Prokaryotic cell Notes

Introduction:

• A prokaryotic cell is a primitive type of cell in which genetic material is not organized in the form of a nucleus but instead lies freely in a naked super-coiled state in the cytoplasm where it is known as the nucleoid. Prokaryotic cells are characterized by their fast proliferation.

• All prokaryotic cells lack a nucleus and other membrane-bound organelles.

• Most prokaryotic cell ranges between 1_10micro meter but they can vary in size from 0.2 micrometers to 750 micrometers (Thiomargarita namibiensis). They belong to two taxonomic domains which are bacteria and archaea.

• Most prokaryotes are unicellular exceptions are myxobacteria which have multicellular stages in their life cycle.

• Prokaryotes lack cell organelles they harbor few internal structures such as the cytoskeleton, ribosomes, etc. In some groups of prokaryotes, membranous organelles are present such as vacuoles.

Characteristics of Prokaryotic Cells:

1. Nuclear components: Nuclear envelope, nucleoplasm, nucleolus, and histone covering of chromatin are absent.

2. Nuclear Material: DNA is naked lies variously coiled in the cytoplasm. It is often called Genophore, nuclear body, or nucleoid. Many prokaryotes also have an additional small circular DNA entity called a plasmid. It carries additional specific factors like nitrogen fixation, resistance, and fertility, etc.

3. Cell Wall: It may be found in bacteria and cyanobacteria. A cell wall is absent in the mycoplasm Or PPLO.

4. Flagella and Fimbriae: Flagella are present in some bacteria only. In some bacteria,non-motile appendages are called pilli Or Fimbriae. They take part in the attachment (Neisseria gonorrhoeae) and conjugation(Escherichia cells).

5. Respiratory enzymes: They usually lie in contact with the cell membrane.

6. One-Envelope system: In a prokaryotic cell, membrane-bound cell organelles are absent so that there is a single membrane in addition to the cell. Hence, prokaryotic cells have a single-membrane or one-Envelope system.

7. Photosynthetic thylakoid: Blue-green algae and some bacteria are photoautotrophic. Their photosynthetic thylakoid lies freely in the cytoplasm. They are not organized into a chloroplast.

Structure of Bacteria Cell:

On the basis of shape, bacterial cells are of the following types:-

1. Coccus: These bacteria are spherical in shape or ovoid. -

2. Bacillus: Rod-Shaped bacteria.

3. Spirillum: Coil or helical.

4. Vibrio: Comma shape bacteria.

Components of Bacterial Cell:

A bacteria cell consists of the following components:-

1. Cell Envelope:

It is the outer covering of the protoplasm of the bacterial cells. Cell envelope consists of three components:

(a).

Gycocalyx: It is the outermost mucilage layer of the cell envelope which consists of non-cellulosic polysaccharides with or without proteins. Glycocalyx may occur in the form of:

a. Loose sheath

b. Capsule: Thick, tough, mucilage covering.

• Glycocalyx gives a sticky character to the cell.

Function:

• Protection from phagocytes

• Attachment

• Protection from viruses.

(b). Cell Wall:

• It is a rigid, solid covering that provides shape and structural support to the cell.

• The cell wall is located between the plasma membrane and the glycocalyx.

• Between the plasma membrane and the cell wall, there is periplasmic space.

• The cell wall protects the bacterial cells against bursting in a hypotonic solution.

(c). Plasma membrane:

• It is the selectively permeable covering of the cytoplasm that forms the innermost component of the cell envelope. Made up of phospholipid bilayer with protein of various types (extrinsic, integral, transmembrane).

• The bacterial membrane is metabolically active as it takes part in respiration, synthesis of lipids, and cell wall components.

2. Cytoplasm:

• It is a crystal colloidal complex that forms the protoplasm excluding its nucleoid.

• Various structure present in the cytoplasm are as follow:

i. Mesosomes: Invagination of the plasma membrane.

Mesosomes is of two types-

a. Septal mesosomes: It connects nucleoid with the plasma membrane. It takes part in the replication of nucleoids by providing points of attachment to the replicated ones.

b. Lateral mesosomes: It is not connected with nucleoid. It contains respiratory enzymes and is often called chondroid.

ii. Ribosomes: They are small membraneless, submicroscopic, ribonucleoprotein entities having sizes 20 nm * 14-15 nm.

Ribosomes are of two types :

* Fixed ribosome- Attached to the plasma membrane

* Free ribosome- Occurs free in the cytoplasm

• The ribosome has two subunit lager 50S and smaller 30S

• Ribosome takes part in protein synthesis.

Polysome/Polyribosome: Some ribosome generally occurs in helical groups.

Note: Free or Matrix ribosome synthesized protein for intracellular use while fixed ribosomes synthesize protein for transfer to outside.

iii. Chromatophores: They are internal membrane system of photosynthetic forms which passes photosynthetic pigments such as bacteriochlorophyll, bacteriopheophytin, and carotenoids.

3. Nucleoid:

It represents prokaryotic genetic material.

Several alternative names of nucleoid are-

• Pro chromosome

• Genophore

• Incipient nucleus

• Chromosome

•• Nucleoid consists of a single circular strand of DNA duplex which is supercoiled with the help of RNA and polyamines.

•• Nucleoid may be directly attached to the plasma membrane or through the mesosomes.

4. Plasmid:

• They are self-replicating extrachromosomal segments of double-stranded, circular, naked DNA.

• They provide unique phenotype characters to bacteria.

• Some of them contain important genes such as fertility factors, resistance factors, etc.

• Plasmids are used as vectors in genetic engineering.

5. Inclusion Body:

• They are non-living structures present in the cytoplasm. The inclusion body may occur freely inside the cytoplasm.

• They stored the material in the form of inclusion bodies such as

• Phosphate granules

• cyanophycean granules

• Poly-beta hydroxybutyrate granules

• sulfur granules.

Inclusion bodies are of three types:

1. Gas vacuole

2. Inorganic inclusion

3. Food reserve

6. Flagella:

• Bacterial flagella are unistranded, equivalent to a single microtubular fiber.

• It is about 20 nm in diameter and 1-7 micrometer in length.

The bacterial flagellum is made up of three parts:-

1. Basal body

2. Hook

3. Filament

7. Pilli and Fimbriae:

Pilli: Long, fewer, and thicker tubular outgrowth which developed in response to fertility factor (F+) in Gram-Positive Bacteria.

• Made up of protein pilin.

• Being long they are helpful in attaching to a recipient cell and forming a conjugation tube.

Fimbriae:

• Small bristles like fibers sprouting from the cell surface in large numbers.

• Fimbriae are involved in attaching bacterial to a solid surface or host tissue.