Prokaryotic and Eukaryotic Cells: Structure, Classification, and Morphology

Prokaryotic and Eukaryotic Cells

Definitions and Origins

Cells are classified as either prokaryotic or eukaryotic based on their structural and genetic characteristics. The term prokaryote comes from the Greek words for "pre-nucleus," indicating the absence of a true nucleus. In contrast, eukaryote derives from the Greek for "true nucleus," reflecting the presence of a membrane-bound nucleus.

• Prokaryotic cells: Lack a nuclear membrane; genetic material is not enclosed within a nucleus.

• Eukaryotic cells: Possess a true nucleus surrounded by a nuclear membrane.

Key Structural Differences

Feature	Prokaryote	Eukaryote
Chromosomes	Single, circular chromosome, not in a membrane	Paired chromosomes in nuclear membrane
Histones	Absent	Present
Organelles	Absent	Present
Cell Wall Composition	Peptidoglycan (Bacteria), Pseudomurein (Archaea)	Polysaccharide (when present)
Cell Division	Binary fission	Mitotic spindle (mitosis)

Classification of Prokaryotes

Domains and Cell Wall Types

Prokaryotes are divided into two domains based on cell wall composition and rRNA signature:

• Domain Bacteria: Cell walls contain peptidoglycan.

• Domain Archaea: Cell walls contain pseudomurein; lack peptidoglycan.

Further classification is based on Gram staining and genetic content:

• Gram-negative bacteria: Includes proteobacteria and nonproteobacteria.

• Gram-positive bacteria: Classified by low or high G+C content.

Proteobacteria

Proteobacteria is a major taxonomic group of Gram-negative bacteria, most of which are heterotrophic. This group is divided into five classes:

• Alpha

• Beta

• Gamma

• Delta

• Epsilon

Proteobacteria include a variety of pathogens important in medical microbiology.

Non-Proteobacteria Gram-Negatives

• Bacteroidetes: Anaerobic; Bacteroides species are found in the mouth and large intestine, with up to 1 billion per gram of feces. Infections are usually from puncture wounds or perforated bowels.

• Fusobacterium: Found in the mouth; involved in dental diseases.

Gram-Positive Bacteria: Firmicutes

Firmicutes are Gram-positive bacteria with low G+C content. They are further classified based on cell wall presence and ability to produce endospores:

• Wall-less: Mycoplasmatales

• Have cell walls: Can produce endospores (Clostridiales - anaerobic; Bacillales - aerobic/facultatively anaerobic) or not (Lactobacillales).

Prokaryotic Cell Morphology

Cell Size and Shape

Prokaryotic cells typically range from 0.2–1.0 μm in width and 2–8 μm in length. Most bacteria are monomorphic (single shape), but some are pleomorphic (variable shapes).

• Bacillus: Rod-shaped

• Coccus: Spherical

• Coccobacillus: Short, oval rod

Arrangements of Cocci and Bacilli

Bacterial cells can be arranged in characteristic patterns:

• Diplococci: Pairs of cocci

• Streptococci: Chains of cocci

• Single bacillus: Individual rod-shaped cells

• Coccobacillus: Short rods, often resembling cocci

Examples:

• Diplococci: Streptococcus pneumoniae

• Streptococci: Streptococcus pyogenes

Arrangement	Description	Example
Diplococci	Pairs of spherical cells	Neisseria species
Streptococci	Chains of spherical cells	Streptococcus species
Single bacillus	Individual rod-shaped cell	Bacillus subtilis
Coccobacillus	Short, oval rod	Haemophilus influenzae

Additional info: Bacterial morphology is important for identification and classification in clinical microbiology. The arrangement and shape can be visualized using scanning electron microscopy (SEM), as shown in the provided images.