Taxonomy

 Introduction

Classification is the orderly arrangement of organisms into groups based on their similarities. Simple criteria used are size, habitat or usefulness e.g. aquatic vs terrestrial plants. Classification can be in terms of either artificial classification or natural system of classification. 

Classification results due to the need to be able to identify organisms in their taxonomic groups, enable easy and systematic study of organisms and show evolutionary relationships of organisms.

Artificial classification: involves reliance on characteristics that relate to man and less of the innate/natural features of the organisms. For instance animals can be grouped as wild or domesticated depending on their usefulness to man.

Natural system of classification involves grouping organisms on the basis of possessing similar natural characteristics. Organisms belonging to the same group are believed to have originated from the same ancestor at some time during evolutionary history.

Taxonomy is the scientific study of classification and it was pioneered by Carolus Linneaus. A group of each classification is called taxon (pl. taxa). In the natural system, organisms can be grouped in a hierarchical system with the following taxa:

Kingdom 

Phylum (Division in plants)

Class

Order

Family

Genus

Species

 

In this system, the species is the most basic unit. A species is a group of organisms that can be interbreed to produce new fertile organisms. Members of a species are identical  in structure, physiology and behavior. Different species that are closely related form a genus. Genera that are closely related create a family. Related families entirely form an order. Orders that are similar form a class while similar classes exist as a phylum or division in the case of plants. Various phyla form a kingdom.

At the kingdom level, there are five classes: Kingdom Prokaryotae / Monera, Kingdom Protista, Kingdom Fungi, Kingdom Plantae and Kingdom Animalia.

 

Importance of the natural system of classification;

• Arrangement of all the knowledge available in an orderly and comprehensible manner.
• To predict the characteristics of organisms that they may not have seen.
• Provides organisms with names that have the same meaning all over the world.
• Understand evolutionary relationship between different organisms.

 

Kingdom classification

1. Kingdom Prokaryotae / Monera

This kingdom comprises of bacteria: they are unicellular and cannot be viewed by naked eyes.

The bacteria cell is enclosed by a cell membrane followed by a cell wall. For some species, the cell wall is surrounded by a capsule. 

It has no organized nucleus or nuclear membrane instead the nuclear material is a long tightly folded DNA forming a nuclear body. They have no membrane bound organelles.

 

 

Bacteria can be grouped into three classes based on their shape:

·         Spherical or ellipsoidal: cocci

Diplococci: pairs of cells

Streptococci: rows of cells bead like or chain like

Staphylococci: irregular clusters of cells

Sarcinae: cuboidal or pocket arrangement

·         Cylindrical  or rod: bacillus

Diplobacillus: pair of cells

Streptobacillus: chains

·         Spiral forms: the cell wound around one or more central axial fibrils. Examples: vibrio: comma shaped and spirilla, spiral like.

 

Economic importance of bacteria

Harmful bacteria

·         Some bacteria are harmful as they end up being disease causing. Such bacteria are said to be pathogenic such as cholera, typhoid, and tuberculosis.

·         Some bacteria are responsible for food spoilage and food poisoning.

Beneficial bacteria

·         Saprophytic bacteria decompose dead bodies to produce simple compounds such as carbon IV oxide, ammonia, and mineral salts.

·         Action of saprophytic organisms in making compost manure: where soluble inorganic materials are made from waste plant and animals.

·         Some bacteria live symbiotically in human gut manufacturing Vitamin K and B12.

·         Some cellulose producing bacteria live in the rumen of ruminants and caecum of herbivores such as horses.

·         Nitrogen fixing bacteria in root nodules of leguminous plants fix nitrogen to ammonia. Ammonia is used by plants to form proteins and nucleic acids. Nitrifying bacteria in the soil converts ammonia to nitrates that can be absorbed by the plants roots.

·         Fermentation of food products to produce products such as yoghurt, sour milk etc.

·         Through genetic engineering, there is production of industrial products such as enzymes, organic acids, hormones.

·         Treatment of sewage and in biogas production.

·         Manufacture of antibiotics.

2.      Kingdom Protista

Unicellular and some multicellular whose cells have an organized nucleus and membrane bound organelles.

2.1 Algae

·         They are autotrophic and contain chlorophyll.

·         Have a simple structure that lacks differentiation into roots, stems and leaves.

·         Some are unicellular and some multicellular.

·         Have a cellulose cell wall and reproduce sexually/asexually.

Examples of algae: Chlamydomonas, Euglena, Spigyra, diatoms, seaweeds.

Economic importance of algae

·         Forms the basic source of food for fish and all aquatic animals.

·         Some seaweed is consumed directly as food by humans in some countries.

·         Seaweeds are the source of agar used in the culturing microorganisms

·         Fossil diatom deposits form diatomite

 

2.2 Protozoa

·         Single-celled, heterotrophic and motile organisms.

·         Have a membrane bound nucleus and several organelles.

·         Can be classified according to the way they move: some use pseudopodia, cilia or flagella.

Examples of protozoa: amoeba, paramecium.

Economic importance of protozoa: can be disease causing / pathogenic.

 Quick question: Describe the process of phagocytosis in amoeba

3.      Kingdom Fungi

Comprises of simple plant like organisms that lack chlorophyll; they are heterotrophic as saprophytes or parasites. It consists of mushrooms, toadstools, yeast and molds.

3.1 Bread mold (Rhizopus and Mucor)

Molds are multi-cellular made up of branching system of filaments called hyphae. The whole structure appears as a white or colored mass called mycelium. Hyphae are enclosed by a cell wall made up of proteinaceous substance called chitin. Hyphae that lie on the surface are called stolons. The rhizoids anchor the mycelium to the substrate. Some hyphae grow vertically and form reproductive structures called sporangia. For both parasitic and saprophytic and parasitic molds the tips of rhizoids produce enzymes that digest the organic matter into soluble substances that are absorbed into the cytoplasm.

Quick question: Describe the mold structure

3.2 Yeast

Exists as a single cell surrounded by cell wall. The cytoplasm contains the nucleus, vacuole and food reserves. Reproduction is through asexual means by budding method. Some of the cytoplasm bulges out forming and which enlarges till it is almost equal to the size of the mother cell. Nuclear division occurs and cross wall is formed between two cells which then separate.

Quick question: budding in yeasts

Economic importance of fungi

·         Saprophytic fungi decompose dead bodies of animals and plants resulting in decay and recycling of nutrients.

·         Production of antibiotics e.g. penicillum

·         The root-fungus complex, mycorrhiza, enable absorption of nutrients.

·         Yeast is used in the making of alcoholic drinks and leavened bread.

·         Some fungi act as sources of food such as edible mushrooms.

·         Some are disease causing such as ringworms

·         Responsible for food spoilage e,g Aspergillus flavum producing aflatoxin

 

4.      Kingdom Plantae

They are multicellular organisms that contain chlorophyll which manufacture food using chlorophyll (autotrophic).

Kingdom plantae division	Characteristics
Bryophyta	Consists of mosses and liverworts.            Have no true roots.            The dominant plant is the gametophyte whereby the male gamete joins with the female gamete to form a sporophyte.            The sporophyte reproduces asexually to produce spores that are stored in the capsule.            Most are found in wet environments.
Pteridophyta e.g. ferns	Have well-structured vascular systems.            Highly differentiated into roots, stems and leaves but no flowers.           Leaves are subdivided into leaflets using fronds.            The stem is called the rhizome and is underground.            Spore forming bodies are called sporangia and are borne in clusters called sori.
Spermatophyta	Highly differentiated into roots, stems, leaves and reproductive parts.            Well-developed xylem and phloem            Photosynthetic            Produce seeds as means of reproduction and dispersal.

Spermatophyta is divided into gymnosperms and angiosperms.

Gymnosperms

·         Has five orders: coniferales, Cycadales, ginkgoales, taxales, gnetales.

·         Conifers: cedars, pines, fir, cypresses, spruce.

·         Reproduction is sexual.

Angiosperms

·         Reproduce asexually.

·         Can be described as herbs (small plants which never become woody i.e. become soft), shrubs (larger than herbs and the main stem divides into many branches close to the ground) and trees (tall, large woody plants).

·         There are two classes of angiosperms:

 

Monocotyledons	Dicotyledons
Leaves are parallel veined	Leaves are net veined
Embryo has one cotyledon	Embryo with two cotyledons
Vascular bundles in the stem scattered and has cambium	Vascular bundles in the stem arranged in a ring surrounding the pith and has cambium.
Flower parts occur in threes or multiples of threes	Flower parts in fours or fives or multiples of fours and fives
Fibrous system	Tap root system

 

 

5.      Kingdom Animalia

Invertebrates

•          Phylum Porifera
•          Phylum Cnidaria
•          Phylum Platyhelminthes
•          Phylum Nematoda
•          Phylum Annelida
•          Phylum Mollusca
•        Phylum Arthropoda: class insecta, class arachnida, class myriapod

Vertebrates

Phylum Chordata: Class Pisces, Class Amphibia, Class Aves, Class Mammalia (Egg laying, pouch mammals, Placental mammals).