Biological Classification


  • Archaebacteria and Eubacteria are the 2 major groups of Monerans. Bacteria and Cyanobacteria are the two types of Eubacteria. Mycoplasma, rickettsiae and actinomycetes are some other groups of Monerans.
  • Basically, they are unicellular with Peptidoglycan cell wall containing 10S ribosomes, which multiply through binary fission.
  • Bacterial cells are of many types on the basis of shape
    (i) Coccus – Spherical / ovoid
    (ii) Bacillus – Cylindrical / rod
    (iii) Spirillum – Cork-screw
    (iv) Vibrio – Comma
    (v) Stalked / budding
  • Based on the reaction of bacteria to Gram’s stain, bacteria can be grouped into either Gram (+) or Gram (−ve). Gram (+ve) bacteria retain blue (or) purple colour. Gram (−ve) bacteria retain safranin.
  • The various form of flagellation in bacteria are as follows
    (i) Atrichous – Flagella absent
    (ii) Monotrichous – Single flagellum
    (iii) Amphitrichous – Flagellum at each end
    (iv) Lophotrichous – A group of Flagella at one end
    (v) Peritrichous – Flagella distributed all over the body
    (vi) Cephalotrichous – A tuft of flagella at each ends.
  • Bacteria has its outermost covering part as cell envelope that contains 3 components glycocalyx, cell wall and cell membrane
  • Glycocalyx if occurs as a loose sheath, will be called a slime layer. But if it is thick and tough, the layer is called a capsule.
  • The cell wall of gram-negative bacteria is 2 layered with 20-30% lipid content that occurs in a wavy format.
  • Gram (−ve) bacteria contain LPS (Lipopolysaccharide) which gives resistance to it against antibiotics and are harmful than Gram (+ ve)
  • Gram (+ve) bacteria have a single-layered cell wall with no outer membrane.
  • Gram (+ve) bacteria do not contain LPS but they possess teichoic acid (forms surface antigens)
  • Peptidoglycan cell wall layer possesses the repeated units of NAG (N-Acetyl Glucosamine) and NAM (N- Acetyl Muramic acid), cross-linked with small peptide chains.
  • Exception: In Mycobacterium and Nocardia, the cell wall possesses mycolic acids, which are long chain fatty acids.
  • The plasma membrane is made of Phospholipids that act as a selectively permeable membrane which is metabolically active that plays a role in respiration, lipid synthesis and cell wall synthesis.
  • Mesosomes are a villiform specialization of a cell membrane, that is of 2 types: septal and Lateral mesosome.
  • Septal mesosome helps in the formation of septum during cell division, whereas lateral mesosome (chondroid) contains respiratory enzymes.
  • Ribosomes generally occur in groups called Polysomes which help in translational amplification
  • Photoautotrophic bacteria contain chromatophores, which are called chlorosomes in green bacteria.
  • The genetic material of prokaryotes is termed as Nucleoid, Incipient nucleus, genophore, Prochromosome (or) chromoneme and it is a circular DNA. It is also called naked as it doesn’t have histone protein.
  • Plasmids are extrachromosomal circular DNA (double-stranded) which impart certain phenotype to bacteria (eg. Fertility factor, resistance, etc.,). Sometimes they get associated temporarily with a chromosome called an episome.
  • Inclusion bodies are the non-living structures which are of 3 types – (i) Gas vacuoles (ii) Inorganic inclusions and (iii) Food reserve.
  • Flagella are uni-stranded with 3 parts – (i) Basal body (ii) Filament and (iii) hook and they are made up of a protein – Flagellin.
  • Pili (longer and fewer) made up of pilin helps in the formation of conjugation tube in F + bacteria.
  • Fimbriae help in adhesion/attachment of a bacterium and also in mutual clinging of bacteria
  • Reproduction in bacteria can be of several methods such as binary fission, sporulation and sexual reproduction. (conjugation that happens between F+/F− bacteria) Where F+ is donor and F− is a recipient.
  • On the basis of respiration, bacteria is of 4 types
    (i) Obligate aerobes (strict): Bacillus subtilis
    (ii) Obligate anaerobes: Clostridium botulinum
    (iii) Facultative anaerobes: Halophiles (generally aerobic)
    (iv) Facultative aerobes: Rhodopseudomonas (generally anaerobic)
  • Photoautotrophic bacteria possess photosynthetic pigments like bacterio-chlorophyll and bacterio-pheophytin, which are anaerobes that do not evolve oxygen during photosynthesis such type is called Anoxygenic photosynthesis (Eg. Purple/green sulphur bacteria).
  • Chemoautotrophic bacteria manufacture their organic food from inorganic raw materials. Eg [Sulphur, Iron, Nitrifying bacteria].
  • Saprophytic bacteria obtain food from organic remains. [Nature’s scavengers]
  • Anaerobic breakdown of carbohydrates and proteins are called Fermentation and Putrefaction respectively.
  • Aerobic breakdown of organic compounds is decay.
  • Symbiotic bacteria has mutual contact with other organism. Eg. E coli in Human intestine produces Vitamin B and K.
  • Parasitic bacteria may or may not cause diseases.
  • Toxins produced by them could be endo (Eg. Vibrio Cholerae) / Exotoxins (Eg. Clostridium tetani)
  • Archaebacteria (lacks peptidoglycan) contains Pseudomurein, which differ in 16S rRNA nucleotides from those of other organisms.
  • They generally live in adverse/extreme environmental conditions.
  • Archaebacteria are of 3 major types – (i) Methanogens (ii) Halophiles (iii) Thermoacidophiles.
  • Mycoplasma are the minute and the simplest organisms of the procaryotes, that are also called MLO (Mycoplasma like Organisms), PPLO (Pluro pneumonia like organisms) (or) Mollicutes.
  • As they lack the cell wall, they are pleomorphic (can change themselves into various shapes).
  • Cyanobacteria (Blue green Algae) perform oxygenic photosynthesis, which had evolved 3 billion years ago.
  • BGA is autotrophic that mastered all sorts of environment. Even the Red Sea is named after its presence. [Trichodesmium erythraeum]
  • It contains a trichome filament with mucilage covering it. Trichome filament can be of homocystous (Oscillatoria) (or) heterocystous (Nostoc) but lacks flagella.
  • BGA possess a heterocyst that is impermeable to O2 but permeable to N2 and so it helps in Nitrogen fixation as it also has an enzyme – Nitrogenase.
  • Some Cyanobacteria produce toxins. Eg. Microcystis aeruginosa, Anaebaena flos aquae.

View this video for the topic from 0:10 to 13:09

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