In this article we will discuss about the structure of bacterial cell. This will also help you to draw the structure and diagram of bacterial cell.
Bacteria are unicellular organisms. The unicell is exceedingly small in size. Little structural detail can be made out in such a small body with an ordinary light microscope.
The use of an electron microscope has revealed details of “fine structure” hitherto unknown (Fig. 18.3). Like other living plant cells, the bacterial cell comprises a cell wall and protoplast. External to the cell wall may be present a thin layer of slime.
1. The Slime:
It is a viscous or gelatinous substance secreted by the cell protoplast. The slime diffuses through the cell wall and deposits in the form of a thin extracellular, viscous layer external to the cell wall.
It is usually composed of polysaccharides or of polypeptides of one or two different amino acids. Under certain condition of growth the slime accumulates form a thick conspicuous layer around the cell wall.
It is called the sheath or capsule (A). The sheath protects the enclosed organism against desiccation and antibodies. The encapsulated pathogen bacteria can withstand phagocytosis.
Some bacteria produce diffuse slime which spreads into the environment and encloses bacterial colonies. This is known as loose slime. The bacterial cell with a capcule normally does not bear flagella.
2. The cell wall (Fig. 18.3A):
In the electron micrograph the cell wall is seen as a thin, sharply defined envelope around the protoplast. It ranges in thickness around 0.02µ. The cell wall is tough though flexible. The inert and somewhat rigid bacterial cell. It shows granular structure and lacks microfibrils.
(a) Structure of cell wall:
The bacterial cell wall proper is composed of 4 layers (Mazanec and Martinec). Of these two are of lower electron density. The other two layers are of higher electron density and thus darker.
The outer darker layer (L4) is wavy. Within it is the lighter layer of low electron density (L3). Next comes the inner dense or darker layer (L2) followed by the innermost layer of low electron density(L1). The L2 darker layer of higher electron density is considered to be mucopeptide.
(b) Chemical Composition:
It differs in different species. Basically it is composed of acetylglucosamine and acetylmuramic acid. They are present in equal amounts and form the main component of the bacterial cell wall.
The other compounds included are polysaccharides, aminoacids and in some species lipids. The polysaccharides which form a major fraction of the bacterial cell wall, in many cases, consist of aminosugars rather than simple sugars (glucose, galactose and mannose).
Cellulose and chitin are absent. Three species of bacteria, namely, Acetohacter xylinum, A. acetigenum and Zymosarcina venticuli are the only exceptions in which the cell wall contains cellulose.
Recently microbiologists have reported that the bacterial cell wall contains an aminoacid diaminopimellic acid.
It commonly occurs in the bacterial cell walls and those of some blue green algae. It is a polypeptide attached to muramic acid, a derivative of glucose.
3. The protoplast:
Within the cell wall is the living stuff of the bacterial cell. It is called the protoplast. The protoplast is a clear watery or slightly viscous substance. It is differentiated into the cell or cytoplasmic membrane (also called plasmalemma) cytoplasm and chromatin or nuclear body.
(a) The cytoplasmic membrane:
At the surface of the protoplast is differentiated a delicate, fine membrane known as the cell membrane or cytoplasmic membrane. It completely encloses the protoplast and lies close against the cell wall pressed by the turgor of the protoplast.
Chemically it consists of phospholipids with proteins and some polysaccharides. It is a differentially permeable membrane which controls the passage of dissolved substances in and out of the cell when active metabolism is going on in the protoplasm.
In addition, the cytoplasmic membrane serves as a matrix upon which many respiratory and other enzymes are organised. In Bacillus subtilis and other gram-positive bacteria the cytoplasmic membrane, at certain places, is seen to form infolding’s which in some cases form complex structures within the cytoplasm. These are called the mesosomes.
(b) The Cytoplasm:
It is a complex mixture of proteins, carbohydrates, lipids, minerals, nucleic acids and water. The latter form as much as 90 percent of the cell. Organic matter is in the colloidal state.
The non-living inclusions are the storage granules of volutin, glycogen, lipid globules or protein crystals. Sulphur and iron are also found in some bacteria. The cytoplasm shows no streaming movements and contains no obvious vacuoles.
It appears rather dense and highly refrigent under light microscope and is fairly homogeneous when free from inclusions. In electron micrographs the bacterial cytoplasm is granular and contains numerous minute, nearly spherical, hollow bodies called the ribosomes.
Ultrastructurally, the ribosome consists of subunits which are nearly spherical but of unequal sizes (Fig. 18.4). Each subunit is composed of roughly equal amounts of RNA and protein.
The ribosomes lie free in the cytoplasm but sometimes they occur in small groups (polyribosomes) held by a strand of messenger RNA. The ribosomes are the sites of protein synthesis.
Other cell organelles such as mitochondria, endoplasmic reticulum and dictyosomes have not been seen in the bacterial cytoplasm. The chloroplasts are absent and so is a definitely organised nucleus.
The pigments in the photosynthetic bacteria (green and purple bacteria) have long been considered to be diffused throughout the cytoplasm.
Recent investigations with the electron microscope have revealed the presence of numerous ultramicrosopic chromatophores in the cells of certain photosynthetic bacteria. In rare cases lamellar structure has been revealed.
(c) The Nuclear Apparatus (Fig. 18.2):
It is the area in which all the chromatin or genetic material (DNA) of the cell is concentrated. Usually it is centrally located in resting bacterial cells and is equivalent of the nucleus in other organisms.
However, it lacks the nuclear membrane and nucleoli. It does not divide by mitosis. Nucleoid or gonophore is the term applied to the nuclear body of bacteria. Lacking the bounding nuclear membrane, the shape of the nuclear body varies.
Often it is irregular, sometimes spherical or oval but elongated (rod-shaped) or dumb-bell-shaped in the dividing cells. The cell in the phase of rapid growth may contain one to four unclear bodies or nuceloids.
In Micrococcus cryophillus the nuclear material is scattered in the form of islands separated by cytoplasm (B) or it may be arranged in the form of a ring (A) enclosing islands of cytoplasm.
In contrast to the granular cytoplasm, the nucleoids appear as light on low density areas containing fine fibrils of DNA (genetic or chromatin material) variously interwoven.
The DNA in the nucleoid of Escherichia coli (Fig. 18.3) occurs in the form of a single, two stranded, thread-like molecule about 1,000 microns (=1mm) long. It is ring-shaped (circular) and tightly packed. No ends are seen.
The DNA molecule contains all the genetic information of the cell. It is often but erroneously referred to as a chromosome.
In its structure the bacterial cell resembles the cyanophycean (blue-green algae) cell in many respects and differs from the eukaryotic cells of other plants in the following respects:
(i) The nuclear membrane and nucleoli are absent in the nuclear body.
(ii) The mitochondria, dictyosomes and endoplasmic reticulum are lacking in the cytoplasm.
(iii) The nuclear body does not divide by mitosis.
(iv) The photosynthetic lamellae when present (phyotosynthetic bacteria) are not organised into chloroplasts.
(v) The cytoplasm shows no protoplasmic streaming movements and contains few, if any, vacuoles.
(vi) The cell wall is complex and multi-layered.
The cell with the above-mentioned characteristics is termed prokaryotic in contradistinction to the eukaryotic cells of plants and animals. The prokaryotic cells are characteristic of bacteria and blue-green algae only.
