Cell and Prokaryotic Cells

    Definition

• Cell is the basic unit of life.
• Cell is the fundamental structural and Functional unit of living organism.
• Anton von Leeuwenhoek 1st saw & describe a live cell.
• Robert brown later discovered the nucleus.
• Unicellular organisms are capable of independent existence and performing the essential function of life.

      Cell Theory

• Cell theory given by Matthias Schleiden (German botanist) and Theodore Schwann (British zoologist).
• But this theory did not explained as to how new cells were formed.
• In 1855, Rudolf Virchow first explained that new cells are formed from pre-existing cells (Omnis cellula-e cellula).
• Rudolf Virchow modified the hypothesis of Schleiden and Schwann to give the cell theory a final shape. 
• Cell theory as understood today is:-

1. All living organisms are composed of cell & product of cells.
2. All cells arises from the pre-existing cells.

   Prokaryotic Cells

• Prokaryotic cells are represented by bacteria, BGA (blue green algae), mycoplasma and pleuro pneumonia like organism (PPLO).
• Bacteria is classified into 2 groups (on the basis of the manner they respond to staining procedure- 

1. Those bacteria that take up the gram stain are Gram positive and 
2. The others that do not take up the gram stain are called Gram negative bacteria.

• Prokaryotes are generally smaller and multiply more rapidly than the eukaryotic cells.
• Prokaryotic cells lack membrane bound nucleus.
• They may vary greatly in shape and size.
• The 4 basic shapes of bacteria are-

1. Bacillus (rod like), 
2. Coccus (spherical), 
3. Vibrio (comma shaped) & 
4. Spirillum.

• Prokaryotes (all) have a cell wall surrounding the cell membrane except in mycoplasma.
• There is no well defined nucleus.
• The genetic material is basically naked, not enveloped by a nuclear membrane.
• In addition to the genomic DNA (single chromosomes & circular DNA) many bacteria have small circular DNA outside the genomic DNA.
• These smaller DNA are called Plasmids.
• The Plasmid DNA confers certain unique phenotypic characters to such bacteria (ex- resistance to antibiotics).
• These plasmid DNA is used to monitor bacterial transformation with foreign (outside) DNA.
• Plasmid (double stranded) have the ability to replicate independently.
• Plasmid term given by Lederberg.
• A specialized differentiated form of cell membrane called mesosome (characteristics of prokaryotes).
• Mesosones are essentially infolding of cell membrane.
• Mesosome is formed by the extension of plasma membrane into the cell.
• These extension are in the form of vesicles, tubules & lamellae.
• In cyanobacteria there is other membranous extensions into the cytoplasm called chromatophore (contains pigment).

  Mesosomes Function

1. Cell wall formation.
2. DNA replication.
3. DNA distribution to daughter cell.
4. Respiration.
5. Secretion process.
6. Increase surface area.
7. Increase enzymatic content.

Bacterial cell

• It may be motile or non motile.
• Motile bacteria have thin filamentous extensions from their cell wall called flagella.
• Bacteria show a range in the number & arrangement of flagella.
• Flagella of bacteria consist of 3 parts;-

1. Filament (it is the longest portion & extends from the cell surface to the outside).
2. Hook & 
3. Basal body.

• Pili (it is elongated tubular structures made of a special protein called pilin protein) & Fimbriae (it is smaller bristle like fibres sprouting out of the cell) are also surface structures of bacteria but pili & fimbriae do not play a role in motility.
• Pili (sex pili) helps in DNA transfer.
• Some bacteria attach to rocks in streams and also to the host tissues with the help of fimbriae.

Inclusion bodies

• Prokaryotes have something unique and I form of inclusion bodies.
• In prokaryotes reserve material are stored in the cytoplasm in form of inclusion bodies.
• Inclusion bodies are not bounded by any membrane and they lie free in the cytoplasm.
• Examples- Phosphate granules, Cyanophycean granules & glycogen granules.
• Gas vacuoles (provide buoyancy) are found in BGA, Purple green, Green photosynthetic bacteria.

Cell Envelope 

• Most of the prokaryotic cells (especially bacterial cells), have chemically complex cell envelope.
• This envelope consists of a tightly bound 3 layered structure (they act together as a single protective unit) that is;

1. Glycocalyx (outermost)- 

• it differs in composition & thickness in different bacteria.
• There is two layer one is slime layer (loose sheath) and second is capsule (which is thick and tough).

       2. Cell wall- 

• It determine the shape of cell & cell wall also provides a strong structural support to prevent the bacterium from bursting or collapsing.

      3. Plasma membrane-

• It is semi-permeable in nature & interacts with outside world.
• Plasma membrane is similar structurally to that of the eukaryotes.