Living World is classified into 3 division or domains, namely:
1) Bacteria (Eubacteria)
2) Archaea (Archae-bacteria)
Bacteria & Archaea are grouped separately under Prokaryotes.
Since, Viruses lack metabolic apparatus & considered non-living outside host cell, they are classified separately from the above divisions.
Bacteria – Escherichia coli, Lactobacillus, Mycoplasma, Cyanobacteria etc.
Archaea – Halobacterium, Methanobacteria, Pyrodictium etc.
Eukaryotes – Humans, Yeast, Plants etc.
Prokaryotic Cell: (pro = before; karyon = nucleus)
- In simple words, prokaryotic cells does not possess membrane bound nucleus & sub-cellular organelles.
- Organisms possessing prokaryotic cell are termed as prokaryotes & are single celled.
Eukaryotic Cell: (Eu = true; karyon = nucleus)
- In simple words, eukaryotic cells possess membrane bound nucleus & other sub-cellular organelles.
- Organisms possessing eukaryotic cells are termed as eukaryotes & can be single or multicellular.
- Single celled (Unicellular).
- Most numerous & widespread organisms on earth.
- They can derive energy from diverse sources.
- They can tolerate environmental extremes, like –
- Hot Springs (Temperature ≈100◦C) : Thermus aquaticus
- Salty Water (Dissolved salts ≥ 50g/L) : Archaeal or Bacterial Halophiles
- Prokaryotic cell size range: 0.1 to 10 μm in length or diameter.
Domains Bacteria & Archaea each are further sub-divided, based on type of habitats they live in:
- Aerobic Habitats:
- Have abundant oxygen supply.
- Residents (Aerobes) derive energy by transfer of electrons from fuel molecules to oxygen.
- Example: Mycobacterium tuberculosis (Bacteria), Halobacterium (Archaea) etc.
- Anaerobic Habitats:
- Devoid of proper oxygen supply.
- Residents (Anaerobes) obtain energy by transferring electrons to nitrate (forming N2), sulfate (forming H2S), or CO2 (forming CH4).
- Example: Escherichia coli (Bacteria), Pyrococcus (Archaea) etc.
- Anaerobes are further divided:
- Obligate anaerobes: Die when exposed to oxygen. Example: Peptostreptococcus (Bacteria), Methanogens (Archaea) etc.
- Facultative anaerobes: Able to live with or without oxygen. Example: Escherichia coli (Bacteria), Haloarchaea (Archaea).
Basic features shared by all prokaryotic cells:
- Plasma Membrane
1. Plasma Membrane: Encloses cell, regulate material in & out of cell. Separates cell cytoplasm from outside environment.
|S.No.||Component||Bacterial Plasma Membrane||Archaeal Plasma Membrane|
|1||Phospholipids||Lipids are attached to glycerol by ester linkages.||Lipids are attached to glycerol by ether linkages. (Ether linkages are more resistant to chemical attack and heat than are ester linkages).|
|2||Lipid layers||Hopanoids form lipid bilayer. Some bilayers have sterols.||Glycerol diethers form lipid bilayer; Glycerol tetraethers form lipid monolayer.|
2. Nucleoid: Irregularly shaped region in the cell where the chromosomes are located.
|S.No.||Component||Bacterial Nucleoid||Archaeal Nucleoid|
|1||Chromosomes||Most bacteria have circular double stranded deoxyribonucleic acid (dsDNA), but some bacteria have a linear chromosome.||All known archaea have circular, double stranded deoxyribonucleic acid (dsDNA).|
|2||Chromosome number||Mostly single copy present. Except few bacteria like Vibrio cholerae and Borrelia burgdorferi which have more than one chromosome.||Some archaea are polyploid, i.e. have more one copy of chromosome – number varying 10 to 25 chromosome copies.|
Enclosed inside plasma membrane. It has two components:
- Cytosol: Composed mainly of water. Contains ions, soluble micro- & macro- molecules, suspended particles/structures/molecules.
- Ribosomes: Prokaryotes have 70S sized ribosomes (S stands for Svedberg units) – made of 50S & 30S subunits. The 50S subunit contains 23S & 5S rRNA, whereas, 30S subunit contains 16S rRNA.
To be continued…