Cells (Structure & Function)

Introduction to Cell Types

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

• 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

General Features of Prokaryotes

Bacteria and Archaea are both prokaryotic, sharing several structural and functional characteristics. However, they also exhibit important differences at the molecular level.

• Simpler structure compared to eukaryotic cells

• Intense genetic and biochemical diversity: Can utilize a wide variety of substances for energy and carbon sources

• Ubiquity: Found in almost all environments, from Antarctic ice to hot springs

• High species diversity: Millions of different species, outnumbering all other types of organisms

• Rapid reproduction: Capable of incredible rates of reproduction

Common Structures in Prokaryotic Cells

Most prokaryotic cells share several key structural features that are essential for their survival and function.

• Nucleoid: Region containing DNA; not surrounded by a membrane

• Ribosomes: Sites of protein synthesis

• Cytoplasm: Gel-like substance filling the cell

• Plasma Membrane: Boundary of the cell, involved in metabolism and transport

• Cell Wall: Provides structural support and protection

• Glycocalyx: Outer layer, may be a capsule or slime layer, offering protection

• Flagella and/or Fimbriae: Appendages for movement and attachment

Bacterial Cell Structure

Genome Organization

Bacterial genetic material is typically organized in a single, circular DNA molecule called the chromosome, located in the nucleoid. Many bacteria also possess plasmids, which are small, circular DNA molecules carrying non-essential but beneficial genes (e.g., antibiotic resistance).

• Chromosome: Main genetic material, circular, located in nucleoid

• Plasmids: Extra-chromosomal DNA, replicate independently, often carry genes for special functions

Internal Structures

• Ribosomes: 70S type, smaller than eukaryotic ribosomes (80S)

• Cytoskeleton: Proteins such as FtsZ and MreB help maintain cell shape and division

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

Cell Membrane

The plasma membrane serves as the boundary of the cell and is involved in metabolic processes such as ATP production. In photosynthetic bacteria, internal membranes derived from the plasma membrane are present.

Cell Wall Composition

Most bacteria possess a cell wall containing peptidoglycan, a polymer consisting of sugar chains cross-linked by peptides. This structure provides rigidity and protection.

• Peptidoglycan: Composed of N-acetylglucosamine (NAG) and N-acetylmuramic acid (NAM) with peptide cross-bridges

Gram-Positive vs Gram-Negative Bacteria

Bacterial cell walls can be classified based on their structure and staining properties:

• Gram-Positive: Thick peptidoglycan layer

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

Glycocalyx

The glycocalyx is an external layer found in some bacteria, which can be a capsule (tight, less water) or slime layer (loose, hydrated). It protects against environmental stress, antibiotics, and immune responses.

Extracellular Appendages

• Flagella: Long, whip-like structures for motility

• Fimbriae: Shorter, hair-like structures for attachment to surfaces or host cells

Fimbriae are structurally distinct from flagella and are not used for motility. They help bacteria adhere to surfaces, such as teeth or tissues.

Archaeal Cell Structure

General Features

Archaea are prokaryotic cells similar in size and morphology to bacteria but possess unique molecular features. Many archaea are found in extreme environments and have diverse metabolic capabilities.

• Nucleoid with circular chromosome

• Ribosomes and cytoskeleton

• Most have cell walls (not peptidoglycan)

• Some have glycocalyx, flagella, and fimbriae-like structures

Similarities and Differences with Bacteria and Eukaryotes

Archaea share some molecular features with eukaryotes (e.g., aspects of transcription and translation) but are structurally distinct from both bacteria and eukaryotes.

• Unique flagella, cell wall composition, and membrane lipids

• Some ribosomal and genetic features resemble eukaryotes

• No identified human pathogens, but some are part of the human microbiome

Distinguishing Features: Bacteria vs Archaea

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

Summary

• Prokaryotic cells (Bacteria and Archaea) are structurally simpler than eukaryotic cells but exhibit remarkable diversity and adaptability.

• Bacteria and Archaea share many features but differ in key molecular and structural aspects.

• Understanding prokaryotic cell structure is essential for studying microbiology, evolution, and biotechnology.