Essay, Research Paper: Protista

PROTISTA
Protista are a group of mostly simple organisms, called protists, which have characteristics of both plants and animals. Most protists are unicellular 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 live in many different environments, such as 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 seaweed, amoebas, and slime molds. The kingdom Protista contains many economically important members, including organisms that cause diseases, such as malaria. Some 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. 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 DNA of protists is organized in chromosomes within the nucleus. When the cell divides, these chromosomes duplicate themselves and then divide in mitosis. Among some protists, the nuclear membrane stays intact during mitosis, whereas in plants, animals, and fungi the nuclear membrane breaks down.
Protists vary greatly in size and shape. 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. The architectural complexity of most protist cells is what sets them apart from the cells of plant and animal tissues. Not only are protists cells, they are also whole, complete, independent organisms, and they must compete and survive as such in the environments in which they live. Adaptations to particular habitats over long periods of time have resulted in both intracellular and extracellular elaborations found at the cellular level in higher eukaryotic species. Internally, complex rootlet systems have evolved in association with the basal bodies, or kinetosomes, of many ciliates and flagellates, and nonhomologous endoskeletal and exoskeletal structures have developed in many protista. Many food-storage bodies are often present, and pigment bodies and chloroplasts are found in some species. In the cortex, just under the pellicle of some protists, extrusible bodies of various types have evolved, with presumably nonhomologous functions, some of which are still unknown. Scales may appear on the outside of the body, and, in some groups, tentacles, suckers, hooks, spines, hairs, or other anchoring devices have evolved.
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.
Most protists contain many mitochondria. 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, like 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, 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, made 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.
Some protists are capable of movement. Protists known as flagellates move by using flagella, which are long whiplike structures that the protists beat to move themselves forward through water. Other protists use numerous and comparatively short structures called cilia. The great number and unusual arrangement of cilia along their surface distinguish the ciliates. The single-celled amoebas and their multicellular relatives the slime molds are protists that use pseudopodia. Pseudopodia are simply an extending piece of cytoplasm that reaches outwards from the cell. It is used both for movement and to engulf prey.
The vast majority of protists reproduce asexually. Sexual recombination has been observed in some protists, but many protists have been only poorly studied.
Because the Protista are so diverse in form, classification within the kingdom has proved difficult. The classification of the Protista is based on the structure and organization of the cell, the presence of organelles, and the pattern of reproduction or life cycles.
Many protists provide humans with benefits. Because they are at the bottom of the food chain, they serve a crucial role in the higher eukaryotes in fresh and marine waters. As well as providing food, the pigmented algae protists provide oxygen from photosynthesis. Algae may supply over half of the world s oxygen. Many nutrients and mineral recycling in oceans and seas comes from the activities of flagellates and ciliates that live there. Seaweeds have been used for fertilizers also.
The plantlike protists include the golden algae (phylum Chrysophyta), dinoflagellates (Pyrrophyta), cryptomonads (Cryptophyta), and euglenoids (Euglenophyta). The animal like protists, which are also called protozoa, include the animal flagellates (Zoomastigina), amoeboid forms (Sarcodina), ciliates and suctorians (Ciliophora), and the parasitic, spore-producing sporozoans (Sporozoa). Fungus-like 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