Cell Biology: Prokaryotes vs Eukaryotes

Cell Membrane and Replication

• Eukaryotic cell membranes contain sterols, whereas prokaryotes (except Mycoplasma) do not.
• Prokaryotic cells replicate by binary fission, while eukaryotic cells replicate by mitosis.

Structure of Bacterial Cells

Shape and Size

• Bacteria are classified by shape into three basic groups: cocci, bacilli, and spirochetes.
• Some bacteria are variable in shape and are said to be pleomorphic (many-shaped).
• The shape of a bacterium is determined by its rigid cell wall.

Arrangement of Bacteria

• Certain cocci occur in pairs (diplococci), some in chains (streptococci), and others in grapelike clusters (staphylococci).
• These arrangements are determined by the orientation and degree of attachment of the bacteria at the time of cell division.

Gram Stain

• It takes approximately 100,000 bacteria/mL to see 1 bacterium per microscopic field using the oil immersion (100×) lens.
• The sensitivity of the Gram stain procedure is low.

Medically Important Bacteria That Cannot Be Seen in the Gram Stain

• Meningococcemia is an exception, where high concentrations of Neisseria meningitidis can occur in the blood.

Special Components of Gram-Positive Cell Wall

• Teichoic acids are found in Gram-positive bacteria and can induce septic shock.
• Teichoic acids mediate the attachment of staphylococci to mucosal cells.
• Gram-negative bacteria do not have teichoic acids.

Periplasmic Space

• The periplasmic space is the area between the inner and outer membranes.
• It contains the peptidoglycan layer and a gel-like solution of proteins.

Cytoplasmic Membrane

• The cytoplasmic membrane is a lipid bilayer with selective permeability.
• It is the site of ATP production.

Cytoplasm

• The cytoplasm has two distinct areas: an amorphous matrix and an inner, nucleoid region composed of DNA.
• The cytoplasm contains ribosomes, nutrient granules, metabolites, and plasmids.

Structures Inside Cytoplasm

• Ribosomes consist of rRNA and protein.
• Protein synthesis occurs at ribosomes and can be inhibited by certain antibiotics.
• Prokaryotic ribosomes (70s) are different from eukaryotic ribosomes (80s), allowing antibiotics to selectively target prokaryotic ribosomes.

Nucleoid

• The nucleoid is the area of the cytoplasm where DNA is located.
• Bacterial DNA is a single, circular molecule that contains about 2000 genes.
• Bacterial DNA has no introns, unlike eukaryotic DNA.

Plasmids

• Plasmids are extrachromosomal, double-stranded, circular DNA molecules that can replicate independently of the bacterial chromosome.
• Plasmids carry genes for antibiotic resistance, heavy metal resistance, UV light resistance, pili, and exotoxins.

Structures Outside the Cell

• Capsule is a gelatinous layer covering the entire bacterium, composed of polysaccharide.
• Capsule is a determinant of virulence, limits phagocytosis, and is used in specific organism identification and vaccine production.