Prokaryotic Cells: Structure, Function, and Morphology – Study Notes

Prokaryotic Cells: Structure, Function, and Morphology

Introduction

Prokaryotic cells are fundamental to the study of microbiology, representing organisms such as bacteria and archaea. Understanding their structure, function, and morphology is essential for grasping microbial physiology, classification, and pathogenicity. This guide summarizes key features and differences between prokaryotic and eukaryotic cells, as well as the diversity of bacterial shapes.

Comparison of Prokaryotes and Eukaryotes

Cellular Characteristics

Prokaryotes and eukaryotes differ in several structural and functional aspects. The following table summarizes these differences:

Characteristic	Prokaryote	Eukaryote
Basic building blocks (lipids, carbs, nucleic acids, proteins)	Present	Present
Size	Smaller (typically 0.2–2.0 μm)	Larger (typically 10–100 μm)
Nucleus	No true nucleus (nucleoid region)	True nucleus with nuclear envelope
Chromosome(s)	Single, circular DNA molecule	Multiple, linear chromosomes
Cytoskeleton	Absent or rudimentary	Present
Membrane-enclosed organelles	Absent	Present (e.g., mitochondria, ER)
Ribosomes	70S (smaller)	80S (larger)
Flagella	Simple structure	Complex structure
Cell wall	Usually present (peptidoglycan in bacteria)	Present in plants/fungi (cellulose/chitin), absent in animals
Cell division	Binary fission	Mitosis/meiosis

Size Relationships

Microbial cells vary greatly in size. The diagram below (described) shows the relative sizes of viruses, prokaryotes, and eukaryotes:

• Viruses: Smallest, visible only with electron microscopy.

• Prokaryotes (bacteria): Intermediate size, visible with light microscopy.

• Eukaryotes: Largest, including cells such as human cells and protozoa.

Additional info: Mycoplasmas are among the smallest bacteria, mitochondria are similar in size to many bacteria, and ribosomes are much smaller than cells.

Bacterial Morphology

Bacterial Shapes

Bacteria exhibit a variety of shapes, which are important for identification and classification. Most bacterial cells are monomorphic, meaning they maintain a single, consistent shape. However, some are pleomorphic and can appear in multiple forms.

• Monomorphic: One normal shape; age or environmental factors may affect shape.

• Pleomorphic: Ability to appear in several shapes.

The three basic shapes are:

• Rods (bacilli)

• Cocci (spherical)

• Spirals

Cocci Arrangements

Cocci are spherical bacteria. Their arrangements depend on the plane of division and whether the cells remain attached after division.

• Diplococci: Pairs of cocci (divide in one plane).

• Streptococci: Chains of cocci.

• Tetrads: Groups of four cocci (divide in two planes).

• Sarcinae: Cubes of eight cocci (divide in three planes).

• Staphylococci: Grape-like clusters.

Key terms: 'Coccus' means berry in Greek. Cocci can vary from round to oval, flattened, or elongated.

Example: Streptococcus pneumoniae forms chains and is a common cause of pneumonia.

Bacilli Arrangements

Bacilli are rod-shaped bacteria. Their arrangements include:

• Single bacilli: Individual rods.

• Diplobacilli: Pairs of rods.

• Streptobacilli: Chains of rods.

• Coccobacilli: Short, oval rods that may resemble cocci.

Key terms: 'Bacillus' refers to the shape, but also to a genus of bacteria.

Example: Bacillus anthracis (causative agent of anthrax) is a rod-shaped bacterium.

Spiral Bacteria

Spiral-shaped bacteria include:

• Vibrios: Curved rods (comma-shaped), e.g., Vibrio cholerae.

• Spirilla: Rigid, spiral-shaped bacteria with external flagella.

• Spirochetes: Flexible, corkscrew-shaped bacteria with axial filaments for movement.

Example: Treponema pallidum (spirochete causing syphilis).

Summary Table: Bacterial Shapes and Arrangements

Shape	Arrangement	Example Organism
Coccus	Diplococci, Streptococci, Tetrads, Sarcinae, Staphylococci	Streptococcus pneumoniae, Staphylococcus aureus
Bacillus	Single, Diplobacilli, Streptobacilli, Coccobacilli	Bacillus anthracis, Escherichia coli
Spiral	Vibrio, Spirilla, Spirochete	Vibrio cholerae, Treponema pallidum

Key Points for Microbiology Students

• Cell structure and morphology are critical for bacterial identification and classification.

• Arrangement of cells after division helps in distinguishing bacterial genera and species.

• Size differences between viruses, prokaryotes, and eukaryotes affect microscopy techniques used for observation.

• Understanding morphology aids in diagnosing infections and understanding bacterial physiology.

Additional info: Some bacteria can change shape (pleomorphism) due to environmental conditions or age. Arrangements such as clusters or chains are often used in clinical microbiology to identify pathogens.