Protista - biology.

Protista - biology. I INTRODUCTION Protista, group of comparatively simple organisms, called protists, that have characteristics of both plants and animals. Most protists are unicellular (consisting of a single cell) and can only be seen with a microscope, although there are some that are composed of more than one cell. There are a wide variety of protists, and they inhabit many different environments--fresh water, seawater, soils, and the intestinal tracts of animals, where they perform crucial digestive processes. Like plants, many species of protists can make their own food by the process of photosynthesis. Like animals, many protists can move around under their own power. Unlike plants and animals, however, protists do not have cells organized into specialized tissues. The protists include such familiar organisms as seaweeds, amoebas, and slime molds. The kingdom Protista contains many economically important members, including organisms that cause diseases, such as malaria. Biologists theorize that members of the kingdom Protista gave rise to the kingdoms Plantae, Animalia, and Fungi about 600 million years ago. All protists are eukaryotes. This means that their cells contain a nucleus, a membrane-bounded structure that encloses the cell's genetic material. (Organisms without nuclei--the bacteria and cyanobacteria, or blue-green algae--are called prokaryotes.) Although most protists have a single nucleus, protists are unique in that some contain multiple nuclei--up to ten thousand in a single cell--and others, such as ciliates, have two different-sized nuclei in a single cell. The deoxyribonucleic acid (DNA) of protists (like that of other eukaryotes) is organized in long molecules called chromosomes within the nucleus. When the cell divides, these chromosomes replicate (duplicate themselves) and then divide in a process called mitosis. Among some protists, the nuclear membrane (the thin layer around the nucleus) stays intact during mitosis, whereas in plants, animals, and fungi the nuclear membrane breaks down. Protists vary greatly in size and shape. Many are minute: The green alga Nanochlorum is only 0.01 mm (0.0004 in) long. Giant kelps can grow to 65 m (210 ft) or more in length. Some protists form structures known as spores that are resistant to chemicals and drying and that disperse in the environment. Spores are often the infectious stage of organisms that cause diseases in humans when ingested. II ANATOMY AND PHYSIOLOGY Most protists contain many mitochondria, membrane-bounded organelles (cellular parts) that break down complex organic molecules and, in the process, release the chemical energy that powers the rest of the cell. The physiology of these protists is very similar to the cellular physiology of plants, animals, and fungi. Mitochondria require oxygen. Protists that live in environments without oxygen--for instance, in black muds or inside the digestive tract of animals--generally do not have mitochondria, and they create cellular energy by processes unique to the kingdom Protista. In addition to mitochondria and nuclei, membrane-bounded organelles called plastids are found in protists that perform photosynthesis. Plastids capture the energy of sunlight and convert the energy into a chemical form available for use in the cell. Plastids are also able to capture inorganic carbon dioxide from the atmosphere and turn it into organic carbon useful for cell growth. Pigments, including chlorophyll, synthesized within plastids capture the sunlight and give photosynthetic protists their distinct colors: The brown algae contain brown plastids; the green algae, green plastids; and the red algae, red plastids. In all likelihood, plastids and mitochondria were once free-living bacteria that became organelles through the process of symbiosis (an intimate and ongoing association of two or more different species). Once controversial, the theory of the symbiotic origin of plastids and mitochondria is generally accepted today. DNA molecules within present-day plastids and mitochondria are probably relics of the DNA of free-living bacteria ancestors. Some protists are capable of movement. Protists known as flagellates move by means of flagella, long whiplike structures that the protists beat to propel themselves through water. Other protists use numerous and comparatively short structures called cilia. One group of protists, the ciliates, are distinguished by the great number and unusual arrangement of cilia along their surface. The single-celled amoebas and their multicellular relatives the slime molds are protists that use pseudopodia, temporary protrusions of the cell, both for movement and to engulf prey. The vast majority of protists reproduce asexually--cells simply grow and divide. Sexual recombination (the production of gametes and the merging of DNA from two individuals to form a zygote) has been observed in some protists. However, many protists have been only poorly studied. III CLASSIFICATION The first detailed descriptions of protists were made in 1676 by the inventor of the microscope, Dutch naturalist Antoni van Leeuwenhoek, who observed microscopic organisms that he called animalcules. Traditionally all organisms that moved had been considered animals, whereas all photosynthetic organisms had been considered plants. The term Protista was first used in 1862 by the German biologist Ernst Haeckel to describe microscopic organisms that were neither clearly animallike nor plantlike. Haeckel recognized that some organisms are both motile and photosynthetic. For almost a hundred years after Haeckel, scientists informally recognized three kingdoms: plants, animals, and protists. The Protista included all the microbes. In the 1930s, it was formally proposed that all single-celled organisms, including bacteria, be placed in their own kingdom, Kingdom Protista. In 1959 American biologist R. H. Whittaker described a classification system of five primary kingdoms: plants, animals, fungi, protists, and bacteria. Because the Protista are so diverse in form, classification within the kingdom has proved difficult. The classification of the Protista is currently based largely on the structure and organization of the cell, the presence of organelles, and the pattern of reproduction or life cycles. The five-kingdom classification system divides the Protista into 27 distinct phyla. More recently, however, classifications based on comparisons of cell physiology and DNA sequences suggest that many protist phyla may be sufficiently large and diverse to be classified as kingdoms. The plantlike protists include the golden algae (phylum Chrysophyta; see Diatom), dinoflagellates (Pyrrophyta), cryptomonads (Cryptophyta), and euglenoids (Euglenophyta). The animallike protists, which are also called protozoa, include the animal flagellates (Zoomastigina), amoeboid forms (Sarcodina; see Foraminifera), ciliates and suctorians (Ciliophora), and the parasitic, spore-producing sporozoans (Sporozoa). Funguslike forms include the hyphochytrids (Hyphochytridiomycota) and the plasmodiophores (Plasmodiophoromycota). The slime molds, which include several disputed phyla here treated as belonging to the Protista, have characteristics of both fungi and protozoans. Contributed By: Gregory Hinkle Microsoft ® Encarta ® 2009. © 1993-2008 Microsoft Corporation. All rights reserved.