Cell Structure and Function: Prokaryotes, Eukaryotes, and the Endosymbiotic Theory

Cell Structure and Function

Comparing Prokaryotic and Eukaryotic Cells

Understanding the differences between prokaryotic and eukaryotic cells is fundamental in microbiology. These two cell types differ in their structural organization, genetic material, and cellular processes.

• Prokaryotic Cells: Lack a membrane-bound nucleus; genetic material is located in the nucleoid region. Examples include Bacteria and Archaea.

• Eukaryotic Cells: Possess a true nucleus enclosed by a nuclear membrane and various membrane-bound organelles. Examples include fungi, protozoa, plants, and animals.

• Key Differences:

  • Size: Eukaryotic cells are generally larger (10–100 μm) than prokaryotic cells (0.1–5 μm).

  • Complexity: Eukaryotes have compartmentalized organelles; prokaryotes do not.

  • Cell Division: Prokaryotes divide by binary fission; eukaryotes by mitosis and meiosis.

• Example: Escherichia coli (prokaryote) vs. Saccharomyces cerevisiae (eukaryote).

Bacterial Cell Structures

Bacterial cells possess unique structures that contribute to their survival and function.

• Cell Wall: Composed primarily of peptidoglycan, providing shape and protection.

• Plasma Membrane: Phospholipid bilayer controlling transport of substances.

• Nucleoid: Region containing circular DNA.

• Ribosomes: 70S type, responsible for protein synthesis.

• Flagella: Used for motility.

• Pili and Fimbriae: Surface structures for attachment and conjugation.

• Capsule: Polysaccharide layer for protection and adherence.

• Example: Streptococcus pneumoniae has a prominent capsule aiding in pathogenicity.

Archaean Cell Structures and Their Differences from Bacteria

Archaea are prokaryotic but possess distinct cellular features compared to bacteria.

• Cell Wall: Lacks peptidoglycan; may contain pseudopeptidoglycan or other polymers.

• Plasma Membrane: Contains ether-linked lipids (as opposed to ester-linked in bacteria).

• Genetic Machinery: Ribosomes and some genetic processes are more similar to eukaryotes.

• Flagella: Structurally different from bacterial flagella.

• Example: Halobacterium species thrive in high-salt environments due to unique membrane adaptations.

Eukaryotic Cell Structures and Their Differences from Prokaryotes

Eukaryotic cells are characterized by compartmentalization and specialized organelles.

• Nucleus: Contains linear DNA organized into chromosomes.

• Mitochondria: Site of aerobic respiration.

• Endoplasmic Reticulum and Golgi Apparatus: Involved in protein and lipid processing.

• Chloroplasts: Present in photosynthetic eukaryotes.

• Cell Wall: Present in plants and fungi, but composition differs (cellulose in plants, chitin in fungi).

• Example: Paramecium (a protozoan) has cilia for movement and a complex internal structure.

Evidence for the Endosymbiotic Theory

The Endosymbiotic Theory explains the origin of mitochondria and chloroplasts in eukaryotic cells as a result of symbiosis between ancestral prokaryotes.

• Double Membranes: Both organelles have inner and outer membranes, consistent with engulfment.

• Own DNA: Mitochondria and chloroplasts contain circular DNA similar to bacterial genomes.

• Ribosomes: Organelle ribosomes resemble prokaryotic (70S) ribosomes.

• Replication: Mitochondria and chloroplasts replicate independently of the cell cycle.

• Phylogenetic Evidence: Genetic analyses show mitochondria are closely related to Alphaproteobacteria, and chloroplasts to Cyanobacteria.

• Example: The presence of genes in mitochondria that are highly similar to bacterial genes supports the theory.