Prokaryotic Cell Structure - 2

Mycoplasma is a wall-less prokaryote that contains sterols in the cell membrane, that offers strength and rigidity.

Plasma membranes of prokaryotes are less rigid because they lack sterols. However,  the prokaryote mycoplasma despite being a prokaryote contains sterols.

Cell wall is common to all bacterial cells except a few like mycoplasmas. Similarly, thylakoids are the specialized membranes of flattened discs that contain light sensitive pigments. These structures are commonly present in cyanobacteria - the photosynthetic bacteria, and they help in photosynthesis. Therefore, these structures are not common to all bacterial cells.

Bacterial capsule lacks cellulose as cellulose is present in the cell walls of plants.

The plasma membrane of a bacterial cell is less rigid as it is devoid of sterols like cholesterol. One exception to this is the prokaryote mycoplasma that contains sterols in the plasma membrane. In the absence of cell wall, sterol containing plasma membrane offers strength and rigidity to this prokaryote.

Flagella help in locomotion and impart antigenic specificity, but not virulence.

Glycocalyx is an important component of biofilms. This structure helps the bacterial cell attach together and colonize. Through attachment, bacteria may grow on diverse surfaces such as rocks, in fast-moving streams, plant roots, and human teeth. Therefore, extracellular polymeric substance is a kind of glycocalyx that helps bacterial cells in a biofilm attach to their target environment to each other.

Histone proteins are associated with the DNA of eukaryotes, but not with the DNA of prokaryotes as they lack histone proteins.

Lipid A is specifically present in gram negative bacteria in the outer lipopolysaccharide membrane. It is the lipid portion of the outer membrane. When the bacteria die, they release lipid A. It is an endotoxin. It is responsible for fever, shock and dilation of blood vessels. From the core polysaccharide, O polysaccharide extends out, which is composed of sugar units. It acts as an antigen and helps distinguish different species of gram negative bacteria like E. coli. These are the only structures that are specifically confined to gram negative bacteria and are absent in gram positive bacteria.

A typical bacterial cell has a well-defined cell wall, and therefore it will not enlarge due to osmosis. The cell wall resists swelling and imparts mechanical strength and rigidity to a baterial cell.

Treponema pallidum is a spirochete because it is spiral shaped and moves with the help of axial filaments or endo-flagella. It is a prokaryote and a pathogenic spirochete - the causative agent of syphilis in human beings.

The peptidoglycan of a gram negative bacterium consists of fewer number of layers and it is thin. In addition to this, the outer lipopolysaccharide layer is not permeable to antibiotics like penicillin. Therefore, a typical gram negative bacterium is less sensitive to antibiotics.

A bacterial capsule is, in fact, a virulent structure as it imparts virulence to a bacterial cell by helping the cell in attachment, adherence and by avoiding phagocytosis.

The antibiotic penicillin removes the cell wall of the cell and water enters the cell by osmosis. The cell is devoid of cell wall, and therefore cannot constrain the swelling of the plasma membrane and it bursts. This is an example of osmotic lysis. This happens when the cell is placed in a hypotonic solution.