The Three-Domain System

The three-domain system is a modern system of classification that recognizes three domains of life: BacteriaA single-celled prokaryotic microorganism., Archaea, and Eukarya.

Bacteria are single-celled organisms that are characterized by their lack of a nucleusA membrane-bound organelle in eukaryotic cells that contains DNA. and their small size. Bacteria are found in almost every habitat on Earth, and they are important for a variety of ecological processes, including decomposition and nitrogen fixation. Bacteria are classified into two major groups based on their cell wallA rigid outer layer that strengthens and supports the cell; made of cellulose in plants and peptidoglycan in bacteria. structureThe organisation and order of information in a text.: Gram-positive bacteria, which have a thick cell wall, and Gram-negative bacteria, which have a thin cell wall.

Archaea are single-celled organisms that are similar to bacteria, but they have some unique characteristics, such as the ability to survive in extreme environments. Archaea are found in a variety of habitats, including hot springs, salt flats, and deep-sea vents. Archaea are classified into several groups based on their metabolismChemical reactions within a cell allowing microorganisms to maintain themselves and multiply. and habitat, including methane-producing archaea, sulfur-reducing archaea, and halophilic archaea.

Eukarya are organisms that have a nucleus and other membrane-bound organelles. Eukarya include all of the multicellular organisms, such as animals, plants, fungi, and protists.