Introduction
For many years, archaea were classified as bacteria. However, when modern techniques
allowed scientists to compare the DNA of the two prokaryotes, they found that there were
two distinct types of prokaryotes, which they named archaea and bacteria. Even though
the two groups might seem similar, archaea have many features that distinguish them from
bacteria.
- The cell walls of archaea are distinct from those of bacteria. In most archaea the cell
wall is assembled from surface-layer proteins, providing both chemical and physical
protection. The cell wall acts as a barrier, preventing macromolecules from coming
into contact with the cell membrane. In contrast to bacteria, most archaea lack pep-
tidoglycan in their cell walls. - The plasma membranes of the archaea also are made up of lipids that are distinct from
those in other organisms. - Furthermore, the ribosomal proteins of the archaea resemble those of eukaryotic cells;
the ribosomal proteins of archaea are different from those found in bacteria.
Although archaea and bacteria share some fundamental differences, they are still similar in
many ways.
- They both are unicellular, microscopic organisms that can come in a variety of shapes
(Figure8.10). - BotharchaeaandbacteriahaveasinglecircularchromosomeofDNAandlackmembrane-
bound organelles. - Like bacteria, the archaea can have flagella to assist with movement.
Obtaining Food and Energy
Most archaea are chemotrophs and derive their energy and nutrients from breaking down
molecules from their environment. A few species of archaea are photosynthetic and capture
theenergyofsunlight; chemotrophsdonotcapturetheenergyfromsunlight. Unlikebacteria,
which can be parasites and are known to cause a variety of diseases, there are no known
archaea that act as parasites. Some archaea do live within other organisms, however, but
form mutualistic relationships with their host, where both the archaea and host benefit. In
other words, they actually assist the host in some way, for example by helping to digest food.
Reproduction
Like bacteria, reproduction in archaea is asexual. Archaea can reproduce through binary
fission, where a parent cell divides into two genetically identical daughter cells. Archaea can