username@email.com
username@email.com
In this lesson, we will review prokaryotes.
The first organisms to live on Earth were probably primitive prokaryotes—the earliest form of simple, single-celled organisms. Archaea are the oldest known living things and are grouped in their own domain. Although there is still debate about what organisms actually evolved first, many scientists agree that today’s prokaryotes are their closest relatives. Among these ancient organisms were methanogens, which synthesize methane from carbon dioxide and hydrogen gases (abundant substances in Earth’s early days); thermoacidophiles, specialized to survive in extremely hot and acidic environment; and halobacteria, which thrive in highly saline conditions. Many of the archaea still living today are extremophiles, living in and adapted to extreme environmental conditions such as deep-sea vents, hot springs, and geysers.
Prokaryotic organisms are some of the simplest forms of life on Earth
The Archaea probably gave rise to other single-celled prokaryotes called Bacteria (or possibly vice-versa). No matter which organisms came first, we do know that the two most primitive of the three domains of life, are prokaryotic. Their most singular characteristic is their lack of a membrane-bound nucleus and membrane-bound organelles. In prokaryotes, the single, continuous strand of DNA (naked DNA) floats freely in the cytoplasm. Both the Archaea and the Bacteria are prokaryotes.
The fundamental characteristics shared by nearly all prokaryotes include:
Most prokaryote locomotion is accomplished by the rapid beating of the cilia that often completely surround the cell. Some prokaryotes can also have one or more flagella.
Reproduction is usually by binary fission, or cell division. The complex series of steps in mitosis (done in eukaryotes) are not needed among prokaryotes because of their simplified, single-stranded DNA. However, mis-matching and other chemical errors frequently lead to mutations during the binary split. For many prokaryotes, particularly infectious bacteria, a new generation may arise every 15 minutes. Combine the short generation time with the occurrence of mutations and you can see why bacteria evolve quickly and are able to develop resistance to antibiotics.
Some prokaryotes reproduce via budding, in which a genetically identical offspring develops out of the parent’s body and eventually buds off from the parent. Some prokaryotes are able to generate spores that remain dormant, but viable, for long periods. The spores of the bacterium Clostridium botulinum, which causes botulism, can survive hours of boiling and still kill you.
Generally, prokaryotes have one of four characteristic shapes:
Two bacteria that have a unique shape are the spirochetes (e.g.,Terponema pallidum, which causes syphilis), which can be incredibly long, and rickettsiae (e.g. Rickettsia prowazeki, which causes typhus), which are the smallest cells known.
Most prokaryotes are heterotrophs that mainly consume dead organic matter. It is to bacteria that we owe the existence of organically rich topsoil, because bacteria decompose organic matter in soil. Some heterotrophic bacteria live symbiotically in hosts, either as commensals, parasites (some pathogenic), or in mutualism that benefits both organisms. There are even heterotrophic bacteria that are parasites of other bacteria. Some prokaryotes are chemotrophs that can use inorganic materials as a source of energy. Archaea are largely chemotrophs, particularly the methanogens that still thrive in anaerobic environments. One group of chemotrophs, vital to the nitrogen cycle, transforms inorganic nitrogen and makes it available to plants.
Some bacteria are photosynthetic. In one group of sulfur bacteria, energy derived from the electrons in sulfur atoms take the place of the electrons derived from water in plants. Nonsulfur photosynthetic bacteria are the ancient cyanobacteria, which were extremely important in the formation of the earliest ecosystems. Cyanobacteria are sometimes called blue-green algae, but they are not related to the true algae (which are eukaryotes). Cyanobacteria contain chlorophyll a and break down water during photosynthesis (as do plants). In the process they release oxygen. The countless cyanophytes that lived in the Earth’s early oceans contributed the oxygen to our atmosphere that we, and most organisms, require to survive. It is probable that cyanophytes gave rise to the first eukaryotic algae.
Which of the following is a characteristic of Archaea?
The correct answer is B. The presence or absence of flagella is not a characteristic of Archaea. One type of archaebacteria, the halobacteria, thrive in saline conditions, but only methanogens synthesize hydrocarbons (methane).
The most common form of reproduction among prokaryotes is
The correct answer is B. Most prokaryotes reproduce asexually via cell division, or binary fission. Fragmentation is the regeneration of a lost part of the organism, as in starfish. Some prokaryotes do reproduce via budding, but it is not common. Similarly, some prokaryotes produce spores, but this is rare among prokaryotes.
Most prokaryotes are heterotrophs that ingest
The correct answer is D. Most prokaryotes live in soil where they ingest and decompose decaying organic matter and release it back into the soil. Some heterotrophic prokaryotes may consume unicellular autotrophs, but this is not widespread, so A is incorrect. Similarly, some heterotrophic prokaryotes may ingest others of their kind, but this is not common, so B is incorrect. Chemotrophs do metabolize inorganic substances, but they are a small sector of the prokaryotic community, so C is not correct.
Which of the following bacteria are photosynthetic?
The correct answer is D. Cyanobacteria are photosynthetic marine bacteria that put oxygen into our atmosphere. Algae are eukaryotic, not bacteria. Vibrios are a category of prokaryotes based on their incomplete spiral shape but are not photosynthetic. Mycoplasmas are heterotrophic, not autotrophic, prokaryotes.
Eukaryotes are characterized by their membrane-bound nucleus and organelles, and the chromosomes that carry their DNA. The first eukaryotes evolved about 1.5 billion years ago (for the previous 3 billion years, the world belonged to prokaryotes). The evolutionary step from prokaryotes to eukaryotes was one of the most revolutionary events in the biological history of the Earth.
One widely supported theory, the endosymbiotic theory, suggests that mitochondria were once free-living bacteria that had developed the ability to use oxygen and make ATP. This conferred such an evolutionary advantage that these bacteria thrived and eventually evolved into aerobic bacteria. It is believed that over time these ATP-producing organisms became incorporated into larger cells, perhaps through ingestion. A prokaryote that incorporates into itself an ATP-generating cell gains a definite energetic, and evolutionary, advantage. Similarly, a large nonphotosynthetic prokaryote that ingests a photosynthetic prokaryote (a proto-chloroplast) also has a survival advantage. The fact that mitochondria have their own very prokaryote-like, single-strand DNA, and that their RNA and ribosomes closely resemble those of prokaryotes supports this theory further.
Single-celled eukaryotes are mostly classified as Protists, which includes algae, slime molds, and protozoa.