UNIT 5: INTRODUCTION TO CLASSIFICATION
Taxonomy is the study of classification of living organisms in taxonomic
levels called taxa (singular: taxon). In biological classification, these taxa
form a hierarchy. Each kind of organism is assigned to its own species, and
similar species are grouped into a genus (plural: genera). Similar genera are
grouped into a family, families into an order, orders into a class, classes into a
phylum (plural: phyla) and phyla into a kingdom. The hierarchy classification
starts from the largest group, the domain.
The eight levels of classification are known as taxa (taxon in singular),
these include: Domain, Kingdom, phylum, class, order, family, genus
and species. As one moves down the taxonomic hierarchy, it follows that
the number of individuals decreases but the number of common featuresincreases.
Three domains are used by biologists to divide organisms into three large
groups based on their cell structure. The domain is the highest taxon in the
hierarchy. The prokaryotes are divided between the domains Eubacteria/
Bacteria and Archaebacterial/ Archea, while all the eukaryotes are placed
into the domain Eukarya.
5.2.1. Domain eubacteria/ bacteria
domain bacteria include prokaryotic organisms as their cells do not have
defined, membrane-limited nuclei.
They are all microscopic that vary in size between 0.2 to 10 micrometers.
The characteristic features of bacteria are:
–– Cells with no true nucleus
–– DNA exists in circular chromosome and does not have histone proteins associated with it.
–– No membrane-bound organelles (mitochondria, endoplasmic reticulum, Golgi body, chloroplasts)
–– Contain mesosomes as infolding of membrane and acts as sites for respiration as they lack mitochondria.
–– Ribosomes (70 S) are smaller than in eukaryotic cells
–– Cell wall is always present and contains peptidoglycans in place of cellulose
–– Cells divide by binary fission– Usually exist as single cells or colonies.
The bacteria are important when they help to fertilize fields, to recycle
nutrients on earth, and to produce food and medicines. The bacteria that
live in soil recycle the nitrogen and carbon contained in the complex organic
molecules that remain in plants and animals after they have died. While
most bacteria is found in many disease, bacteria is very useful to our lives
because is found in the digestive system to help break down food.
a. Domain Archaea (Archaebacteria)
This contains bacteria that live in extreme environments where few other
organisms can survive, like in volcanic hot springs and black organic mud
totally devoid of Oxygen.
Types and economic importance
They are classified according to the environments they live in:
–– Methanogenic bacteria that live in habitats deprived of oxygen and give
off methane as a product of metabolism for example those that live in
the guts of ruminant animals such as cows.
–– Halophilic bacteria live only in water with high concentration of salt.
–– Thermoacidophilic bacteria tolerate extreme acid and temperature that
exceed boiling point of water and a pH below 2.They are autotrophic
producer for a unique animal community’s food chain.
b. Domain Eukarya
All the organisms classified into this domain have cells with true nuclei
and membrane-bound organelles. It include the four remaining kingdoms:
protists, fungi, plantae and Animalia. Their characteristic features are:
–– Cells with a nucleus and membrane-bounded organelles
–– linear DNA associated with histones arranged within a chromosome inthe nucleus
–– Ribosomes (80S) in the cytosol are larger than in prokaryotes, while chloroplasts and mitochondria have small ribosomes (70S ribosomes), like those in prokaryotes.
–– Chloroplast and mitochondrial DNA is circular as in prokaryote suggesting an evolutionary relationship between prokaryotes and eukaryotes
–– A great diversity of forms: unicellular, colonial and multicellular organisms
–– Cell division is by mitosis.–– Many different ways of reproduction including asexually and sexually.
5.3.1. Protoctista
This kingdom is made up of a very diverse range of eukaryotic organisms,
which includes those that are often called protozoans and algae. Living
things such as paramecium, amoeba, euglena, algae and plasmodia belongto the kingdom Protoctista.
The characteristic features of protoctists are listed according to the different
phyla due to their diverse range:
–– Rhizopus that have pseudopodia for locomotion. Example, amoeba.
–– Flagellates which are protoctista which move by using flagella. Example, Trypanosoma.
–– Sporozoans which are mainly parasitic organisms that reproduces by multiple fission. Example plasmodium.
–– Ciliates are protoctista which move with cilia. Example paramecium.
–– Euglenoid flagellates which are organisms with flagella but with a biochemistry quite distinct from that of flagellates. Example Euglena.
–– Green algae are photosynthetic protoctista with chlorophyll pigments. Example chlorella.
–– Red algae are photosynthetic protoctista with red pigment as well as chlorophyll. Example, chondrus
–– Brown algae which are photosynthetic protoctista with brown pigments as well as chlorophyll. Example Fucus and sea weed.
NB: Some protists are used in food industry. eg saccharomyces cerevisiae
( yeast). The plants protists produce almost one half of the oxygen on the
planet through photosynthesis. They participate in decomposition andrecycling of nutrients that humans need to live.
5.3.2. Fungi
Fungi are all heterotrophic, obtaining energy and carbon from dead and
decaying matter or by feeding as parasites on living organisms. There is avast range in size from the microscopic yeasts to macroscopic fungi.
–– Heterotrophic nutrition.
–– They use organic compounds made by other organisms as their sourceof energy and source of molecules for metabolism
–– Reproduce asexually by means of spores and sexually by conjugation.
–– Simple body form, which may be unicellular or made up of long threads called hyphae (with or without cross walls).
–– Large fungi such as mushrooms produce large compacted masses of hyphae known as fruiting bodies to release spores.
–– Cells have cell walls made of chitin or other substances.
NB: As economic importance some mushrooms are used as food,
saprophytic fungi such as mucor spp/Rhizopus spp are used in the curing
of tea and tobacco; the fungi decompose organic matter helping to clean the
environment and recycle materials.
5.3.3. Plantae
Plants are all multicellular photosynthetic organisms. They have complex
bodies that are often highly branched both above and below the ground.
Characteristic features of plants are:
–– Multicellular eukaryotes with cells that are differentiated to form tissues and organs.
–– Few specialized cells.
–– Cells have large and often permanent vacuoles for support with cell walls made of cellulose.
–– Autotrophic living organisms (most plants contain chlorophyll and store carbohydrates as starch or sucrose).
–– Usually plants are green
–– Roots ,stems and leaves
–– Sexual and asexual reproduction
NB: People depend upon plants to satisfy such basic human need as food,
clothing, shelter and health care.
5.3.4. Animalia
Animals are multicellular organisms that are all heterotrophic with differentmethods of obtaining their food.
Organisms in Animalia kingdom share the following features:
–– Multicellular (different types of specialized cells).
–– Eukaryotic
–– Heterotrophic (cells do not have chloroplasts and cannot photosynthesize,
although some, such as coral polyps have photosynthetic protoctistsliving within their tissues).
–– Cell vacuoles are small and temporary (for example lysosomes and
food vacuoles).
–– Cells do not have cell walls.
–– Sense organs (communication is by the nervous system)
–– Motile, at least for part of their life
NB: Many animals are helpful to humans; many varieties of livestock are
kept because they add protein to our diets in the form of meat, milk products,
and egg. Fiber bearing animals such as sheep provide material for making clothing
5.3.5. Monera
Organisms in this kingdom are unicellular, that do not have a nucleus. They
are prokaryotic. They are the smallest and simplest organisms. Some of
them stick together to form chains or clusters while others are single cells.
The figure below shows a typical structure of a bacterial cell which contains
all the main features of prokaryotes.
Although some of them are harmful in causing human diseases, others are
beneficial species that are essential to good health, as they are involved infood industry, medicine and in pharmacy.
NB: About economic importance of monera kingdom, many of Nostoc
species fix atmospheric nitrogen and thus increase soil fertility. They are
also important in the manufacturing and services industries ( eg production
of many dietary supplements and pharmaceuticals)
–– Presence or absence of the envelope: Plant viruses’ bacteriophage
are no enveloped while animal viruses like HIV and influenza virus are
enveloped.
5.4.2. Characteristics of viruses
Viruses are microorganisms whose structure is only visible with electron
microscopes. A typical virus consists of DNA or RNA within a protective
protein coat called capsid which provides protection. Viruses become active
in metabolism only once inside the host cell. When they infect cells, they
use biochemical machinery and proteins of the host cell to copy their nucleic
acids and to make proteins coats often leading to destruction of the host
cells. The energy for these processes is provided by the ATP from the host
cell. Because viruses do not consist of cells, they also lack cell membranes,
cytoplasm, ribosomes, and other cell organelles. Without these structures,
they are unable to make proteins or even reproduce on their own. Instead,
they must depend on a host cell to synthesize their proteins and to make
copies of themselves. Viruses infect and live inside the cells of living
organisms. They are also regarded as parasites since they depend entirely
on living cells for their survival. Although viruses are not classified as living
things, they share two important traits with living things: They have geneticmaterial, and they can evolve.
