appear red (red tides). Toxins released in such large
numbers may even kill other marine animals such as
fish.
3 Euglenoids
The majority of them are freshwater organisms found in
stagnant water. Instead of a cell wall, they have a protein-rich layer called a pellicle which makes their body flexible.
They have two flagella, a short and a long one. Though
they are photosynthetic in the presence of sunlight, 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.
4 Slime Moulds
Slime molds 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 unfavorable 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.
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.
Amoeboid protozoans: These organisms live in freshwater, seawater, or moist soil. They move and capture their prey by putting out pseudopodia (false feet) as in Amoeba. Marine forms have silica shells on their surface. Some of them such as Entamoeba are parasites. Flagellated protozoans: The members of this group are either free-living or parasitic. They have flagella. The parasitic forms cause diseases such as sleeping sickness. Example: Trypanosoma.
KINGDOM FUNGI
The fungi constitute a unique kingdom of heterotrophic organisms. They
show a great diversity in morphology and habitat. When your bread
develops a mold or your orange rots it is because of fungi. The common
mushroom you eat and toadstools are also fungi.
White spots seen on
mustard leaves are due to a parasitic fungus. Some unicellular fungi,
e.g., yeast are used to make bread and beer. Other fungi cause diseases
in plants and animals; wheat rust-causing Puccinia is an important
example. Some are the source of antibiotics, e.g., Penicillium. Fungi are
cosmopolitan and occur in air, water, soil and on animals and plants.
They prefer to grow in warm and humid places. Have you ever wondered
why we keep food in the refrigerator?
Yes, it is to prevent food from going
bad due to bacterial or fungal infections.
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. Others have septae or cross walls in their
hyphae. The cell walls of fungi are composed of chitin and polysaccharides.
Most fungi are heterotrophic and absorb soluble organic matter from
dead substrates and hence are called saprophytes. Those that depend
on living plants and animals are called parasites. They can also live as
symbionts – in association with algae as lichens and with roots of higher
plants as mycorrhiza.
KINGDOM PLANTAE
Kingdom Plantae includes all eukaryotic chlorophyll-containing
organisms commonly called plants. A few members are partially
heterotrophic such as insectivorous plants or parasites. Bladderwort
and Venus fly trap are examples of insectivorous plants and Cuscuta is a
parasite. The plant cells have a eukaryotic structure with prominent
chloroplasts and cell walls mainly made of cellulose. You will study the
eukaryotic cell structure in detail in Chapter 8. Plantae includes algae,
bryophytes, pteridophytes, gymnosperms, and angiosperms. The 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.
This phenomenon is called the alternation of generation.
KINGDOM ANIMALIA
This kingdom is characterized by heterotrophic eukaryotic organisms
that are multicellular and whose 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 the 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 mechanisms. Most of them are
capable of locomotion.
VIRUSES, VIROIDS, AND LICHENS
In the five kingdom classification of Whittaker, there is no mention of some
acellular organisms like viruses and viroids and lichens. These are briefly
introduced here.
All of us who have suffered the ill effects of the common cold or ‘flu’ know
what effects viruses can have on us, even if we do not associate it with our
condition.
Viruses did not find a place in classification since they are not
truly ‘living’ if we understand living as those organisms that have a cell
structure. Viruses are non-cellular organisms that are characterized
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. Would you call viruses living or non-living?
The name virus which means venom or poisonous fluid was given by
Pasteur. D.J. Ivanowsky (1892) recognized certain microbes as causal
organisms 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 crystallized and crystals consist
largely of proteins.
They are inert outside their specific host cell. Viruses
are obligate parasites.
In addition to proteins, viruses also contain genetic material, which could
be either RNA or DNA. No virus contains both RNA and 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 bacteria) are
usually double-stranded DNA viruses. The protein coat
called capsid made of small subunits called capsomeres protects the
nucleic acid. These capsomeres are arranged in helical or polyhedral
geometric forms. Viruses cause diseases like mumps, smallpox, 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.
“Binomial nomenclature is the biological system of naming the organisms in which the name is composed of two terms, where, the first term indicates the genus and the second term indicates the species of the organism.”
Question on Biological Classification
Discuss how classification systems have undergone several changes over a period of time?
The classification systems have undergone several changes with time. The first attempt at classification was made by Aristotle. He classified plants as herbs, shrubs, and trees. Animals, on the other hand, were classified on the basis of the presence or absence of red blood cells. New organisms are constantly being discovered and for classifying them, a new system needs to be used. This is why the new classification system is constantly developed to classify organisms based on the similarity in characters.
Organize a discussion in your class on the topic – Are viruses living or nonliving?
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 called the capsid. The presence of nucleic acid (DNA or RNA) suggests that viruses are alive.
What are the characteristic features of Euglenoids?
Euglenoids have certain features like-
- They are unicellular protists.
- They are found in freshwater.
- They have a pellicle present- which is a protein-rich membrane.
- They lack a cell wall.
- They have two flagella on the anterior of the body.
- They can prepare their food with photosynthetic pigments called chloroplasts.
- In the absence of light, they acquire food as heterotrophs. They will then feed on other organisms for feed.
- They are the connecting bridge between plants and animals as they have featured both plants and animals.
- They have a contractile vacuole to regulate the fluid in the cytoplasm.
- They are usually red due to the presence of carotenoids.
- They attack their food and eat via phagocytosis in which the entrapment of food takes place by an outer membrane.
- They reproduce by longitudinal cell division, a form of asexual reproduction mode.
- They generate cysts that can withstand the drying period.
Give a brief account of viruses with respect to their structure and nature of genetic material. Also, name four common viral diseases
Viruses are sub-microscopic, infectious, nucleoprotein particles that can infect all living organisms. A virus consists of genetic material either in the form of RNA or DNA surrounded by a protein coat. 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. AIDS, smallpox, mumps, and influenza are some common examples of viral diseases.
What do the terms phycobiont and mycobiont signify?
They both are components of a lichen. Phycobiont signifies the algal component of lichen. On the other hand, mycobiont signifies the fungal constituent of a lichen. Lichens are symbioses between fungi (mycobionts) and photoautotrophic green algae or cyanobacteria (photobionts). Many lichens occupy large distributional ranges covering several climatic zones.
Plants are autotrophic. Can you think of some plants that are partially heterotrophic?
However, some insectivorous plants are partially heterotrophic. They have various means of capturing insects so as to supplement their diet with the required nutrients derived from insects, causing the proliferation of growth. Examples include pitcher plants (Nepenthes), Venus fly trap, bladderwort, and sundew plant.
Describe briefly the four major groups of Protozoa.
Protozoa are unicellular, heterotrophic, eukaryotic organisms comprising four organization types: amebae, flagellates, ciliates, and parasitic sporozoans. Protozoa can be divided into four phyla based on their locomotion: Mastigophora, Sarcodina, Ciliophora, and Sporozoa. Members of the phylum Mastigophora move about by using one or more whiplike flagella. The genus Euglena contains flagellated species.
For our purposes, there are only 4 groups of protozoa that will be covered here: these groups are separated by motility and cell structure.
- Amebas (representative: Ameba proteus)
- Flagellates (representative: Trypanosoma, Euglena)
- Ciliates (representative: Paramecium)
- Apicomplexa (representative: Plasmodium)
What is the nature of cell walls in diatoms?
Diatoms are algae that dwell in glass houses. They are the only organisms with cell walls made of transparent, opaline silica.
Classification of Organism
1. There are millions of organisms. It is impossible to study each individual separately. Classification means to categories organisms into different groups. The study of an individual in a group gives us an idea of the rest of the members of that group.
2. Linnaeus divides all organisms into two kingdoms - Planate and Animalia in his book “System of Nature”. The foundation of the modem classification system was laid in the line of classification system started by Linnaeus. Therefore Linnaeus is called the ‘Father of Taxonomy’. Due to the disputed positions of organisms like bacteria, viruses, fungi, and euglena, there is a need for reconsideration of the system of classification.