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Concept of Classification

The Concept of Classification

Explain the concept of classification

Have you ever visited a library? How are the books arrangedand kept? Are they arranged randomly or systematically? Obviously the books are arranged systematically in the shelves. Science books are kept separately in their respective shelves. Science books are kept separately from social or commercial books. Biology books are separated from physics books or chemistry books. Likewise, in the shop, laboratory or pharmacy items are sorted and placed on the basis of their similarities.

In the world, there are numerous varieties of living organisms.These organisms do vary in size, structure, shape, habitat, modeof feeding and even mode of reproduction. The organisms can be sorted out and placed into different groups based on their similarities. The system of sorting out and placing organisms into different groups on the basis of their similarities and differences is called classification.

Living Things According to Their Similarities and Differences

Group living things according to their similarities and differences

Activity 1

Study the external features of the following organisms:Lizard, sugar cane plant, maize plant, shark, Tilapia, cactusplant, bat, cockroach, butterfly, hen, goat, toad, frog, spider andorange plant

Activity 2

  1. Sort out the above organisms into plants and animals
  2. Classify the animals on the following basis
  3. Those with wings and those without wings
  4. Those with tails and those without tails
  5. Those with antennae and those without antennae
  6. Terrestrial and aquatic organisms

The Importance of Classifying Living Things

Explain the importance of classifying living things

The following are importances of classification

  1. Classification makes the study of living things easy
  2. Classification makes communication easy among biologists from different parts of the world
  3. It provides good organized system in which a newly identified organism can be easily fitted in future.
  4. It makes it easier to identify organisms
  5. It can be used to predict characteristics that are present in the members of the same group.

Classification Systems

Types of Classification Systems and Their Differences

Outline types of classification systems and their differences

There are two types of classification

  1. Artificial classification
  2. Natural classification

Artificial classificationAn artificial classification is based on one or a few easilyobservable characteristics and usually designed for practicalpurpose with an emphasis on convenience and simplicity. In thistype of classification, unrelated organisms are often placed inthe same group while closely related organisms are often placedin different groups.Linnaeus included all

Linnaeus included all worm-like organisms in a single group,the vermes. This included wide a range of animals as fromnematodes, earthworms to snakes. This was an artificialclassification since it did not consider natural relationships suchas the fact that snakes have backbones and earthworms do nothave. Thus, snakes have more features in common withvertebrates than with worms.

Bats and birds would be placed in the same group because allhave the ability to fly since they posses wings. But bats sharemany features with mammals. They have features like hair,mammary glands, sweat glands and different types of teethwhich are characteristics of mammals. Therefore, bats are morerelated to mammals than birds.Natural

Natural system of classificationThis type of classification system tries to use natural relationshipsbetween organisms. It considers many features in commonincluding internal as well as external features.Similarity of embryology, morphology, anatomy, biochemistrycell, structure and behaviour are all considered relevant. It isbased on evolutionary relationship in which organismsbelonging to the same group are believed to have a commonancestor.

Characteristic features which show homology are distinguishedfrom those which show analogy. Example the fore limbs ofmammals, whales, birds and bats have the same basic patternand similar bone arrangement, i.e. homologous. This suggeststhat these organisms are coming from the same ancestor and thatcan be placed in the same group.

The wings of birds and grasshoppers are analogous. The wingsof birds have internal skeletons while the wings of grasshoppersdo not have bones but they perform similar function. Theseorganisms cannot be placed in the same group since they aredifferent in many ways.

Merits and Demerits of Each Type of Classification System

Explain merits and demerits of each type of classification system

Advantages of artificial system of classification

  1. It is easy to classify organisms since it is based on few observable characteristics.
  2. It does not take much time to classify organisms based on this system (not time consuming).
  3. It does not need expertise (even a layman can do).
  4. It is relatively stable i.e. not easily changing from time to time.

Disadvantages of artificial system of classification

  1. It tends to place closely related organisms into different groups instead of being grouped together e.g. a bat can be placed in a group of birds instead of mammals.
  2. Different or unrelated organisms may be placed in the same group e.g. bats placed in a group of birds, worms placed with snakes in the same group
  3. The system does not provide enough information about its members.
  4. It is difficult to incorporate additional information.

Advantages of natural system of classification

  1. Closely related organisms are placed in the same group.
  2. It reflects evolutionary relationships.
  3. Unrelated organisms cannot be placed in the same group.
  4. It makes it easy to incorporate additional information.

Disadvantages of natural system of classification

  1. It is difficult since it considers many features.
  2. It requires expertise i.e. more knowledge about an organism.
  3. It is time consuming.
  4. It is relatively unstable i.e. it changes from time to time.
  5. It is more expensive since more data are required.

Differences between natural classification and artificial classification

Artificial classification Natural classification
(i) Considers few features in common Considers many features in common
(ii) Does not reflect on evolutionary relationships Reflects on evolutionary relationships
(iii) It is easy to classify It is difficult to classify
(iv) Not time consuming It is time consuming
(v) Does not require expertise Requires expertise
(vi) New information cannot be added New information can be added.

Practical Activities of Classifying Living Things According to Artificial and Natural Classification Systems

Carry out practical activities of classifying living things according to artificial and natural classification systems

Activity 3

Collect the under named organisms and keenly examine their external and internal features:Bat, grasshopper, rat, earthworm, fish, ascaris, cockroach, lizard, hen, hawk, and toad.

Activity 4

Classify the organisms using artificial classification system.

Activity 5

Classify the organisms using natural classification systems.

Major Groups of Living Things

Major Groups of Living Things

Mention major groups of living things

The major groups of living things are the kingdoms. Previously, living things were categorized into two main groups; plantae and animalia kingdom. But this classification caused difficulties since some organisms seemed to posses some of the characteristics of both groups. For example, euglena is capable of feeding like an animal and locomote like an animal. Therefore, it is placed in animalia kingdom. But the same euglena has chlorophyll and it is capable of manufacturing its own food. Therefore, it should also be placed in plantae kingdom. Such an organism does not seem to fit exactly in animalia or plantae kingdom. Thus, euglenas are assigned in the major group of their own.

Currently, there are five major groups (kingdoms) of living things.

  1. Kingdom monera
  2. Kingdom protoctista
  3. Kingdom fungi
  4. Kingdom plantae
  5. Kingdom animalia

Ranks of Classification

Outline ranks of classification

In the millions of organisms found on earth some are more similar while others are less similar. For example, all human beings resemble each other more closely than they resemble with the chimpanzees. Humans have more resemblance with chimpanzee than cats and dogs, and more like dogs than birds.Therefore when classifying the organisms, groups are established which are called ranks or taxa.

There are seven ranks of classification

  1. Kingdom
  2. Phylum/division
  3. Class
  4. Order
  5. Family
  6. Genus
  7. Species

The highest rank of classification is the kingdom. The lowest rank of classification is the species. Every known organism has particular place in each group.

  1. Kingdom– this is the highest rank (taxon). It comprises of several related taxa. It comprises of many organisms than any other taxon.
  2. Phylum/division– this is the second largest rank of classification. It consists of several closely related classes.
  3. Class – members or this group have more characteristics in common than do members of division or phylum.
  4. Order– it consists of groups that are more alike than those in a class.
  5. Family –this is made up of groups that are more alike than those in the order. Wolves and cats are both in the order Carnivore but wolves are in the family Canidae while cats belong to the family Felidae.
  6. Genus – it consists of very similar species but members of different species cannot breed one another.
  7. Species – Species can be defined as a group of closely related organisms which are capable of interbreeding and produce fertile offspring.

It is the basic unit of scientific classification. Organisms that need to be placed in the same species must have the following characteristics:

  1. Must have many features in common.
  2. Must be able to breed one another to produce fertile offspring.
  3. Must be distinct and different from other organisms.

N.B: The term division is used by botanists instead of phylum when classifying plants or organisms related to plants.

Classification of some members of animals

Human Leopard Domestic cat
Kingdom Animalia Animalia Animalia
Phylum Chordata Chordata Chordata
Class Mammalia Mammalia Mammalia
Order Primates Carnivore Carnivore
Family Hominidae Felidae Felidae
Genus Homo Panthera Felis
Species sapiens padus catus

Scientific naming of living organisms

The scientific process of naming organisms is called nomen- clature. Biological nomenclature is based on the binomial system (double naming system) pioneered by the work of a Swedish naturalist Carl Linnaeus (1707-1778).

Biologists assign scientific names to organisms so as to avoid confusion among themselves since scientists from different countries use different languages. The scientific names are uniform in all languages.

Latin language is used in assigning the scientific name because it is an official language and that this language is no longer subject to changes; it is considered to be a dead language hence names once given remain unchanged.

Rules of binomial nomenclature

  1. Scientific names of organisms must be in Latin language and if the names are derived from other languages, they must be latinized.
  2. A scientific name of an organism has two parts, genus name and species name.
  3. A genus name always starts with a capital letter and a species name follows with a small letter.
  4. In typed scripts, a scientific name must be written in italics or underlined if hand written.
  5. A specific name is sometimes accompanied with the name of the author who first described and named the organism.
  6. When an organism is known by several names, the valid name is the one which was established after the work of Linnaeus.

Example 1

Examples of scientific names

Human being:Homo sapiens

Homo is the generic name and sapiens is the specific name.

Other examples of organisms with their scientific names

Common name Scientific name
Earthworm Lumbricus terrestris
Cockroach Periplaneta americana
Amoeba Amoeba proteus
Coffee Coffea arabica
Maize Zea mays
Bean Phaseolus vulgaris
Domestic cat Felis catus
Sisal Agave sisalana
Ashok tree Polyanthia longifolia
Housefly Musca domestica
Neem tree Azadirachta indica
Flamboyant (Christmas tree) Delonix regia
Tropical almond (mkungu) Terminalia catapa

Practical Activities of Grouping Organisms into their Respective Major Groups

Carryout practical activities of grouping organisms into their respective major groups

Activity 6

Group Organisms into their Respective Major Groups


Viruses were discovered by a Russian botanist D. I. Ivanovsky and a Dutchman Beijerink.In 1852 Ivanovsky prepared an infectious extract from tobacco plants that were suffering from mosaic disease. When the extract was passed through a filter able to prevent the passage of bacteria, the filtered fluid was still infectious. 1898 Beijerink gave the name “virus” (in latin means, “poison”) to describe the infectious nature of certain filtered plant fluids.

General and Distinctive Features of Viruses

Explain general and distinctive features of viruses

Characteristics of Viruses

  1. They are the smallest living organisms ranging from 20-30nm. On average, they are about 50times smaller than bacteria.
  2. Viruses do not have cellular structures, which mean that they lack certain important organelles like nucleus, cytoplasm, golgi bodies, etc.
  3. They can only reproduce inside the living cells hence they are parasitic.
  4. They have a simple structure consisting of either DNA or RNA but not both, surrounded by a protein or lipoprotein coat.
  5. They can be described as living or non-living.
  6. They are highly specific to their hosts i.e. each virus recognizes only certain types of cells.
  7. Viruses are capable of replicating themselves only when they are inside the host cell.

Viruses as living things

  1. They possess genetic material (RNA or DNA).
  2. They reproduce when they are in the host cell (replication).
  3. They are capable of identifying their hosts and attack them.
  4. They can undergo mutation (they mutate) i.e. they undergo different changes in shape.
  5. Like other parasites, they are specific to host.

Viruses as non-living things

  1. They can crystallize outside the host.
  2. They are metabolically inert in isolation.
  3. They are non-cellular i.e. they lack cell organelles.
  4. They do not perform necessary life processes such as respiration, excretion nutrition etc.

The Structure of Viruses

Describe the structure of viruses

Generally viruses have a very simple structure consisting of the following:

  1. RNA or DNA which may be single stranded or double stranded. They form a structure called core.
  2. A protective coat of protein surrounding the core called capsid.
  3. A nucleocapsid which is a combined structure of core and capsid.
  4. Envelope – an additional layer of lipoprotein layer around the capsid.
  5. Capsids are made up of identical repeating units known as capsomeres.


Bacteriophage is a virus that attacks and kills bacteria. Some of them have head with a tail sheath.


Advantages and Disadvantages of Viruses

Outline advantages and disadvantages of viruses

Advantages of viruses

  1. Viruses are used in developing vaccines, for example, vaccines for measles, polio and rubella are made from viruses that have been attenuated (weakened).
  2. Viruses are used as biological weapons to kill organisms.
  3. They are used as vectors in genetic engineering to transfer genes from one organism to another for improving or treating the defective genes.
  4. Bacteriophages are viruses that attack bacteria and hence they help in controlling infections and diseases.
  5. Viruses are used as biological weapons in wars and in biological pest control.

Disadvantages of viruses

  1. Most viruses cause diseases to both plants and animals. Plant disease such tomato mosaic, cassava mosaic and tobacco mosaic; and animal diseases e.g. measles, small pox, poliomyelitis and yellow fever are caused by viruses.

Kingdom Monera

This kingdom is made up of organisms known as bacteria (singular: bacterium). They are the most ancient and smallest organisms with a cellular structure. They are mainly single celled. Bacteria occupy many environments such as soil, dust, water and in the bodies of plants and animals

Some bacteria are found in hot springs where temperatures are 78ºC or more. Others can withstand very low temperatures. Some are found in very deep cracks in the ocean floor, at very high temperatures, about 360ºC (extreme thermophiles).

General and Distinctive Features of the Kingdom Monera

Explain general and distinctive features of the kingdom monera

General characteristics of the kingdom monera

  1. They are mainly unicellular and very small.
  2. They are all prokaryotic (nucleus not bound by membrane)
  3. They reproduce by binary fission.
  4. Some members of the kingdom are autotrophs while others are heterotrophs
  5. They have cell wall made up of protein material and sometimes lipids.
  6. Some bacteria form spores during adverse conditions i.e. extreme conditions e.g. high or low temperatures, drought etc. The spores allow them to survive as they have very thick resistant walls.
  7. Some are aerobes while others are anaerobes.
  8. The genetic material (DNA) is scattered in the cytoplasm and they lack internal membrane bound organelles such as mitochondria, chloroplasts, golgi bodies etc.

Structures of the Representative Organisms of the Kingdom Monera

Describe structures of the representative organisms of the kingdom monera

General structure of the bacteria

  1. Bacteria have strong and rigid cell walls due to the presence of murein. The wall prevents the cell from bursting when it absorbs much water (as a result of osmosis).
  2. Bacterial cells are bounded by partially permeable membranes.
  3. Bacteria possess capsules which are slimy or gummy.They have flagella which aid motility of the bacteria. Motile bacteria can move in response to a certain stimulus i.e. tactic movement. Flagella can easily be seen by electron microscope.
  4. They have small self replicating circle of extra DNA called a plasmid.

General structure of bacterium

Forms of bacteria

Bacterial shape is an important aid to classification. There are four main shapes

Coccus (plural: cocci)

They are spherical in shape and can be of the following types.

  • Micrococcus – exist singly. They cause sore throat.

  • Diplococci – this type of bacteria exist in pairs. The pneumococci (Diplococcus pneumoniae) are the only members. They cause pneumonia.

  • Streptococci – this type of bacteria stick together and form a chain. Most of them infect upper respiratory surface and cause diseases e.g. sore throat.

  • Staphylococci – These bacteria form a grape like bunch. They cause boils, pneumonia, food poisoning and other diseases.

Bacilli (singular: bacillus)

These are rod shaped bacteria. They can be:

  • Single rods, for example, Escherichia coli commonly living in the gut and Salmonella typhi which cause typhoid fever.

  • Rods in chain, for example, Azotobacter, a nitrogen-fixing bacteria, and Bacillus anthracis which cause the disease anthrax.

  • Bacilli with endospores showing various positions, shapes and sizes of spores. They can be:
  1. Central, not swollen e.g. Bacillus anthracis causes anthrax
  2. Spherical spore, terminal swollen e.g. Clostridium tetani causes tetanus
  3. Sub-terminal, swollen e.g. Clostridium botulism causes botulism.Spores may also be central

Spirilla (singular: spirillum)

These are spiral-shaped bacteria e.g. Treponema pallidum which causes syphilis



These are comma shaped bacteria, for example Vibrio cholerae.

Vibrio cholerae

The Advantages and Disadvantages of the Kingdom Monera

Outline the advantages and disadvantages of the kingdom monera

Economic Importance of Bacteria

In Agriculture, the bacteria are mostly important for following reasons

  1. Nitrogen fixation:Plants cannot trap nitrogen from the atmosphere but bacteria can fix nitrogen and change it into nitrogenous compounds. The phenomenon is called nitrogen fixation. The bacteria, which fix atmospheric nitrogen, are called nitrogen-fixing bacteria. Some of the nitrogen fixing bacteria are found freely in soil are called free-living bacteria, eg. Clostridium and Azotobacter. Some of them are found in root nodules making association with leguminous plants called symbiotic bacteria, eg. Rhizobium.
  2. Nitrification:Some bacteria convert ammonium compounds into nitrates in the soil. The process is called nitrification. The bacteria that take part in this process are called nitrifying bacteria, eg. Nitrobacter and Nitrosomonas. Nitrosomonas NitrobacterAmmonia—————>Nitrites————->Nitrates
  3. Ammonification:Some bacteria utilize proteins of dead bodies and convert it into amino acids. These amino acids are converted into ammonia by some bacteria. The process is called ammonification and the bacteria involve in the process are called ammonifying bacteria. Then the ammonia reacts with CO2 and H2O and gives ammonium carbonate which is absorbed by plants.
  4. Bacteria decompose dead bodies. They convert complex organic compounds into simple inorganic compounds. Therefore they’re called natural scavengers.
  5. Bacteria make the milk sour and produce flavor. They are responsible for coagulation of milk. E.g. Lacto bacillus.
  6. Bacteria convert sugary substances into alcohol, acids, acetones; etc the process is called fermentation.
  7. Bacteria also help to produce different types of enzymes like Amylase secreted from Bacillus, Protease from Bacillus, Streptokinase from Streptomyces.
  8. Bacteria are useful for vitamin production like Vitamin B (Cabalmin) is secreted from Pseudomonas, Vitamin B (Riboflavin) is secreted from Clostridium.
  9. They are important for antibiotic production like Terramycin from Streptomyces rimosus, Streptomycin from S. griseus, Neomycin from S. fradiae.
  10. They are also important to produce hydrogen commercially. During the process they ferment carbohydrate and hydrogen gas is produced.
  11. Bacteria decompose waste products.

Disadvantage of Bacteria

  1. Some of the species cause food poisoning. They secret some toxic chemical substances on out food stuff which cause food poisoning, eg Staphylococcus and Clostridium.
  2. Some are responsible for human diseases: Cholera: Vibrio cholera, Pneumonia: Staphylococcus pneumoniae, Diarrohea: Escherechia coli, Tuberclosis: Mycobacterium tuberclosis Leprosy: M. leprae, Meningitis: Nisseria meningitides.
  3. Some bacteria are responsible for plant diseases. Red stripe in sugarcane, Leaf streak in rice, black rot in cabbage and yellow rot in wheat are caused by Xanthomonas and Pseudomonas.

The Characteristics of Pathogenic and Non-pathogenic Bacteria

Outline the characteristics of pathogenic and non-pathogenic bacteria

Pathogenic bacteriaare bacteria that can cause infection. This article deals with human pathogenic bacteria.

Although most bacteria are harmless or often beneficial, several are pathogenic. One of the bacterial diseases with the highest disease burden is tuberculosis, caused by the bacteriumMycobacterium tuberculosis, which kills about 2 million people a year, mostly in sub-Saharan Africa. Pathogenic bacteria contribute to other globally important diseases, such as pneumonia, which can be caused by bacteria such asStreptococcusandPseudomonas, and foodborne illnesses, which can be caused by bacteria such asShigella,Campylobacter, andSalmonella. Pathogenic bacteria also cause infections such as tetanus, typhoid fever,diphtheria,syphilis, and leprosy. Pathogenic bacteria are also the cause of high infant mortality rates in developing countries.

Nonpathogenic organismsare those that do not cause disease, harm or death to another organism and is usually used to describe bacteria.It describes a property of a bacterium – its ability to cause disease. Most bacteria are nonpathogenic. It can describe the presence of non-disease causing bacteria that normally reside on the surface of vertebrates and invertebrates as commensals. Some nonpathogenic microorganisms are commensals on and inside the body of animals and are called microbiota. Some of these same nonpathogenic microorganisms have the potential of causing disease, or being pathogenic if they enter the body, multiply and cause symptoms of infection.

Immunocomprised individuals are especially vulnerable to bacteria that are typically nonpathogenic but because of a compromised immune system, disease occurs when these bacteria gain access to the body’s interior. Genes have been identified that predispose disease and infection with nonpathogenic bacteria by a small number of persons.Nonpathogenic colistrains normally found in the gastrointestinal tract have the ability to stimulate the immune response in humans, though further studies are needed to determine clinical applications.

Kingdom Protoctista

General and Distinctive Features of the Kingdom Protoctista

Explain general and distinctive features of the kingdom protoctista

This Kingdom comprises of unicellular and simple multicellular organisms whose cells have organized nucleus and membrane bound organelles. It includes the algae and protozoa.

Algae have chlorophyll hence make their own food through the process of photosynthesis.Protozoa are heterotrophic taking in ready-made food.

Other organisms, which are included in this Kingdom are red algae, Brown algae, Amoeba, Euglena, plasmodium, trypanosome and paramecium.

General features of Kingdom Protoctista

  • Some are autotrophs, for example Euglena while others are multicellular.
  • They are all eukaryotes with most of them having locomotery structure.
  • Most of them live in moist places or in water.

Distinctive features

  • Most are unicellular eukaryotes
  • They reproduce by binary fission

Phyla of the Kingdom Protoctista

Mention phyla of the kingdom protoctista

The phyla of kingdom protoctists include the following:

  1. Euglenophyta e.g. Euglena
  2. Rhizopodia e.g. Amoeba
  3. Zoomastigma e.g. Trypanosoma
  4. Apicomplexa e.g. Plasmodium
  5. Oomycota e.g. White root
  6. Chlorophyta e.g. Spyrogyra
  7. Cilliaphora e.g. Paramecium
  8. Rhodophyta e.g. Red algae

Structure of Amoeba and Paramecium

Describe structure of amoeba and paramecium

Amoeba belong to phylum Rhizopoda, organisms with the following extra features:

  • They are eukaryotes
  • Unicellular
  • Parasitic
  • They move using pseudopodia

Pseudopodia are projection of the cytoplasm that extend and pull the amoeba forward or engulf food particles.

The Advantages and Disadvantages of Amoeba, Euglena Paramecium and Plasmodiu

Explain the advantages and disadvantages of amoeba, euglena paramecium and plasmodium

Advantages of Paramecium:Are eaten by small water animals

Disadvantages of Paramecium:Causes diseases of bulanterdium eoli destroying the lining of intestines

Osmoregulation:Paramecium has two contractile vacuoles and each is associated with permanent system of collecting channels, which empty, into the main vacuole.

Feeding:Paramecium feed on bacteria. These are obtained from the surrounding water by the beating of the cilia-lining o the oral grove.

Reproduction:Paramecium reproduces both asexually and sexually. The asexual method is more common and it is binary fission. Sexual method is called conjugation.

Movementin paramecium is caused by cilia beating. This movement is called cilliary movement.


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