ABSTRACT

Microbes have been evolving for between three and four billion years, resulting in a huge diversity of metabolic types. Organisms have evolved to obtain energy from light or various inorganic substances, or by breaking down different organic compounds to their basic constituents. Some species obtain key elements for building cellular material directly from inorganic minerals, while others require complex “ready-made” organic compounds. One of the original defining features of bacteria and archaea as “prokaryotic” cells was that they have a simple cellular organization with no membrane-bound nucleus or organelles such as mitochondria or chloroplasts. However, many bacterial cells do show extensive internal compartmentalization. Members of the phylum Planctomycetes clearly belong to the Bacteria, but have a membrane-bound nucleus and some have a membrane-bound organelle. Almost all bacteria and archaea possess some type of cell wall external to the cytoplasmic membrane. The cell wall is responsible for the shape of the cell and provides protection in an unfavorable osmotic environment.