In the last few decades, several taxonomic schemes have been used to classify the world's organisms. One of the simplest divides life into prokaryotes and eukaryotes; that is, those organisms without nuclei went into one group and those with nuclei went into another, respectively. Another commonly used scheme divides life into five kingdoms: Monera (prokaryotes), Protista, Plantae, Fungi, and Animalia. You may see one of these schemes in an older textbook or website.
Keep in mind that classification schemes strive to show the evolutionary relationships between groups, and in recent years it has become apparent that the evolutionary relationships of prokaryotes are quite complex. One prokaryotic group, the Archaea, have some features that are more eukaryotic-like than prokaryotic-like. For example, Transcription and Translation (the stages of producing a protein) are more similar between the Archaea and the Eukarya than either is to the Bacteria. The replication of DNA in Archaea is also more similar to that in Eukarya than in the Bacteria.
To reconcile new data, the taxonomic scheme of life has been revised. The most current scheme proposes that life be divided into three domains. In this scheme prokaryotic organisms can belong to the domain Archaea or the domain Bacteria, while those organisms that have a nucleus comprise the third domain, Eukarya. Organisms that belong to each of these three domains are sometimes referred to as the archaebacteria, eubacteria and eukaryotes, respectively. This figure illustrates the relationship among the three domains. Keep in mind that the last universal common ancestor ( LUCA ) of life on earth most likely arose over 3.5 billion years ago. This organism probably had a fairly complex cell structure.
Archaea are sometimes referred to as extremophiles, and many species live in extreme environments (e.g., hot springs, salt ponds, Arctic ice, deep oil wells, acidic ponds that form near mines, and hydrothermal vents); however, these environments are not extreme to those archaea. In fact, many extremophiles die when moved to our more temperate environment.
The archaea have several features that distinguish them from the bacteria. First, the archaea do not have the compound peptidoglycan associated with their cell membranes. Peptidoglycan is a polymer of sugars and amino acids that is unique to the bacteria. Secondly, the machinery that bacteria and archaea use to read the organism's genetic material is different (we will not focus on the differences). Thirdly, members of the archaea have unique lipids associated with their cell membranes (we will not focus on the characteristics of these lipids).
Watch the video below (from Moo Moo Science) to review the key differences between the three different domains of life.
To watch this video on YouTube (and see closed captioning) - press the arrow icon in the bottom right corner of the video player.
Bacterial Shape and Structure of the Cell Wall
There are millions of different types of bacteria in the world. That’s why it’s important to have a way to classify them. Scientists usually classify bacteria based on two characteristics:
The shape of individual cells is used to classify prokaryotes; they can be either spherical ( coccus ), rod-shaped ( bacillus ), or helical ( spirillum ).
The cells of all organisms (including prokaryotes) are encased in a plasma membrane, which consists of a phospholipid bilayer that is selectively permeable. This bilayer keeps salts and liquids in balance inside the cell by engulfing needed particles and ridding the cell of wastes. In addition to a plasma membrane, many organisms (including plants, fungi, and some protists) also have cell walls. These walls are involved in maintaining cell shape and protection. Most prokaryotes have cell walls, but they are structurally different than those of the plants, fungi, and protists (Archaea have unique molecules in their cell walls and cell membranes that we won’t discuss here).
Bacteria containing a lot of peptidoglycans in their cell walls also tend to have less complex cell walls and are called gram-positive bacteria. Conversely, gram-negative bacteria have less peptidoglycan, but have more complex cell walls overall. In particular, the gram-negative bacteria have an additional outer membrane with attached lipopolysaccharides.
A procedure known as a Gram's stain is used to differentiate gram-positive and gram-negative bacteria. Gram staining is a procedure that involves staining and washing bacteria with a series of stains. Bacteria Bacteria that stain purple contain a lot of peptidoglycans in their cell walls and also tend to have less complex cell walls. They are gram-positive bacteria. Conversely, bacteria that stain pink have less peptidoglycan, but have more complex cell walls overall. They are gram-negative bacteria. In particular, the gram-negative bacteria have an additional outer membrane with attached lipopolysaccharides (LPS).
Some of these lipopolysaccharides are toxic, serving to counteract the natural defenses of the host organism. Also, the additional membrane of gram-negative bacteria can make them more resistant to antibacterial medications (such as antibiotics ). For these two reasons, a gram-negative bacterial infection can sometimes be far more severe than a gram-positive infection.