Origin of Prokaryotes and Eukaryotes: Protists
Protists represent in excess of 100,000 species and are so varied in their structure and function that originally some were considered plants, others animals, others fungi, and some, a combination. As such, the kingdom Protista is often described as representing those organisms that are eukaryotes, not plants, not animals, and not fungi. Although most are unicellular, several, such as the giant kelp, are multicellular but lack specialized tissues. The protists may also represent the ancestors of modern-day plants, animals, and fungi.
There are four separate phyla of protists with animal characteristics. In early classification schemes, they were clumped together and called protozoa to separate them from the more plantlike protists:
Malaria is still a problem in developing tropical countries where millions of people become infected annually.
Sporozoa are among the best known protists because they are all parasites, including human parasites. They usually live in a host organism and reproduce by spores, which are dormant cells enclosed in a protective membrane. Whenever the spores land on an appropriate host, they are able to enter by various means and then grow to maturity as parasites. The parasitic cells have specialized organelles for penetrating host cell membranes. More exotic sporozoa have life cycles that involve two hosts.
The plasmodium is the parasite that causes malaria by entering human red blood cells and digesting their nutritional contents until the red blood cells become nonfunctional. The plasmodium then grows, reproduces, and infects neighboring red blood cells. Occasionally a female anopheles mosquito will withdraw some plasmodium-infected blood as part of her normal dietary requirement and then transfer the plasmodium to another unsuspecting victim.
The phylum Sarcodina is best known for their bloblike structures, called pseudopodia, that provide a means of locomotion. Pseudopods are temporary membrane-bound cytoplasm projections that direct the motion of the sarcodines. This innate flexibility allows the sarcodines to assume virtually any shape. Amoebas are typical sarcodines that use pseudopods to locate, surround, and engulf food sources. Other interesting examples include the foraminifera, which are aquatic protists mostly known by the calcium carbonate shells they secrete, which sometimes accumulate in large deposits when they die, such as the famous White Cliffs of Dover, England. Because foraminifers only inhabit warm waters, whenever a geologist discovers a strata containing their fossilized shells, the climate for that aquatic environment at that time can be estimated fairly accurately.
The ciliates (phylum Ciliophora) exhibit several advancements not associated with the previous protists. They exist as free-living, nonparasitic, fresh- or saltwater, unicellular or colonial organisms. They also have developed short, hairlike structures, called cilia that move in rhythm for locomotion. Cilia are often described as functioning similar to oars for movement of a ship, which is accurate except cilia sometimes surround the entire organism. They allow directed movement toward a food source and away from inhospitable territories. Paramecia are a common example of a ciliate and exhibit another interesting phenomenon—they have two nuclei. A large macronucleus controls the everyday activities of the cell, and a smaller micronucleus (often more than one) functions during gamete exchange. Under normal conditions, paramecia reproduce asexually by binary fission (refer to Cell Theory, Form, and Function); during periods of stress, however, they conjugate, meaning they exchange haploid micronuclei with another paramecium. Refer to the illustration Paramecium conjugation for a pictorial representation.
Because no offspring or fertilized eggs are produced, technically, sexual reproduction did not occur, but gametes were exchanged by mature adults, resulting in a new genetic complement for both paramecia! Paramecia also contain most organelles that more advanced life-forms utilize. For instance, in addition to the mitochondria and nucleus, they also use food vacuoles containing digestive enzymes, an anal pore for waste removal, and contractile or water vacuoles for water transportation.
Zoomastigina, also known as flagellates, are known for their specialized flagella, which are whiplike structures that propel the flagellates through their aquatic environment. Typically flagellates have only one flagella, but may have up to four working in sync. Although most flagellates are harmless, simply surrounding and engulfing their food, others are human parasites. One of the most interesting parasites is the trypanosome, which causes African sleeping sickness. The symptoms are well known: fever, chills, and skin rash. Affected individuals become very weak, unconscious, and may fall into a fatal coma.
The trypanosome is transmitted by the tsetse fly and lives in the bloodstream, and continually change their surface molecular structure to gain invisibility to the host's immune system and remain undetected in attacks on the host. The disease attacks the nervous system of infected individuals.
The three plantlike protists all contain chlorophyll, and as autotrophs utilize photosynthesis to make their own energy. They are generally multicellular and mobile, usually flagella assisted, inhabit wet or aquatic territories, and do not have true roots, stems, or leaves, but are considered to be a type of algae:
Euglenophytes are also known as green algae and are structurally similar to Zoomastigina because both utilize flagella and have common structures. However, the euglenophytes also contain chloroplasts and undergo photosynthesis. The euglena is a typical euglenophyte, even lending its name to the phylum. Euglenas contain an eyespot, which does not focus as an eye, but does differentiate light from dark and allows a euglena to move toward a light source for greater photosynthetic opportunities. They are also capable swimmers with two flagella, which is important because they can then inhabit diverse aquatic territories. Living as both a photosynthetic autotroph and, in the absence of light, as a nutrient absorbing heterotroph, the euglenas are quite versatile in their ter-ritory requirements. Their range is also not limited by reproduction, which they accommodate both sexually, whenever possible, and asexually the rest of the time. Refer to the illustration Typical euglena.
Volvox is an example of a colonial euglenophyte consisting of individual cells resembling a hollow ball, which is also capable of producing daughter colonies. Volvox cells and their cousin chlamydomonas contain features in common with more complex plants such as cell walls made of cellulose, starch as an energy-storing compound, and chloroplasts, suggesting these protists may have evolved into modern-day plants.
Diatoms are the principle species found in the phylum Chrysophyta. Diatoms are unique because they store their food in a light, less-dense oil form that allows them to float on top of water to be closer to sunlight. They also substitute the carbohydrate pectin for cellulose in their cell wall, which is rich in the element silicon, the main ingredient in glass. Diatoms are an important freshwater and marine food source; when dead, their shells often accumulate in large strata known as diatomaceous earth, which is still used as an abrasive and for filtering purposes.
Dinoflagellates are unicellular photosynthetic algae that are the principal member of the phylum Pyrrophyta. They exhibit three unusual properties. First, several species are luminescent and give off light when disturbed. Second, their DNA is not packed with histones, making them separate from all other eukaryotes. Third, dinoflagellates are subject to population explosions whenever conditions are favorable. This “bloom” is sometimes called a “red tide” because the algae are red in color and are so numerous that the water appears reddish. They are often so thick they block the absorption of oxygen by less-mobile aquatic life, such as flounder and crabs. Often these animals move to the more oxygen-rich waters near the shore, where they contribute to the human jubilee on the shore.
Excerpted from The Complete Idiot's Guide to Biology © 2004 by Glen E. Moulton, Ed.D.. All rights reserved including the right of reproduction in whole or in part in any form. Used by arrangement with Alpha Books, a member of Penguin Group (USA) Inc.
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