Cells that lack a membrane-bound nucleus are called prokaryotes
(from the Greek meaning before nuclei). These cells have few internal
structures that are distinguishable under a microscope. Cells in the
monera kingdom such as bacteria and cyanobacteria (also known as
blue-green algae) are prokaryotes.
Prokaryotic cells differ significantly from eukaryotic cells. They
don't have a membrane-bound nucleus and instead of having chromosomal
DNA, their genetic information is in a circular loop called a
plasmid. Bacterial cells are very small, roughly the size of an animal
mitochondrion (about 1-2µm in diameter and 10 µm
long). Prokaryotic cells feature three major shapes: rod shaped, spherical, and spiral. Instead of going through elaborate replication
processes like eukaryotes, bacterial cells divide by binary fission.
Diagram of a prokaryotic cell. Notice
the internal organelles are not easily distinguishable.
Bacteria perform many important functions on earth. They serve as
decomposers, agents of fermentation, and play an important role in
our own digestive system. Also, bacteria are involved in many nutrient
cycles such as the nitrogen cycle, which restores nitrate into the
soil for plants. Unlike eukaryotic cells that depend on oxygen for
their metabolism, prokaryotic cells enjoy a diverse array of metabolic
functions. For example, some bacteria use sulfur instead of oxygen in