Cellular Characteristics and Metabolism Across the Three Domains of Life

Characteristics of Cells in All Three Domains

Shared Features of Bacteria, Archaea, and Eukarya

All living cells, regardless of their domain, share several fundamental characteristics that are essential for life. These features reflect the basic requirements for cellular function and survival.

• Basic Building Blocks: Cells are composed of chemical molecules such as proteins, lipids, carbohydrates, and nucleic acids, which are assembled to create functional cellular structures.

• Genetic Information Flow: All cells use the central dogma of molecular biology, where genetic information flows from DNA to RNA to protein.

• Plasma Membrane: The plasma membrane encloses the cell, maintaining the internal environment and mediating interactions with the external environment.

• Use and Need for Energy: Cells require energy to drive metabolic processes and maintain life. Energy metabolism is a universal feature.

Example: The presence of a plasma membrane is observed in Escherichia coli (Bacteria), Halobacterium (Archaea), and human cells (Eukarya).

Energy and Metabolism

Universal Requirements for Cellular Life

Energy and carbon are fundamental requirements for all cells. These resources are used to build macromolecules and fuel cellular processes.

• Source of Carbon and Energy: All cells must acquire carbon and energy from their environment.

• Macromolecule Synthesis: Energy is used to synthesize proteins, nucleic acids, lipids, and carbohydrates.

• ATP as Energy Carrier: Adenosine triphosphate (ATP) is the universal energy currency in cells, used to power biochemical reactions.

Example: Muscle contraction in animals and active transport in bacteria both require ATP.

Modes of Nutrition and Metabolism

Diversity of Nutritional Strategies

Organisms are classified based on how they obtain energy and carbon. Prokaryotes (Bacteria and Archaea) exhibit the greatest diversity in metabolic modes.

• Chemoautotrophs: Use inorganic molecules for energy and CO2 as a carbon source. Found in some bacteria and archaea.

• Photoautotrophs: Use light for energy and CO2 as a carbon source. Includes plants, some bacteria, and some protists.

• Chemoheterotrophs: Use organic molecules for both energy and carbon. Includes animals, fungi, many bacteria, and protists.

• Photoheterotrophs: Use light for energy and organic molecules for carbon. Found in some bacteria.

Example: Cyanobacteria are photoautotrophs, while Escherichia coli is a chemoheterotroph.

Carbon Source	Energy Source
	Oxidation of Molecules (Chemo-)	Light (Photo-)
CO2 (Auto-)	Chemoautotroph Some bacteria and archaea; no eukaryotes	Photoautotroph Some bacteria, some protists, most plants
Organic molecules (Hetero-)	Chemoheterotroph Some bacteria, archaea, protists, fungi, animals, some plants	Photoheterotroph Some bacteria

Additional info: Inorganic molecules are used by chemoautotrophs, organic molecules by chemoheterotrophs.

Comparing the Three Domains of Life

Bacteria, Archaea, and Eukarya: Key Differences and Similarities

The three domains of life—Bacteria, Archaea, and Eukarya—differ in cellular structure, genetic organization, and metabolic diversity.

• Bacteria and Archaea: Both are prokaryotes, lacking a nucleus and membrane-bound organelles. They are generally smaller and simpler than eukaryotic cells.

• Genetic and Biochemical Diversity: Prokaryotes exhibit extensive diversity in metabolic pathways and can utilize a wide range of substances for energy and carbon.

• Environmental Adaptation: Prokaryotes thrive in diverse environments, including extreme conditions such as hot springs and polar regions.

• Reproduction: Prokaryotes reproduce rapidly, often outnumbering eukaryotes in many habitats.

Example: Thermophilic archaea live in hot springs, while bacteria are found in soil, water, and as part of the human microbiome.

Prokaryotic Cell Structure and Function

Common Features of Bacterial and Archaeal Cells

Prokaryotic cells share several structural features that distinguish them from eukaryotic cells.

• Nucleoid: Region containing the cell's DNA, not enclosed by a membrane.

• Ribosomes: Complexes of RNA and protein responsible for protein synthesis.

• Cytoplasm: Gel-like substance filling the cell interior.

• Plasma Membrane: Encloses the cell and regulates transport.

• Cell Wall: Provides structural support and protection.

• Glycocalyx: Capsule or slime layer outside the cell wall, offering protection.

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

Example: Streptococcus pneumoniae has a thick capsule, while Escherichia coli has flagella for movement.

Bacterial DNA and Plasmids

Genetic Organization in Bacteria

Bacterial genetic material is organized differently from eukaryotes, with most bacteria possessing a single, circular chromosome and additional plasmids.

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

• Plasmids: Small, circular DNA molecules carrying non-essential but often beneficial genes (e.g., antibiotic resistance).

• Replication: Plasmids replicate independently of chromosomal DNA.

Example: Plasmids in Staphylococcus aureus can confer resistance to antibiotics.

Cell Division in Prokaryotes

Binary Fission and the Role of FtsZ

Prokaryotic cells divide by binary fission, a process involving the FtsZ protein, which forms a contractile ring at the future site of division.

• FtsZ Protein: Forms the Z-ring, guiding cell division.

• Binary Fission: Results in two genetically identical daughter cells.

Example: Binary fission allows rapid population growth in bacteria.

Bacterial Cell Membrane and Cell Wall

Structure and Function

The bacterial cell membrane serves as a boundary and is involved in metabolism, while the cell wall provides structural support and protection.

• Cell Membrane: Site of ATP production and transport; photosynthetic bacteria may have internal membranes derived from the plasma membrane.

• Cell Wall: Composed of peptidoglycan, with differences between Gram-positive and Gram-negative bacteria.

Example: Photosynthetic bacteria have thylakoid membranes for light absorption.

Gram-Positive vs. Gram-Negative Bacteria

Cell Wall Structure and Implications

Bacterial cell walls can be classified as Gram-positive or Gram-negative based on their structure and staining properties.

• Gram-Positive: Thick peptidoglycan layer; stains purple in Gram stain.

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

• Peptidoglycan: Composed of glycan chains cross-linked by peptides.

Example: Bacillus subtilis is Gram-positive; Escherichia coli is Gram-negative.

Glycocalyx: Capsule and Slime Layer

Protective Structures Outside the Cell Wall

Some bacteria possess a glycocalyx, which can be a capsule or slime layer, providing protection against environmental stress and immune responses.

• Capsule: Dense, well-organized layer of polysaccharides (sometimes proteins).

• Slime Layer: Looser, hydrated layer.

• Functions: Protection from desiccation, antibiotics, viruses, and antibodies.

Example: The capsule of Streptococcus pneumoniae enhances its virulence.

Extracellular Appendages: Flagella, Fimbriae, and Pili

Motility and Attachment in Prokaryotes

Bacteria and archaea may possess flagella for movement and fimbriae or pili for attachment to surfaces or other cells.

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

• Fimbriae: Short, rigid, hair-like structures for attachment.

• Pili: Similar to fimbriae, used for attachment and conjugation (DNA transfer).

Example: Neisseria gonorrhoeae uses pili to attach to host cells.

Archaea: Unique Features and Comparison

Distinctive Characteristics of Archaea

Archaea share some features with bacteria and eukaryotes but also possess unique molecular structures.

• Prokaryotic Cell Structure: Smaller than eukaryotes, with diverse morphologies.

• Nucleoid and Circular Chromosome: Like bacteria, archaea have a nucleoid and circular DNA.

• Ribosomes and Cytoskeleton: Present in archaea, with some similarities to eukaryotes.

• Cell Wall: Not composed of peptidoglycan; unique molecular composition.

• Membrane Lipids: Distinct from both bacteria and eukaryotes.

• Extracellular Structures: Unique flagella, glycocalyx, and fimbria-like structures.

Example: Halobacterium has unique membrane lipids adapted to high-salt environments.

Additional info: No identified human pathogens among archaea, but some are part of the human microbiome.