Comparing Prokaryotic and Eukaryotic Cells

Overview and Definitions

Cells are classified as either prokaryotic or eukaryotic based on structural and genetic differences. Understanding these differences is fundamental in microbiology.

• Prokaryote: The term comes from Greek words meaning "prenucleus." Prokaryotic cells lack a true nucleus.

• Eukaryote: The term comes from Greek words meaning "true nucleus." Eukaryotic cells have a membrane-bound nucleus.

Key Differences Between Prokaryotic and Eukaryotic Cells

Example: Escherichia coli is a prokaryote; human cells are eukaryotes.

The Prokaryotic Cell

Basic Shapes of Bacteria

Bacteria exhibit a variety of shapes, which are important for identification and classification.

• Bacillus: Rod-shaped

• Coccus: Spherical-shaped

• Spiral: Includes three forms:

  • Vibrio: Comma-shaped

  • Spirillum: Rigid, spiral-shaped

  • Spirochete: Flexible, corkscrew-shaped

• Star-shaped: Rare, star-like appearance

• Rectangular: Box-shaped, uncommon

Key Point: Most bacteria are monomorphic (single shape), but some are pleomorphic (variable shapes).

Size of Bacterial Cells

• Average size: 0.2 to 2.0 μm in diameter, 2 to 8 μm in length

Arrangement of Bacterial Cells

Bacterial cells can be arranged in characteristic patterns, which aid in identification.

• Pairs: Diplococci (spherical), diplobacilli (rod-shaped)

• Clusters: Staphylococci

• Chains: Streptococci (spherical), streptobacilli (rod-shaped)

• Groups of Four: Tetrads

• Cubical Groups of Eight: Sarcinae

Example: Streptococcus pyogenes forms chains of cocci; Staphylococcus aureus forms clusters.

Visual Examples of Bacterial Shapes

• Spiral Bacteria: Includes vibrio, spirillum, and spirochete forms (see Figure 4-4 in textbook for images).

• Star-shaped and Rectangular Prokaryotes: Rare forms, sometimes found in aquatic environments.

Additional info: The arrangement and shape of bacteria are determined by patterns of cell division and whether cells remain attached after division.