Biochemistry - The Chemical Reactions of Living Cells, Volume 1 - D. Metzler 1980

The Setting
The Simplest Living Organisms
Mycoplasmas

The simplest organisms on Earth are Bacteria and blue-green Algae, which make up the kingdom of prokaryotes (Procariotae, Monera) [1, 2]. The primary distinguishing feature of prokaryotes is the absence of a membrane-bound Cell Nucleus. The Cells of all other organisms, known as eukaryotes, contain nuclei separated from the Cytoplasm by a membrane. Some biologists also classify Viruses as living organisms, yet these remarkable entities (Box 4-B) cannot be considered fully alive in the strict sense, as they generally lack an independent METABOLISM.

Mycoplasmas belong to the simplest prokaryotes and are the most primitive of all known living organisms. Unlike most other bacteria, these tiny organisms lack a rigid Cell wall. As a result, they easily change shape and frequently pass through filters that retain other bacteria.

In terms of Nutrition, mycoplasmas are rather fastidious and typically (if not always) parasitic. Some inhabit human mucous membranes without causing any harm, whereas others cause diseases, such as primary atypical Pneumonia (pathogen: Mycoplasma pneumoniae).

Mycoplasma cells are typically spherical, with a diameter of 0.33 µm (0.0003 mm) [3], and are enclosed by a thin cell membrane about 8 nm (80 Å) thick. Inside is the cytoplasm—a fluid matrix containing a multitude of dissolved substances as well as submicroscopic particles. A single, extremely tightly coiled DNA molecule, which constitutes the bacterial chromosome, is localized in the center of The Cell. The region of the cell containing the chromosome and its surrounding area can be referred to as a nucleus¹ or nucleoid. In addition to DNA, the cell contains about 400 roughly spherical particles, approximately 20 nm in diameter, called Ribosomes². These are the centers of Protein Synthesis. The cytoplasm also houses various Proteins, though their number does not exceed 50,000. Furthermore, there are several types of RNA and numerous lower-molecular-weight compounds.

What is the minimum quantity of proteins, DNA, and other macromolecular structures sufficient to sustain cellular life? This remains unknown, but it is entirely clear that in the tiny mycoplasma cell, these compounds are present in sufficient measure.

¹ Many biologists reserve the term nucleus strictly for Eukaryotic cells.

² The stated number of particles is merely a rough estimate based on a comparison of mycoplasma and E. coli sizes and their RNA content.