Prokaryotic Cells

Bacteria are examples of the prokaryotic cell type. An example is E. coli. In general, prokaryotic cells are those that do not have a membrane-bound nucleus. In fact "pro-karyotic" is Greek for "before nucleus". Besides bacteria, the cyanobacteria (blue-green algae) are a major group of prokaryotes. There are few well-distinguished internal structures in prokaryotes, in contrast to the multitude of distinct organelles in eukaryotes.

The other domain of the prokaryotes is composed of the organisms called archaebacteria, which are ancient life forms which may live in extreme environments.

The genetic information of prokaryotes (their DNA) is typically in nucleoid of DNA strands, but they may have additional DNA in a circular loop called a plasmid.

Comparison of Eukaryotic and Prokaryotic Cells

All living things are composed of cells which are classified as either prokaryotic or eukaryotic cells. The different cell types have many things in common. Enger & Ross's perspective of the common characteristics lists:

1. Cell membranes
   细胞膜
2. Cytoplasm
   细胞质
3. Genetic material
   遗传物质
4. Energy currency
   能量货币
5. Enzymes and Coenzymes
   酶与辅酶

French biologist Edouard Chatton proposed the names "prokaryote" and "eukaryote" in 1938 based solely on the absence or presence of a nucleus.

Bacteria

Bacteria exist as single prokaryotic cells. The cells are very small, roughly the size of an animal mitochondrion (about 1-2μm in diameter and 10 μm long). Bacteria commonly occur in three major shapes: rod shaped, spherical, and spiral. Instead of going through elaborate replication processes like eukaryotes, bacterial cells divide by binary fission.

This general description of bacterial function was accessed at Thinkquest :"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 their metabolism."

Bacteria are incredibly varied, and incredibly abundant. To give some kind of organization to the multitude, bacteria are typically divided into four phyla: the cyanobacteria, spirochetes, gram-positive bacteria, and proteobacteria.

Cyanobacteria are photosynthetic, like plants, which means that they use the sun's energy to make food for themselves.

Spirochetes are gram-negative , spiral-shaped, and heterotrophic. Some of them live in the presence of oxygen, others don't. They may be parasitic, living symbiotically (where two organisms live off each other), or free-living. One type of spirochete causes syphilis.

Gram-positive bacteria are not all gram-positive, despite the same. They are grouped together due to other similarities. This phylum includes the strain of streptococcus bacteria that causes strep throat. It also includes the bacteria that produces yogurt, by growing and fermenting in milk (producing lactic acid). These bacteria also produce many of our antibiotics.

Proteobacteria is one of the largest phyla of all the bacteria. Many are gram-negative. They are divided into several subgroups, such as enteric bacteria, chemoautotrophs, and nitrogen-fixing bacteria. The enteric bacteria live mainly in intestinal tracts, like E. coli. The chemoautotrophs oxidize chemicals in minerals to obtain energy. The nitrogen-fixing bacteria are essential to many ecosystems - one type converts the unusable nitrogen in the atmosphere to ammonia, the form plants can use most easily.

This material on bacteria was largely taken from essortment)

Archaea

Archaea were first called "archaeabacteria", suggesting "ancient bacteria", but they are considered distinct from ordinary bacteria. Partly to avoid confusion with other bacteria, this domain is currently called archaea. The Archaea were originally found in extreme environments like hot springs (e.g., Yellowstone National Park) and hot, deep ocean vents where ordinary bacteria could not survive and were dubbed "extremophiles". They have since been found in all environments. They are considered to be the oldest living organisms on the Earth. They are single-celled organisms, classified as prokaryotes. They operate in the absence of oxygen only, and thus are dubbed "obligate anaerobes".

The three phyla of archaebacteria are the methanogens, the halophiles and the thermoacidophiles.

Methanogens can harvest energy by converting H₂ and CO₂ into methane gas. This must be done in the absence of oxygen. They operate deep in marshes, and are found in the intestinal tracts of humans, cows, and some other animals.

Halophiles thrive in the presence of extremely high salt levels, which would kill ordinary bacteria. They are found in the Dead Sea, the Great Salt Lake, and other areas with a high salt content.

Thermoacidophiles are found in extremely acidic conditions and in areas with very high temperatures. "They can survive in areas with temperatures as high as 230 degrees Fahrenheit and with pHs below 2 (hydrochloric acid, which is incredibly strong, has a pH of 1). These locations include volcanic vents and hydrothermal vents (cracks in the ocean floor where scalding water leaks out)."(Accessed at essortment)