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Biology LibreTexts

5.1: Cells

Both prokaryotic cells and eukaryotic cells have the following components:

  1. cytoplasmic membrane (aka plasma membrane, cell membrane): living boundary of cell. Made of phospholipid bilayer with inserted proteins. Important role in controlling what flows into and out of cell. Surface contains important molecule used in recognition, triggering immune responses and cell signaling
  2. cytoplasm: water-rich substance filling interior of cell; rich in nutrients, enzymes, ribosomes, etc.
  3.  chromosome(s): genetic information of cells; encodes all information to direct cell functions and biosynthesis. Made of double stranded DNA. DNA base sequence determines amino acid sequence of all proteins produced by cell.

4. ribosomes: made of RNA and proteins, sites of protein synthesis

-Prokaryotic cells have “small” ribosomes called 70S ribosomes

-Eukaryotic cells in their cytoplasm have “large” ribosomes, called 80S ribosomes.

-Key Idea*: Bacterial 70S ribosomes as targets of antibiotics

-bacterial  70S ribosomes are the sites of protein synthesis within cells

-(remember eukaryotes such as humans have larger 80S ribosomes)

-several antibiotics bind to and inhibit the bacterial 70S ribosome consequently stopping protein synthesis in the bacteria 

                                70S ribosome inhibitors

                                                Chloramphenicol

                                                Macrolides  e.g. erythromycin

                                                Tetracycline

                                                Aminoglycosides e.g. gentamicin, kanamycin

 

- remember these antibiotics do not inhibit the eukaryotic  80S ribosomes

Flagellum” motility

Fimbriae: attachment

Figure 1 : composite bacterial cell

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Structures common to all bacteria

1.Cytoplasmic membrane

2. Cytoplasm

3. Genetic info= DNA  (usually) in  a single, circular chromosome

                        -NOT contained within a membrane-bound nucleus

4. 70S ribosomes (smaller than eukaryotic 80S ribosomes)

Structures common to most (but not all) bacteria

5. Bacterial Cell Wall: outside cytoplasmic membrane,

            -contains unique substance peptidoglycan

                        =prevents bacteria from bursting open (“osmotic lysis”)

            -beta-lactam antibiotics such as penicillin, ampicillin and

antibiotic vancomycin interfere with peptidoglycan synthesis so that bacteria burst and die

-Note: some bacteria lack cell walls examples bacterial pathogens Mycoplasma and Ureaplasma (can cause respiratory tract infections, genital infections). These cell wall-less bacteria would not be killed by penicillins or vancomycin

            -more on cell walls later in chapter

6. Capsule: outside of cytoplasmic membrane

                        usually made of carbohydrates; some made of polypeptides

                        -inhibit phagocytosis by host leukocytes

7. Fimbriae and Pili:  (singular: pilus)

            a. attachment pili also known as fimbriae

                        -hollow protein tubes projecting form surface of cell

                         - used for attachment to surfaces in environment

                        -permit attachment to host cells

            b. sex pili aka conjugation pili

                        -permit attachment of one bacterium to another

                        -function in transfer of genetic information between gram-negative bacteria

8. Flagellum/Flagella:

            -bacterial protein “propeller” used to move bacterium through fluids

9. Endospore:

            -tough resistant structures formed to protect bacterial DNA when conditions deteriorate; may survive centuries.

-process of forming endospore is called “sporulation”, triggered by deterioration of environment (lack of nutrients, water etc)

-when conditions improve, endospore “germinates’ forming vegetative bacteria which grow and divide, damage host  e.g. Clostridium tetani, Clostridium botulinum, Bacillus anthracis endospores