Prokaryotes I Online Biotech Notes

Online Biotech Notes 

Prokaryotes 

Prokaryotes are the most numerous and widespread organisms on earth, and are so classified because they have no defined membrane-bound nucleus.

Prokaryotes Definition

Prokaryotes are any cellular organism that has no nuclear membrane, no organelles in the cytoplasm except ribosomes, and has its genetic material in the form of single continuous strands forming coils or loops, characteristic of all organisms in the kingdom Monera, as the bacteria and blue-green algae.

Prokaryotes Examples 

• Examples of prokaryotes are Archaea and Bacteria.  

Prokaryotes Features 

• Prokaryotes range in size from 0.1 to 10 m, and have one of three basic shapes: spherical (cocci), rodlike (bacilli) or helically coiled (spirilla). 
• They can be divided into two separate groups: the eubacteria and the archaebacteria. 
• The eubacteria are the commonly encountered bacteria in soil, water and living in or 
• on larger organisms, and include the Gram-positive and Gram-negative bacteria, and cyanobacteria (photosynthetic blue-green algae). 
• The archaebacteria grow in unusual environments such as salt brines, hot acid springs and in the ocean depths, and include the sulfur bacteria and the methanogens.
• prokaryotic cells do not have membrane-bound structures, they do have distinct cellular regions

Cell Structure of Prokaryotes

• Like all cells, a prokaryotic cell is bounded by a plasma membrane that completely encloses the cytosol and separates the cell from the external environment.
• The plasma membrane, which is about 8 nm thick, consists of a lipid bilayer containing proteins (see Topic E1). Although prokaryotes lack the membranous subcellular organelles characteristic of eukaryotes, their plasma membrane may be infolded to form mesosomes. 
• The mesosomes may be the sites of deoxyribonucleic acid (DNA) replication and other specialized enzymatic reactions. 
• In photosynthetic bacteria, the mesosomes contain the proteins and pigments that trap light and generate adenosine triphosphate (ATP). 
• The aqueous cytosol contains the macromolecules [enzymes, messenger ribonucleic acid (mRNA), transfer RNA (tRNA) and ribosomes], organic compounds and ions needed for cellular metabolism. Also within the cytosol is the prokaryotic ‘chromosome’ consisting of a single circular molecule of DNA which is condensed to form a body known as the nucleoid.
• Many bacterial cells have one or more tail-like appendages known as flagella which are used to move the cell through its environment.

Cell Structure of Prokaryotes

Bacterial cell Walls of Prokaryotes

• To protect the cell from mechanical injury and osmotic pressure, most prokaryotes are surrounded by a rigid 3–25 nm thick cell wall (Fig. 1). 
• The cell wall is composed of peptidoglycan, a complex of oligosaccharides and proteins. 
• The oligosaccharide component consists of linear chains of alternating N-acetylglucosamine
• (GlcNAc) and N-acetylmuramic acid (NAM) linked. Attached via an amide bond to the lactic acid group on NAM is a D-amino acid-containing tetrapeptide. 
• Adjacent parallel peptidoglycan chains are covalently cross-linked through the tetrapeptide side-chains by other short peptides.
• The extensive cross-linking in the peptidoglycan cell wall gives it its strength and rigidity. 
• The presence of D-amino acids in the peptidoglycan renders the cell wall resistant to the action of proteases which act on the more commonly occurring L-amino acids, but provides a unique target for the action of certain antibiotics such as penicillin. 
• Penicillin acts by inhibiting the enzyme that forms the covalent cross-links in the peptidoglycan, thereby weakening the cell wall. 
• The (1–4) glycosidic linkage between NAM and GlcNAc is susceptible to hydrolysis by the enzyme lysozyme which is present in tears, mucus and other body secretions.
• Bacteria can be classified as either Gram-positive or Gram-negative depending on whether or not they take up the Gram stain. 
• Gram-positive bacteria (e.g. Bacillus polymyxa) have a thick (25 nm) cell wall surrounding their plasma membrane, whereas Gram-negative bacteria (e.g. Escherichia coli) have a thinner (3 nm) cell wall and a second outer membrane. 
• In contrast with the plasma membrane, this outer membrane is very permeable to the passage of relatively large molecules (molecular weight > 1000 Da) due to porin proteins which form pores in the lipid bilayer. Between the outer membrane and the cell wall is the periplasm, a space occupied by proteins secreted from the cell.

Cell wall Structure of Gram positive and gram negative bacteria

Thank you!!!