Prokaryotic cells

• 1. Flagellum
  2. Capsule or slime coat
  3. Mesosome
  4. Photosynthetic membranes
  5. Plasmid
  6. Pili
  7. Glycogen granules, lipid droplets
  8. 70S ribosomes
  9. Nucleoid - a long circular strand of DNA
  10. Cytoplasm
  11. Cell surface membrane
  12. Cell wall

  
• All bacterial cells have a cell wall. The contents of bacterial cells are hypertonic to the medium around them, so water tends to move into the cells by osmosis. The cell wall prevents the cell swelling and bursting. It also maintains the shape of the shape of the bacterium, and gives support and protection to the content of the cell. All bacteria cell walls have a layer of peptidoglycan - consists of many parallel polysaccharide chains with short peptide cross-linkages producing an enormous molecule with a net-like structure.
• Some bacteria have a capsule (slime layer if it is very thin and can diffuse) around their cell walls. This may be produced from starch, gelatin, protein, or glycolipid, and protects the bacterium from phagocytosis by white blood cells. It also covers the cell markers on the cell membrane that identifies the cell, making it easier for the bacterium to be pathogenic.
• Some bacteria have from 1-100 thread-like protein projections found on their surface. These are called pili and seem to be used for attachement to a host cell and for sexual reproduction. But, they can also make the bacteria more vulnerable to virus infections.
• The cell surface membrane is like the eukaryotic membrane and controls what enters and exits the cell. It is also the site of some of the respiratory enzymes.
• Mesosomes are artefacts from the process of preparing the cell for an electron micrograph, others believe they are associated with enzyme activity, particularly during the separation of DNA and the formation of new cell walls when the bacteria divide.
• The genetic material of prokaryotic cells consists of a single circular strand of DNA which is not contained in the membrane-bound nucleus. The DNA is folded and coiled to fit into the bacterium and is contained in the nucleoid.
• A plasmid codes for a particular aspect of the bacterial phenotype in addition to the genetic information. For example, plasmids can code for the production or a particular toxin or resistance to a particular antibiotic. Plamids can reproduce themselves independently of the nucleoid.
• Prokaryotes have ribosomes where protein synthesis occurs. 70S ribosomes have two subunits, the smaller is the 30S and the larger is 50S.
• All bacteria cell walls contain peptidoglycan but there are types which can be distinguished by Gram staining. Different types of disease-causing bacteria are vulnerable to different types of antibiotic and the type of cell wall they have is of the factors that affects how vulnerable they are, so Gram staining is valuable to determine different types of cell wall.
• The cell wall of Gram-positive bacteria have a thick layer of peptidoglycan containing chemical substances such as techoic acid within the net-like structure. The crystal violet/iodine complex in the Gram stain is trapped in the thick peptidoglycan layer and resists decolouring when the bacteria are dehydrated using alcohol. As a result, the bacteria do not pick up the red safranin counterstain and appear purple/blue when viewed in a light microscope.
• The cell walls of Gram negative bacteria have a thin layer of peptidoglycan with no techoic acid between two layers of membrane. The outer membrane is made up of lipopolysaccharides. This layer dissolves when the bacteria are dehydrated in ethanol. This exposes the thin peptidoglycan layer and the crystal violet/iodine complex is washed out. The peptidoglycan then takes up the red safranin counterstain. The cells appear red when viewed in a light microscope
• Bacteria can also be grouped based on their respiratory requirements. Obligate aerobes need oxygen for respiration. Facultative anaerobes use oxygen if it is available, but can manage without it. Obligate anaerobes can only respire in the absence of oxygen - in fact oxygen will kill them
• Another way in which bacteria can be identified is by their shape. Some bacteria are spherical (cocci), rod-shaped (bacilli), twisted (spiralla) or comma-shaped (vibrios)