All prokaryotes are single-celled organisms, but so are many eukaryotes. In fact, the vast majority of organisms on earth are single-celled, or “unicellular.” The prokaryotes are split into two taxonomic domains: the Bacteria and Archaea. All eukaryotes fall under the domain Eukarya. Within the Eukarya, the only groups that are dominated by multiple-celled organisms are land plants, animals and fungi. The rest of the Eukarya are part of a large, diverse group of organisms called the protists, most of which are unicellular organisms.
Prokaryotes versus Eukaryotes
Prokaryotic organisms exist as a single prokaryote cell, while eukaryotes consist of one or more eukaryote cells. There are several key differences between prokaryote and eukaryote cells. Most DNA in a eukaryote cell is enclosed within a membrane-bound nucleus, while prokaryote cells do not have a true cell nucleus. Eukaryote DNA consists of strands with ends, while prokaryote cells have circular DNA with no ends. Cellular machinery is spread throughout prokaryote cells, while the machinery of eukaryote cells is contained within membrane-bound compartments called organelles. This compartmentalization allows eukaryote cells to regulate cell functions more efficiently than their prokaryote ancestors. Finally, eukaryote cells are around 10 to 20 times larger than prokaryote cells.
Prokaryotes were the first life-forms to colonize Earth and remain the most numerous organisms on the planet. They are highly adaptable, surviving extreme conditions that no other organism can withstand. Their small size and simple structure allows them to reproduce very rapidly and hence evolve survival mechanisms much faster than other organisms. Prokaryotes give the Grand Prismatic Spring in Yellowstone National Park -- which can reach 87 degrees Celsius (188 degrees Fahrenheit) at its center -- its distinctive bright colors. Bacteria have been found living in Arctic permafrost, where they survive at -25 degrees Celsius (-13 degrees Fahrenheit). Prokaryotes move through their environment using long, rotating hair-like tubes called flagella. Prokaryotes obtain nutrients and energy from a range of different sources, but can be classified into two broad groups: autotrophs and heterotrophs. Autotrophs obtain carbon by photosynthesis and heterotrophs obtain carbon from organic matter.
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Unicellular protists also occur as autotrophs and heterotrophs. A well known heterotroph is the carnivorous amoeba, which engulfs smaller protists and bacteria. Other heterotrophs include paramecium, and molds, rusts and mildews. Autotrophic protists include dinoflagellates, diatoms and algae. Many protists have the ability to actively move around their environments using flagella or cilia, shorter but more numerous tubes that beat rather than rotate. Others, like the amoeba, move by rapidly changing the shape of their cell using fluid transfer, a process known as pseudopodia. Some protists are less mobile, relying on wind or water currents for distribution. These include some diatoms and many types of mold and slime. Some unicellular protists, such as dinoflagellates and slimes, form colonies that make them appear as if they are a multiple-celled organism. However, each cell functions independently within the colony.
Role in the Environment
The prokaryotes decompose dead organic matter, and are an important component of the carbon and nitrogen cycles. Decomposers release carbon dioxide, methane, oxygen and soluble nitrogen into the environment. Photosynthetic prokaryotes fix, or sequester, carbon within their cells and nitrogen-fixing bacteria do the same for nitrogen. Photosynthetic protists also play an important role in carbon fixation and oxygen production. Prokaryotes and protists enter into symbiotic relationships with plants and animals. Most are helpful -- for example, bacteria in the human gut help to digest food -- while others are parasites that cause damage to the tissues of plants and animals.