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Topic: Classification of Living Things
Lesson Objectives: At the end of the lesson, learners should be able to:
- State the characteristic features of the kingdoms and give specific examples of representative organisms,
- Explain binomial system of nomenclature.
Classification of living things deals with placing together in categories those living things that resemble each other. It involves placing living things into groups that have certain features in common which distinguish them from other groups.
The system of classification of living things used today is based on that introduced by a Swiss scientist called Car Von Linne (1707 – 1778). His name was latinized to Carolus Linnaeus. He published the classification of plants in 1753 and that of the animals in 1758.
Living Things are first split into kingdoms. The kingdoms are further split into a large number of smaller groups called Phyla (singular, Phylum) for animals and divisions for plants.
All the members of a phylum or division have certain features in common. Each phylum or division is broken down into classes. Each class is further broken down into orders, orders into families, families into genera (singular genus), genera into species.
The arrangement of living things in this hierarchy is summarised from the highest to the lowest level is summarised in figure 2.1
There are seven major groups used in classification of living things. These are:
The basic unit of classification of living things is the species. Species is the smallest unit containing members which have the largest number of features in common and usually interbred among themselves but a member of a species cannot interbreed with a member of another species. For example, all human beings belong to one species and all monkeys belong to a different species. So both human beings and monkeys cannot interbreed.
Binomial System of Nomenclature
Carolus Linnaeus also introduced a system of naming living things which is popularly used by biologists today. This system is called binomial system of nomenclature. In this system, each organism or living thing is given two names. Hence, the term binomial nomenclature. The first name is the generic name (common to the genus) and it always begins with a capital letter.
The second name is the specific name which begins with a small letter. These scientific names are written in italics or are underlined. Examples of scientific name of some organisms are given below;
i. Man. : Homo sapiens
ii. Lion : Panthera Leo
iii. Maize : Zea mays
iv. Rat : Rattus rattus
v. Dog : Canis domestica
Carolus Linnaeus classified all living organisms into two major kingdoms namely plant kingdom and animal kingdom. Under this arrangement, lots of one-celled organisms could not fit in properly.
Many biologists then decided to place all living things into five kingdoms. These kingdoms are;
In the classification of living things, virus specifically could not fit into any of the five kingdoms. As s result of this, it has to be treated separately.
Virius: This is a microscopic organism which cannot be seen by an ordinary microscope but an electronic microscope. It doesn’t have a cell structure but is just made up of a coiled strand of nucleic acid i.e Ribonucleic acid (RNA) or deoxyribonuclei acid (DNA) enclosed within a protein coat. Virus is seen as being on the borderline between living and non-living things. When outside the living cells, it forms a crystal and becomes non-living but within the cell, it replicates (produces) and becomes living organism.
The diagram below shows the structure of a virus:
Characteristics Of Virus
i. Virus is microscopic in nature
ii. It possess either RNA or DNA
iii. It cannot reproduce by binary fission.
iv. It does not have structures used in the synthesis of protein.
v. It doesn’t respire, feed, excrete, etc
vi. It is responsible for the causes of many chronic diseases like AIDS, small box, zl influenza and measles.
Virus As A Living (Animate ) Thing
Virus is often regarded as a living or animate thing because of the following reasons;
i. Virus can reproduce when present in another living cell
ii. It possess characteristics which can be transmitted from one generation to the next.
Virus as a non-living (Inanimate) Thing
Virus is often regarded as a non-living (inanimate) thing because of the following reasons;
i. When a virus is extracted from a living cell and place in a non-living medium, it assumes a crystalline form and thus becomes non-living.
ii. Virus cannot respire, excrete or respond to stimuli.
i. They are unicellar; though some form filaments of cells.
ii. The cells are prokaryotic
iii. The cells have no organised nucleus, with nuclear membrane.
iv. They do not have complex chromosomes
v. There is no sexual reproduction
Monera Kingdom: This is divided mainly into two Phyla namely;
Characteristics of Schizophyta
i. They are simple living things that belong to Monera.
ii. They have prokaryotic cells with no definite nucleus.
iii. They are microscopic and non-green plants.
iv. They reproduce by asexual means (binary fission)
v. They are unicellar organisms
vi. They lack mitochondria
vii. The cell has a rigid cell wall which is complex.
viii. They lack cellulose but consist of polysaccharide and amino acids.
Examples of organisms that belong to this phylum is bacteria.
Diagram of Bacterium: Wikimedia.org
Characteristics Of Cyanophyta
i. They are microscopic in nature
ii. They do not have cilia, flagella or other locomotive organelles but some move by gliding.
iii. They reproduce by cell division
iv. Their cells contain chlorophyll but not in chloroplast
v. Their cell walls do not contain cellulose like that of bacteria
vi. Examples of organisms in this group is the blue-green algae.
Fig 2.5 Blue-green algae
i. They are unicellar organisms
ii. The organisms are all eukaryotic
iii. They move either by cilia, flagella or may be amoeboid by nature
iv. Some of the Protista are heterotrophic while some are both heterotrophic and photosynthetic.
v. Mode of reproduction is usually asexual by mitosis while some have sexual reproduction by fusion of gametes. There are four phyla in this kingdom. They are;
Protozoa and Euglenophyta can be used as representative of this group.
Characteristics Of Protozoa
i. They belong to the group of organisms called protista
ii. They are microscopic organisms.
iii. They have eukaryotic cells, i.e cells with membrane.
iv. They reproduce asexually by binary fission
v. They are mainly aquatic organisms while few are parasitic
vi. The organisms in this group move by different organelles e.g Amoeba move by Pseudopodia, Paramecium move by cilia.
vii. Examples of Protozoa include Amoeba, Paramecium, Trypanosome, Plasmodium etc.
Diagram Of Paramecium
Diagram Of Amoeba
Diagram of Trypamosome
Characteristics of Euglenophyta
Euglena viridis is a Protista and a typical example of an organism sharing both the characteristics of plants and animals. However, it is a microscopic, unicellar and aquatic organism.
Diagram Of Euglena
Animal Characteristics of Euglena
The characteristics of Euglena which makes it an animal include:
i. Possession of flagellium used for movement
ii. Possession of gullet for passage of food
iii. Possession of contractile vacuole used for osmoregulation and Excretion
iv. Presence of eyespot which enables it to detect light intensity.
v. Possession of pellicle which makes its body flexible or give body definite shape.
vi. Presence of myonemes which do aid movement.
Plant charactersitics of Euglena
The charactersitics of Euglena which make it a plant include:
i. Possession of chloroplast which enables it to carry out photosynthesis
ii. Possession of pyrenoids where starch is stored.
iii. Presence of paramylum granules – forms in which starch is stored.
iv. It has holophytic (autotrophic) mode of nutrition.
The fungi were for a long time classified with the plants. They however differ from plants in the composition of their cell walls. Most of their cell walls are made up of chitin rather than cellulose.
Diagram of Rhizopus
i. They are eukaryotes (i.e cells with membrane)
ii. Some are unicellar e.g yeast while others are multicellular e.g mushroom
iii. They have no true roots, stems and leaves.
iv. They are non-green plants i.e they lack chlorophyll.
v. They are mainly saprophytes while others are parasites.
vi. They store excess food in form of glycogen
vii. The vegetative body parts are made up of fine and delicate threads called hyphae.
viii. They reproduce asexually by formation of spores and some sexually by conjugation.
ix. Examples of fungi are bread mould, rhizopus, mushroom, mucor, mildrews, yeast and toad stools.
x. They are mainly found in moist environments.
Done studying? See previous lessons in biology.
Take a quick test for this lesson.
- Use representative organisms to discuss the characteristics of each classes of living things.
- Explain binomial system of nomenclature.
Questions answered correctly? Kudos!
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https://commons.m.wikimedia.org/wiki File:Simple diagram of bacterium_(en).svg.
Turrill, W.B. (1938). “The Expansion Of Taxonomy With Special Reference To Spermatophyta”. Biological Reviews. 13 (4): 342–373.
Walker, P.M.B., ed. (1988). The Wordsworth Dictionary of Science and Technology. W.R. Chambers Ltd. and Cambridge University Press.