Classification of living organisms

• The modern classification of five Kingdoms system of living organisms, according to Whittaker (1969), classify the living organisms to five kingdoms:

Living Organisms

Monera ProtistaFungi Plantae Animalia

KINGDOM MONERA Two groups were included in Kingdom Monera: Eubacteria (true bacteria) and

Cyanobacteria (blue-green algae). Some texts consider these groups as subkingdoms and others consider them as divisions of Kingdom Monera. Bacteria are unicellular organisms which lack chlorophyll. They possess certain biological properties and they multiply by binary fission. Members of Cyanobacteria reproduce in the same way and thus bacteria and blue greens are grouped under this kingdom due to this primitive type of reproduction. This kingdom includes all primitive forms of organisms by being very simple. This simplicity is due to :

a) Nucleus absent, or present in a primitive form lacking a nucleolus and a nuclear membrane .

b) Absence of sexual reproduction.

c) Absence of motile reproductive units.

d) Reproduction is mainly by fission which is a very primitive method.

Eubacteria

• Bacteria (singular, bacterium), which belong to group Eubacteria are small, relatively simple, single-celled organisms. They have existed on earth longer and are more widely distributed than any other organisms. They are found in almost every imaginable habitat: in air, soil, water, in extreme temperatures, and in harsh chemical environments. Some bacteria can be photosynthetic, using H2S rather than H2O, as a source of electrons, but most are heterotrophic, absorbing nutrients from the surrounding environment.

Structure of bacterial cell • In true bacteria, the cell consists of cytoplasm, a nucleus and the wall . Chemically the cell wall is

constituted mainly of amino acids, sugars, and lipids. These substances are joined together to form the complex polymeric substances (proteins, carbohydrates and lipids). Cell wall was found to consist of 3 layers, the outermost being lipoprotein, the middle being lipopolysaccharide, and a rigid innermost layer of mucopolymers.

• Some bacteria are able to accumulate on their cell surface a thick slime layer known as a capsule which protects the cell. The flagella which are thin hair-like appendages originate from the cytoplasm just beneath the cell wall. Chemical analysis of flagella shows that it is composed of protein called flagellin. Many bacteria possess filamentous appendages other than flagella which are called fimbriae. These are shorter and more numerous than flagella.

• The cytoplasmic membrane (plasma membrane) lies immediately beneath the inner surface of the cell wall. It is composed of lipoprotein and functions as osmotic barrier. The cytoplasm contains proteins, carbohydrates, lipids, mineral compounds and other substances such as lipoprotein bodies and RNA, oxidizing and reducing enzyme system and mitochondria are also found. A vacuole containing cell sap appears only when the cell becomes old.

• Bacterial cells do not contain the nucleus characteristic of the cells of higher plants and animals. They do , however , contain bodies within the cytoplasm that are regarded as a nuclear structure DNA (primitive nucleus). It may assume various shapes of which spherical, elongated, dumbbell or irregular shapes can be mentioned. There is no evidence of nuclear membrane separating the nuclear substance from the cytoplasm.

• Bacteria are called "prokaryotes", from the Greek for "prenucleus". Their one distinguishing characteristic is that they do not contain membrane bound "organelles". Particularly their genetic material is not bound by a nuclear envelope. Bacteria do not have chromosomes. Instead, their genetic material is a single circular loop of DNA (Fig. 1). They reproduce by the process of "binary fission", where the cell duplicates its components and divides into two cells. In other words, the cell pinches into two cells without the complex movement of chromosomes seen in mitosis. Newly produced cells usually become independent, but they may remain attached in linear chains or grapelike clusters. In favourable environments, individual bacterial cells rapidly proliferate, forming colonies consisting of millions of cells.

Fig. 1. Typical prokaryotic cell (non-photosynthetic bacterial cell).

Structure in points: The structure of most eubacterial cells is simple. Many eubacteria have layer called a capsule which protect the cell . Then they have outer cell wall and plasma membrane. Cell wall is made up of three layers:

Outermost Lipoprotein Middle Lipopolysaccharide Rigid innermost mucopolymers

Plasma membrane is composed of Lipoprotein and act as osmotic barrier

Cell is filled by cytoplasm Cytoplasm contains

Ribosomes Circular loop of DNA called plasmids. Carbohydrates, proteins, lipids, enzymes and other compounds