Archaea and bacteria are both single-celled microorganisms, but they are classified into different domains due to significant differences in their genetic, biochemical, and structural characteristics.
One of the primary reasons for this classification is the difference in their genetic makeup. Archaea have unique genes and metabolic pathways that are not found in bacteria. For example, the enzymes involved in transcription and translation in archaea are more similar to those in eukaryotes (organisms with complex cells, like plants and animals) than to those in bacteria.
Another key difference lies in the structure of their cell walls. Bacterial cell walls are made of peptidoglycan, a substance that is absent in archaea. Instead, archaea have cell walls composed of different materials, such as pseudopeptidoglycan or other unique polymers.
Additionally, archaea have distinct membrane lipids. While bacterial membranes are made of fatty acids linked to glycerol by ester bonds, archaeal membranes consist of isoprenoid chains linked to glycerol by ether bonds. This difference in membrane composition contributes to the ability of many archaea to survive in extreme environments, such as high temperatures, high salinity, or highly acidic conditions.
These genetic, structural, and biochemical differences led scientists to classify archaea into a separate domain, distinct from bacteria, highlighting the diversity and complexity of life on Earth.