We all have heard about bacteria. Do you know under which kingdom are they placed and why? Since bacteria are the simplest, primitive living organisms without a well defined nucleus that is not bound by a nuclear membrane they are considered as prokaryotes. The term prokaryotes comes from ‘pro’ meaning primitive and ‘karyon’ meaning nucleus. All prokaryotes of the living world are placed under the kingdom Monera.
But apart from lacking a well defined nucleus, what else distinguishes the kingdom Monera from the other kingdoms? Are bacteria the only organisms included in this kingdom? Let us find the answers to these questions.
Table of contents
This kingdom includes unicellular, microscopic, prokaryotic organisms which mostly live in aquatic environments. Bacteria are, however, omnipresent and are present everywhere.
Members of this kingdom can be autotrophic or heterotrophic in nature. Autotrophic prokaryotes can prepare their own food either by photosynthesis (using light as the source of energy) or chemosynthesis (using the energy obtained from oxidation of chemicals). Photosynthetic monerans have photosynthetic pigments such as chlorophyll, bacteriochlorophyll, etc. Chemosynthetic prokaryotes are mostly decomposers, mineralisers and nitrogen fixers.
Heterotrophic bacteria cannot prepare their own food and depend on others for nutrition. These can either be saprophytes (feeding on dead decaying organisms), parasites (feeding on living hosts) or symbionts (which exist mutually with other organisms).
Cell wall is usually made up of peptidoglycan (or murein) which is a type of mucopeptide. Peptidoglycan is a polymer made up of alternating subunits of N-acetyl glucosamine (NAG) and N-acetyl muramic acid (NAM) crosslinked with short amino acid chains.
Archaebacteria, except for methanogens, do not have peptidoglycan in their cell wall. Their cell wall is made up of polysaccharides and glycolipids which allow them to thrive in harsh conditions. Prokaryotes such as Mycoplasma lack a cell wall.
Like eukaryotes, the cell membrane of prokaryotes is made up of phospholipid bilayer, proteins and carbohydrates. It is selectively permeable in nature.
The space between cell wall and cell membrane is known as periplasmic space. This space is analogous to lysosomes because in this space digestion of complex substances takes place.
The cytoplasm of prokaryotes lacks membrane bound organelles. In prokaryotic cells, the nucleus is indistinct. Nuclear membranes are absent around genetic material. It also lacks nucleolus. Prokaryotes lack the true chromosome. Instead, a false chromosome is present which is made of double stranded circular naked DNA and non-histone protein like polyamines. The false chromosome coils and forms a chromosomal region which is known as a nucleoid. A single copy of the nucleoid is present.
Beside this DNA, another small and double stranded circular DNA is also present in bacterial cells which is known as the plasmid. Plasmids have the ability to replicate independently and are usually associated with antibiotic resistance. These are present in multiple copies.
In prokaryotes, ribosomes are 70S type and are made up of a heavier 50S and lighter 30S subunit. Ribosomes are the sites of protein synthesis.
Cytoplasm may also include inclusion bodies which help in storage of reserve food materials.
Reproduction in the members of kingdom Monera usually occurs asexually by means of binary fission in which the parent cell divides amitotically into two daughter cells.
Some bacteria show parasexual methods of reproduction under unfavourable conditions. These methods include -
Respiration in members of the kingdom Monera can either be aerobic (requires oxygen) or anaerobic (does not require oxygen).
All members of archaebacteria are anaerobic in nature.
Aerobic prokaryotes can be obligate (cannot respire without oxygen) or facultative anaerobes (normally respire in the presence of oxygen but can also respire in its absence).
Anaerobic prokaryotes can be obligate (cannot respire in presence of oxygen) or facultative aerobes (normally respire in absence of oxygen but can also respire in its presence).
The kingdom Monera includes Archaebacteria (primitive bacteria), Eubacteria (true bacteria), Cyanobacteria, Actinomycetes, Rickettsia and Mycoplasma.
Archaebacteria are further classified as halophiles, thermoacidophiles and methanogens.
Q 1. You are interested in studying the cell structure of bacteria. During your studies, you found a space between the cell wall and cell membrane in bacteria. Name this space.
a. Peri-plastidial space
b. Peri-mitochondrial space
c. Pericardial space
d. Periplasmic space
Answer: The space between the cell wall and cell membrane of a bacteria is known as the periplasmic space. It has digestive enzymes and help in digestion of complex substances. Thus, the correct option is d.
Q 2. Select the incorrect statement about cell structure of prokaryotes.
a. They possess both 80S and 70 S ribosomes for protein synthesis.
b. They do not have well defined nuclear membranes, instead they have nucleoid.
c. Cell membrane is made up of lipids and proteins.
d. The cell wall of methanogens is not made up of peptidoglycan.
Answer: Bacteria are prokaryotes and have only 70 S ribosomes for protein synthesis. It is made up of a smaller 30S and larger 50S subunits.
Prokaryotes lack a nuclear membrane and hence do not have a well-defined nucleus. The genetic material exists as DNA associated with some non-histone proteins, lyingnaked in the cytoplasm in the form of a nucleoid.
Cell membrane of prokaryotes is similar to that of eukaryotes and is composed of a bilayer of phospholipids with proteins arranged in a mosaic fashion.
The cell wall of methanogens is made up of pseudopeptidoglycan.
Thus, the correct answer is option a.
Q 3. Which of the following characteristics is significantly different in eubacteria and archaebacteria?
a. Cell shape
b. Cell membrane structure
c. Mode of nutrition
d. Mode of reproduction
Answer: Cell membrane composition of archaebacteria is different from eubacteria. It is made up of branched chain lipids unlike eubacteria who have their cell membrane made up of lipids and proteins. Hence, option b is correct.
Q 4. Define nucleoid.
Answer: Nucleoid is a region where bacterial chromosomal double stranded circular DNA is present along with non-histone proteins.
Q 5. What other type of DNA does bacteria possess other than chromosomal DNA?
Answer: Some bacteria possess plasmid DNA which is an autonomously replicating, small, circular double stranded DNA. It has some antibiotic resistant genes.
Q 1. Who evolved earlier, prokaryotes or eukaryotes?
Answer: The prokaryotes have a much simpler cellular organisation when compared to eukaryotes. They lack well-defined nucleus and membrane bound organelles. Eukaryotes on the other hand have a much more complex cellular organisation with genetic material bound within a define nucleus with a nuclear membrane. They also have membrane bound organelles which perform specific functions for the cell. According to the endosymbiotic theory, the eukaryotic plant cells must have evolved when larger heterotrophic prokaryotes engulfed smaller photosynthetic cyanobacteria which eventually established a symbiotic relationship with the host cell. Similarly, aerobic prokaryotes which were engulfed and started living symbiotically within the larger heterotrophic prokaryote eventually gave rise to mitochondria. Thus, we can safely say that eukaryotes evolved after prokaryotes.
Q 2. Why was the kingdom Monera further divided in the six kingdom classification?
Answer: In Carl Woese’s six kingdom classification, the kingdom Monera was further divided into Kingdom Archaebacteria and kingdom Eubacteria because archaebacteria and eubacteria vary a lot with respect to their cellular structure. The cell wall of eubacteria is composed of peptidoglycan whereas in archaea peptidoglycan is absent. Their cell wall is made up of polysaccharides and glycolipids which allow them to thrive in harsh conditions.
Cell membrane of eubacteria is composed of lipids and proteins while that of archaea is composed of branched chain lipids.
Q 3. Who was the first person to consider Monera as an independent kingdom?
Answer: Herbert Copeland was the first person to consider Monera as a separate kingdom for unicellular prokaryotic organisms in his four kingdom classification system.
Q 4. What is the main difference between the kingdom Monera and Protista?
Answer: Kingdom Monera includes unicellular prokaryotic organisms whereas kingdom Protista includes unicellular eukaryotic organisms
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