Cellular Diversity 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 structure and function.

• Basic Building Blocks: Cells are composed of chemical molecules (such as proteins, lipids, carbohydrates, and nucleic acids) that are assembled to create a functional cell.

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

• Plasma Membrane: Every cell is surrounded by a plasma membrane that separates the internal environment from the external surroundings.

• Use and Need for Energy: Cells require energy to maintain life processes, which is managed through energy metabolism.

Example: The plasma membrane in Escherichia coli (Bacteria), Halobacterium (Archaea), and human cells (Eukarya) all serve as selective barriers and sites for metabolic activity.

Energy and Metabolism

Universal Requirements for Cellular Life

Energy metabolism is a core aspect of cellular function, enabling cells to grow, reproduce, and respond to their environment.

• Source of Carbon and Energy: All cells must acquire carbon (for building macromolecules) and energy (to drive cellular processes).

• Macromolecule Synthesis: Energy is used to synthesize macromolecules such as proteins, nucleic acids, and polysaccharides.

• 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. This diversity is most pronounced among prokaryotes (Bacteria and Archaea).

• Chemoautotrophs: Use inorganic molecules as an energy source and CO2 as a carbon source. Found in some bacteria and archaea; no eukaryotes.

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

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

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

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 for chemoautotrophs include hydrogen sulfide, ammonia, and ferrous iron. Organic molecules for chemoheterotrophs include glucose and amino acids.