CHAPTER OUTLINE
29.1 Protists
Protists are eukaryotes. For the most part, protists are unicellular and microscopic. Protists are any eukaryotic organism that is not a plant, animal or fungus and are most likely related to the first eukaryotic cell to have evolved.
Biology of Protists
Protists can be unicellular, colonial, or multicellular. They are structurally diverse. The variability of protists also extends to their lifestyles and modes of reproduction. Most carry out asexual reproduction with sexual spores or cysts as an option if environmental conditions are unfavorable. Some protists are nonmotile, others use flagella, cilia, or psuedopods for locomotion. Protozoan is a common term for a heterotrophic, usually motile, unicellular protist.
Diversity of Protists
In this chapter, we will group the protists into six groups according to some of their major shared characteristics: photosynthetic protists, flagellates, ciliates, amoeboids, sporozoans, and water molds and slime molds.
Photosynthetic Protists
Algae are photosynthetic eukaryotes that can be unicellular or form colonies or filaments. Some types are multicellular seaweeds. In aqueous environments, small algae are often a component of suspended, photosynthesizing organisms termed phytoplankton. Algae contain chloroplasts and perform photosynthesis. Algae generally have a rigid cell wall, sometimes made of cellulose. Green algae are believed to be closely related to the first plants. Diatoms are the most numerous unicellular algae in the oceans and are important primary producers at the base of marine food chains. Diatoms have a wide variety of elaborate shells made of silica. Dinoflagellates are best known for the red tide they cause when they greatly increase in number. One species in red tides produces a very potent toxin. Red algae are multicellular and produce a number of useful gelling agents. The brown algae are the conspicuous multicellular seaweeds that dominate rocky shores along cold and temperate coasts.
Flagellates
Flagellates are heterotrophic protists that propel themselves using one or more flagella. Most are symbiotic, and many are parasitic. Euglenoids are freshwater unicellular organisms that typify the problem of classifying protists. Many have chloroplasts, but some do not.
Ciliates
Ciliates are the largest group of protozoans. All of them have cilia, hairlike structures that rhythmically beat, moving the cell forward or in reverse. Paramecium is the most widely known ciliate.
Amoeboids
Amoeboids move by pseudopods, processes that form when cytoplasm streams forward in a particular direction. They usually live in aquatic environments. Foraminiferans and radiolarians are related amoeboid groups that have an external skeleton.
Sporozoans
Sporozoans produce spores and are either intercellular or extracellular parasites.
WaterMolds and Slime Molds
Water molds and slime molds are classified as protists.
Water Molds
Most water molds are saprotrophs, meaning that they feed on dead organic matter. They usually live in water.
Slime Molds
Slime molds feed on dead plant material in forests and woodlands.
Cellular Slime Molds
Cellular slime molds can exist as individual amoeboid cells. They are common in soil, where they feed on bacteria and yeasts.
Protozoal Diseases of Humans
A relatively few protists cause human diseases.
Malaria
The most widespread and dangerous sporozoan disease is malaria. It can be caused by several sporozoan parasites in the genus Plasmodium, which have a complex life cycle that involves transmission by a mosquito vector.
Other Sporozoan Diseases
Toxoplasma gondii is commonly transmitted by cat feces and can be harmful to a developing fetus. Cryptosporidium parvum is common in surface waters and in the feces of animals and birds, it usually causes self-limiting gastroenteritis.
Diseases Caused by Flagellates
African sleeping sickness, Chagas disease, giardiasis, and vaginitis are all caused by flagellates.
Diseases Caused by Amoeboids
Amoebic dysentery, corneal inflammations and serious infections are caused by amoeboids and more rarely, an amoeboid protest can invade and attack the human nervous system, nearly always resulting in death.
29.2 Fungi
Fungi are a structurally diverse group of eukaryotes that are strict heterotrophs. Most fungi are saprotrophs, which decompose dead plants, animals, and microbes.
Biology of Fungi
The body of a fungus is composed of a mass of individual filaments called hyphae; collectively, the mass of filaments is called a mycelium. The cell walls of fungal cells contain chitin, not cellulose. The energy reserve of fungi is not starch, but glycogen. Fungi are nonmotile and most do not have flagella. Fungi produce windblown spores, haploid reproductive cells, during both asexual and sexual reproduction.
Diversity of Fungi
Fungi are traditionally classified based on their mode of sexual reproduction.
Chytrid Fungi
The chytrid fungi may have been the first type of fungi to evolve. They are unique among the fungi because they are aquatic and they produce flagellated reproductive cells.
Zygospore Fungi
The zygospore fungi are mainly saprotrophs.A sporangium is a capsule that produces spores. Rhizopus stolonifer, the common bread mold, reproduces sexually, the ends of + and – hyphae join, haploid nuclei fuse, and a thick-walled zygospore results.
Sac Fungi
The sac fungi are named for their characteristic cuplike sexual reproductive structure called an ascocarp. Many sac fungi reproduce by producing chains of sexual spores called conidia. Cup fungi, morels and truffles have conspicuous ascocarps. The original source of penicillin and other species of Penicillium that are necessary to the production of blue cheeses are examples of sac fungi.
Yeasts
The term yeasts is generally applied to unicellular fungi, and many of these organisms are sac fungi. Brewer’s yeast is representative of budding yeasts.
Club Fungi
Club fungi are named for their characteristic sexual reproductive structure called a basidium. Mushrooms and shelf or bracket fungi are basidiocarps. Some are posisonous, others produce a hallucinogenic chemical. Smuts and rusts are club fungi that parasitize cereal crops.
Symbiotic Relationships of Fungi
Fungi commonly form symbiotic relationships with other organisms.
Lichens
Lichens are associations between fungi and cyanobacteria or green algae. Lichens are efficient at acquiring nutrients and moisture, and therefore can survive in poor soils as well as on rocks. The body of a lichen has three layers and the fungus forms a thin, tough upper layer and a loosely packed lower layer.
Mycorrhizae
Mycorrhizae are mutualistic relationships between soil fungi and the roots of most plants, helping them grow more successfully in dry or poor soils. AM fungi are a recently recognized group whose name stands for arbuscular mycorrhial fungi. Arbuscules are branching invaginations the fungus makes when it invades plant roots.
Fungal Diseases of Humans
Fungi tend to cause disease mainly in people who immune system isn’t working properly. Mycoses, fungal diseases of humans, vary in levels of seriousness.
Superficial Mycoses
Most superficial mycoses are confined to the outer layers of the skin, hair, or nails.Fungi called dermatophytes cause infections of the skin called tineas. Athlete’s foot, ringworm, yeast infections, and thrush are all examples.
Systemic Mycoses
Systemic mycoses refer to fungal infections affecting the internal organs, mainly the lungs. Histoplasmosis, valley fever, and “sick building syndrome” are all examples.
Antifungal Drugs
The similarities between fungal cells and human cells make it a challenge to design antimicrobials against fungi that do not also harm humans. Fungal skin infections can be treated with a topical medication, but systemic fungal infections must have medication taken into the body, resulting in a problem with side effects.
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