The cell is the basic unit of life. Though it may not appear so in larger, more complex organisms, life begins and ends with the cell. Every single living thing is a collection of anywhere from one to trillions of cells, from a microscopic henitalium bacterium to the honey fungus spread over 2.4 miles of Oregon’s Blue Mountains.
French biologist Édouard Chatton was the first to characterize the distinction between the prokaryote and the eukaryote. He coined the terms in his 1925 paper Pansporella perplex: Reflections on the Biology and Phylogeny of the Protozoa, calling them prokaryota and eukaryota, the two empires of life. The primary difference between the two is the lack or presence of a true nucleus and membrane bound organelles.
The word prokaryote comes from the Greek pro, meaning before, and karyon, meaning kernel. Prokaryotes are believed to be the first forms of life to have evolved on Earth. While there are many prokaryotic species that seem to aggregate together and divide labor across the colony, each prokaryotic cell is considered a separate organism. There are two types, or domains, of prokaryote. While bacteria and archaea are different in some ways, they do share two fundamental traits that make them prokaryotes. They both exist only as single cell organisms and they both lack nuclei or any other membrane bound organelles.
Bacteria are the more common prokaryotes. They are both the most familiar single celled organisms to most people and the most abundant form of life on Earth. Bacteria are found all across the planet, and many believe they exist throughout the universe. Earth bacteria exist in a variety of shapes and sizes, but typically they are roughly 10 times smaller than eukaryotic cells.
While bacteria don’t have the membrane bound organelles found in eukaryotic cells, they do have their own cellular components that carry out similar functions. A single bacterium has a fairly limited amount of deoxyribonucleic acid, or DNA, which is stored roughly in the center of the cell in the nucleoid region. The nucleoid region is not a true nucleus, but it is somewhat similar. Bacteria also contain ribosomes, which carry out protein synthesis, but they are simpler than those of multicellular organisms.
Bacteria are quite diverse, and the behavior and function of each individual species varies greatly. They can act as predators to other microorganisms. They can partake in symbiotic relationships, either mutual or parasitic in nature. They can act as pathogens for humans and animals, and can secrete chemicals, both harmful and beneficial, into the environment.
If a bacterium can acquire enough nutrients, it will grow and multiply. Bacteria reproduce through the process of binary fission, which is the replication of essential components followed by division into two cells. Errors often occur during DNA replication, which result in bacterial evolution. Since bacterial life cycles tend to be very short from the human perspective, bacterial evolution happens rapidly. While this offers a unique opportunity to study evolution, it also enables harmful bacterial to rapidly evolve to avoid or resist both the human immune system and medical treatment.
As prokaryotes, archaea share many similarities with bacteria. It is for that reason that archaea weren’t recognized as separate from bacteria until 1977. Thanks to the experiments of Carl Woese and George E. Fox, we now know that archaea, previously called archaebacteria and considered a type of bacteria, are actually a different type of organism all together. This discovery brought about an update to Édouard Chatton’s two domain classification system, and the old two domain system of prokaryote and eukaryote was eventually replaced with the three domains bacteria, archaea, and eukaryotes.
Archaea come in a variety of shapes, and range in size from 0.1 to 150 micrometers in length. Some archaea have metabolisms quite different from those of bacteria, and their gene structure actually shows that they’re closer on the evolutionary ladder to eukaryotes than to bacteria. Archaeal cell walls do not contain peptidoglycan, the polymer of sugars and amino acids that forms the mesh-like layer outside bacterial plasma membranes. Their lipids also lack the fatty acids found in bacteria and eukaryotes. Instead, they have side chains composed of repeating units of isoprene. The archaeal ribosome is shaped differently, and the RNA polymerases are more complex than those of bacteria.
The cell is the basic unit of life, and there are two basic types of cells. Eukaryotic cells have nuclei and membrane bound organelles, while prokaryotic cells do not. Prokaryotes are believed to have evolved much earlier than eukaryotes, and exist in much greater numbers and in far more places. However, they are much simpler than eukaryotes and only exist as single cell organisms. As both the most abundant forms of life on Earth and probably the most commonly known prokaryotes, bacteria were long believed to be the only prokaryote. However, since 1977, scientists have known about archaea. Domain Archaea was added as the third domain because they have several physical differences from bacteria and seem closer on the evolutionary ladder to eukaryotes than to bacteria.