Scientists classify living things into systems based on shared characteristics to understand their relationships and evolutionary history. This framework helps organize the immense diversity of life on Earth.
Understanding Prokaryotes
Prokaryotes are single-celled organisms characterized by their simple cellular structure. A defining feature of prokaryotic cells is the absence of a membrane-bound nucleus; instead, their genetic material, typically a single circular DNA molecule, resides in a region called the nucleoid within the cytoplasm. They also lack other membrane-bound organelles, such as mitochondria or chloroplasts, which are common in more complex cells.
Prokaryotes possess essential cellular components like ribosomes, which are responsible for protein synthesis. Their small size, generally ranging from 0.1 to 5 micrometers in diameter, contributes to their efficiency in nutrient exchange and rapid reproduction. Most prokaryotes are encased by a cell wall, which provides structural support and protection, though its composition can vary. These ubiquitous organisms are found in nearly every environment on Earth, from soil and water to the bodies of other living organisms and extreme habitats.
The Kingdom Monera
Historically, all prokaryotic organisms were grouped together under the Kingdom Monera. This classification was part of the widely accepted five-kingdom system, which also included Protista, Fungi, Plantae, and Animalia. This kingdom encompassed a broad range of microorganisms, including what were then known as bacteria and archaebacteria.
However, scientific advancements, particularly in molecular biology, revealed significant genetic and biochemical differences among these organisms. Consequently, the Kingdom Monera is no longer considered a valid classification because it grouped evolutionarily distinct organisms. These fundamental differences led to a re-evaluation of how life is classified.
The Domain System
The current scientific understanding classifies life into a three-domain system: Bacteria, Archaea, and Eukarya. This system was proposed by Carl Woese in 1977, based on extensive analysis of ribosomal RNA (rRNA) sequences, which provided a more accurate reflection of evolutionary relationships. In this system, the prokaryotes are divided into two distinct domains: Bacteria and Archaea. This division acknowledges that Bacteria and Archaea, while both prokaryotic, are as genetically different from each other as they are from eukaryotes.
Bacteria
Bacteria, often referred to as true bacteria, constitute a vast and diverse group of prokaryotes found in nearly all environments. Their cell walls primarily consist of peptidoglycan, a unique polymer providing structural strength. Their cell membranes are typically composed of ester-linked lipids. Bacterial ribosomal RNA sequences also possess specific characteristics that distinguish them from Archaea and Eukarya.
Archaea
Archaea, once mistakenly classified as archaebacteria, are now recognized as a separate domain due to their unique biochemical and genetic characteristics. Their cell walls lack peptidoglycan, instead being composed of substances like pseudopeptidoglycan, glycoproteins, or S-layer proteins.
A notable difference is found in their cell membranes, which contain ether-linked lipids, providing greater stability and allowing many Archaea to thrive in extreme environments such as hot springs and highly saline waters. While archaeal ribosomes are structurally similar to bacterial ribosomes, their ribosomal RNA sequences and the organization of their genes show distinct differences, with some features being more akin to eukaryotes.