Prokaryotic Cells: Structure and Function

Cells (Structure & Function): Prokaryotic Cells

Overview of Cell Types in the Three Domains

All living organisms are classified into three domains: Bacteria, Archaea, and Eukarya. These domains are distinguished by their cell types:

• Bacteria: Prokaryotic cells

• Archaea: Prokaryotic cells

• Eukarya: Eukaryotic cells

Prokaryotic cells lack a membrane-bound nucleus and other organelles, while eukaryotic cells possess these structures.

Prokaryotic Cell Structure and Function

Bacteria and Archaea are both prokaryotic, sharing fundamental structural features but also exhibiting important differences. Prokaryotic cells are generally smaller and simpler than eukaryotic cells, yet they display remarkable diversity and adaptability.

• Smaller/Simpler Structure: Prokaryotic cells are typically 1 micrometer in diameter, about 1/10 the size of a typical eukaryotic cell.

• Genetic and Biochemical Diversity: They utilize a wide variety of substances for energy and carbon sources.

• Global Distribution: Prokaryotes thrive in nearly all environments, from Antarctic ice to hot springs.

• Species Diversity: There are millions of different prokaryotic species, outnumbering all other types of organisms.

• Reproductive Capacity: Prokaryotes reproduce rapidly, allowing for quick adaptation.

Common Structures in Prokaryotic Cells

Most prokaryotic cells share several key structural components:

• Nucleoid: Region containing DNA, not surrounded by a membrane.

• Ribosomes: Sites of protein synthesis.

• Cytoplasm: Gel-like substance filling the cell.

• Plasma Membrane: Selective barrier regulating entry and exit of substances.

• Cell Wall: Provides structural support and protection.

• Glycocalyx: Outer layer (capsule or slime) for protection and adhesion.

• Flagella and/or Fimbriae: Appendages for motility and attachment.

Bacterial Genome Organization

Bacterial genetic material is typically organized as follows:

• Chromosome: Usually a single, circular DNA molecule located in the nucleoid.

• Plasmids: Small, circular DNA molecules carrying non-essential but beneficial genes (e.g., antibiotic resistance, metabolism, toxins). Plasmids replicate independently of the chromosome.

Internal Structures of Prokaryotic Cells

• Cytoplasm: Contains ribosomes and sometimes a cytoskeleton (proteins such as FtsZ and MreB involved in cell shape and division).

• Internal Membrane Structures: Some bacteria have internal membranes for specialized functions (e.g., photosynthesis, calcium storage).

Plasma Membrane Functions

The plasma membrane serves as the boundary of the cell and is involved in:

• Regulating transport of substances

• Metabolic processes (e.g., ATP production)

• In photosynthetic bacteria, internal membranes derived from the plasma membrane facilitate photosynthesis.

Cell Wall Composition

Most bacteria possess a cell wall containing peptidoglycan, a polymer consisting of:

• Carbohydrate backbone: Alternating units of N-acetylglucosamine (GlcNAc) and N-acetylmuramic acid (MurNAc).

• Peptide cross-bridges: Link carbohydrate chains, providing strength.

Gram-Positive vs. Gram-Negative Bacteria

Bacterial cell walls are classified based on their structure:

• Gram-Positive: Single, thick peptidoglycan layer.

• Gram-Negative: Thin peptidoglycan layer surrounded by an outer membrane.

Glycocalyx: Capsule and Slime Layer

The glycocalyx is an external layer found in some bacteria:

• Capsule: Dense, tightly bound, less water content.

• Slime Layer: Looser, more hydrated.

• Functions: Protection from environmental stress, antibiotics, viruses, and immune responses; aids in adhesion.

Extracellular Appendages: Flagella and Fimbriae

• Flagella: Long, whip-like structures for motility.

• Fimbriae: Shorter, hair-like structures for attachment to surfaces or host cells; not used for motility.

Archaea: Unique Features and Comparison

Archaea are prokaryotic cells similar in size and basic structure to bacteria but possess unique molecular features. Many archaea inhabit extreme environments and have distinct metabolic pathways.

• Share features with both bacteria (prokaryotic cell type, nucleoid, circular chromosome, ribosomes, cytoskeleton, cell wall, glycocalyx, flagella, fimbriae-like structures) and eukaryotes (some aspects of information processing systems).

• Unique molecular structures: flagella, cell wall (not peptidoglycan or cellulose), membrane lipids.

• Some archaea are part of the human microbiome; few are known pathogens.

Distinguishing Features of Bacteria and Archaea

The following table summarizes key molecular differences between bacteria and archaea:

Molecular Feature	Bacteria	Archaea
RNA polymerase	Just one type (five subunits)	Just one type (13 subunits; similar to eukaryotic RNA polymerase II)
Peptidoglycan in cell wall	Present	Absent
First amino acid incorporated during translation	Formylmethionine	Methionine
Histones associated with DNA	No	Yes
Plasma membrane lipids	Phospholipid tails made of a pair of fatty acids	Phospholipid tails made of a pair of isoprene chains

Summary

• Prokaryotic cells (bacteria and archaea) are structurally simpler than eukaryotic cells but highly diverse and adaptable.

• Key features include nucleoid, ribosomes, cytoplasm, plasma membrane, cell wall, glycocalyx, flagella, and fimbriae.

• Bacteria and archaea differ in molecular details such as cell wall composition, membrane lipids, and genetic machinery.

Example: Escherichia coli is a well-studied Gram-negative bacterium with a single circular chromosome, plasmids, and flagella for motility.