The “true nucleus” of eukaryotes (from Gr karyon, “nucleus”) is only one of their distinguishing features. The membrane bound organelles, the microtubules, and the microfilaments of eukaryotes form a complex intracellular structure unlike that found in prokaryotes. The organelles responsible for the motility of eukaryotic cells are flagella or cilia—complex multistranded structures that do not resemble the flagella of prokaryotes. Gene expression in eukaryotes takes place through a series of events achieving physiologic integration of the nucleus with the endoplasmic reticulum, a structure that has no counterpart in prokaryotes. Eukaryotes are set apart by the organization of their cellular DNA in chromosomes separated by a distinctive mitotic apparatus during cell division.

In general, genetic transfer among eukaryotes depends on fusion of haploid gametes to form a diploid cell containing a full set of genes derived from each gamete. The life cycle of many eukaryotes is almost entirely in the dip loid state, a form not encountered in prokaryotes. Fusion of gametes to form reproductive progeny is a highly specific event and establishes the basis for eukaryotic species. This term can be applied only metaphorically to the prokaryotes, which exchange fragments of DNA through recombination. Currently, the term protist is used informally as a catch-all term for unicellular eukaryotic microorganisms. Because protists as a whole are paraphyletic, newer classification systems often split up traditional sub divisions or groups based on morphological or biochemical characteristics.

Traditionally, microbial eukaryotes—protists—are placed in one of the four following major groups: algae, protozoa, fungi, and slime molds. These traditional sub divisions, largely based on superficial commonalities, have been largely replaced by classification schemes based on phylogenetics. Molecular methods used by modern taxonomists have been used to generate data supporting the redistribution of some members of these groups into diverse and sometimes distantly related phyla. For example, the water molds are now considered to be closely related to photosynthetic organisms such as brown algae and diatoms.

Algae

The term algae has long been used to denote all organisms that produce O2 as a product of photosynthesis. One former subgroup of these organisms—the blue-green algae, or cyanobacteria—are prokaryotic and no longer are termed algae. This classification is reserved exclusively for a large diverse group of photosynthetic eukaryotic organisms. Formerly, all algae were thought to contain chlorophyll in the photosynthetic membrane of their chloroplast, a subcellular organelle that is similar in structure to cyanobacteria. Modern taxonomic approaches have recognized that some algae lack chlorophyll and have a free-living heterotrophic or parasitic life style. Many algal species are unicellular microorganisms. Other algae may form extremely large multicellular structures. Kelps of brown algae sometimes are several hundred meters in length. Several algae produce toxins that are poisonous to humans and other animals. Dinoflagellates, a unicellular alga, are responsible for algal blooms, or red tides, in the ocean (Figure 1). Red tides caused by the dinoflagellate Gonyaulax species are serious because this organism produces potent neurotoxins such as saxitoxin and gonyautoxins, which accumulate in shellfish (eg, clams, mussels, scallops, and oysters) that feed on this organism. Ingestion of these shellfish by humans results in symptoms of paralytic shellfish poisoning and can lead to death. Some algae (eg, Prototheca and Helicosporidium) are parasites of metazoans or plants. Protothecosis is a disease of dogs, cats, cattle, and rarely humans caused by a type of algae, Prototheca, that lacks chlorophyll. The two most common species are P. wickerhamii and P. zopfii; most human cases, which are associated with a defective immune system, are caused by P. wickerhamii.

Fig1. The dinoflagellate Gymnodinium scanning electron micrograph (4000×). (Reproduced with permission from Dr. David Phillips/Visuals Unlimited.)

Protozoa

Protozoa is an informal term for single-celled nonphoto synthetic eukaryotes that are either free-living or parasitic. Protozoa are abundant in aqueous environments and soil. They range in size from as little as 1µm to several millimeters, or more. All protozoa are heterotrophic, deriving nutrients from other organisms, either by ingesting them whole or by consuming their organic tissue or waste products. Some protozoans take in food by phagocytosis, engulfing organic particles with pseudopodia (eg, amoeba), or taking in food through a mouth-like aperture called a cytostome. Other protozoans absorb dissolved nutrients through their cell membranes, a process called osmotrophy.

Historically, the major groups of protozoa included: flagellates, motile cells possessing whip-like organelles of locomotion; amoebae, cells that move by extending pseudopodia; and ciliates, cells possessing large numbers of short hair-like organelles of motility. Intermediate forms are known that have flagella at one stage in their life cycle and pseudopodia at another stage. A fourth major group of protozoa, the sporozoa, are strict parasites that are usually nonmotile; most of these reproduce sexually and asexually in alternate generations by means of spores. Recent taxonomic studies have shown that only the ciliates are mono phyletic, that is, a distinct lineage of organisms sharing common ancestry. The other classes of protozoa are all polyphyletic groups made up of organisms that, despite similarities in appearance (eg, flagellates) or way of life (eg, endoparasitic), are not necessarily closely related to one another.

Fungi

The fungi are nonphotosynthetic protists that may or may not grow as a mass of branching, interlacing filaments (“hyphae”) known as a mycelium. If a fungus grows simply as a single cell it is called a yeast. If mycelial growth occurs, it is called a mold. Most fungi of medical importance grow dimorphically, that is, they exist as a mold at room temperature but as a yeast at body temperature. Remarkably, the largest known contiguous fungal mycelium covered an area of 2400 acres (9.7 km²) at a site in eastern Oregon. Although the hyphae exhibit cross walls, the cross walls are perforated and allow free passage of nuclei and cytoplasm. The entire organism is thus a coenocyte (a multinucleated mass of continuous cytoplasm) confined within a series of branching tubes. These tubes, made of polysaccharides such as chitin, are homologous with cell walls.

The fungi probably represent an evolutionary offshoot of the protozoa; they are unrelated to the actinomycetes, mycelial bacteria that they superficially resemble. The major subdivisions (phyla) of fungi are Chytridiomycota, Zygomycota (the zygomycetes), Ascomycota (the ascomycetes), Basidiomycota (the basidiomycetes), and the “deuteromycetes” (or imperfect fungi). The evolution of the ascomycetes from the phycomycetes is seen in a transitional group, whose members form a zygote but then transform this directly into an ascus. The basidiomycetes are believed to have evolved in turn from the ascomycetes. The classification of fungi and their medical significance are discussed further in Chapter 45.

Slime Molds

These organisms are characterized by the presence, as a stage in their life cycle, of an ameboid multinucleate mass of cytoplasm called a plasmodium. The plasmodium of a slime mold is analogous to the mycelium of a true fungus. Both are coenocytic. Whereas in the latter, cytoplasmic flow is confined to the branching network of chitinous tubes, in the former, the cytoplasm can flow in all directions. This flow causes the plasmodium to migrate in the direction of its food source, frequently bacteria. In response to a chemical signal, 3′, 5′-cyclic AMP, the plasmodium, which reaches macroscopic size, differentiates into a stalked body that can produce individual motile cells. These cells, flagellated or ameboid, initiate a new round in the life cycle of the slime mold (Figure 2). The cycle frequently is initiated by sexual fusion of single cells.

Fig2. Slime molds. A: Life cycle of an acellular slime mold. B: Fruiting body of a cellular slime mold. (Reproduced with permission from Carolina Biological Supply/DIOMEDIA.)

The growth of slime molds depends on nutrients provided by bacterial or, in some cases, plant cells. Reproduction of the slime molds via plasmodia can depend on intercellular recognition and fusion of cells from the same species. The life cycle of the slime molds illustrates a central theme of this chapter—the interdependency of living forms. Full understanding of any microorganism requires both knowledge of the other organisms with which it coevolved and an appreciation of the range of physiologic responses that may contribute to survival.

اخر الاخبار

اخبار العتبة العباسية المقدسة

متحف الكفيل يباشر صيانة المجموعة الرابعة من مقتنيات العتبة العسكرية المقدسة

ضمن برنامجها التبليغي اليومي.. شعبة الخطابة تنظم مجلسًا للوعظ والإرشاد

قسم الشؤون الفكرية يواصل إقامة مسابقة النور الزكي الرمضانية في ذي قار

المجمَع العلمي يواصل إقامة 18 ختمة قرآنية رمضانية في الديوانية

المزيد

الآخبار الصحية

دراسة: تناول القهوة يوميا يخفض خطر الإصابة بالاضطرابات النفسية

عرض شائع في الفم قد يكون مرتبطا بخطر الإصابة بأمراض خطيرة

نصائح عامة لاستخدام سماعات الرأس بأمان

تأثير اللياقة البدنية على الدماغ

المزيد

الآخبار التكنلوجية

ديدان الأرض تكشف تلوث التربة بالمعادن الثقيلة

خبير أرصاد جوية: ظاهرة النينيو القادمة قد تكون الأقوى خلال 10 سنوات

"ناسا" تكشف موعد إعادة إطلاق رحلتها المأهولة إلى القمر

علماء الجيولوجيا يفكون لغز الصدع العملاق تحت الماء في المحيط الأطلسي

المزيد

مواضيع ذات صلة

Prokaryotes

Microbial Metabolism: Focal Metabolites and Their Interconversion

Relationship of Biocide Concentration and Time on Antimicrobial Killing

Specific Actions of Selected Biocides

Flow cytometry

البروتوبلازم Protoplasm

غشاء الخلية The cell membrane

مملكة البدائيات (Monera)

كيس الصفار ( المح ) Yolk sac

اقسام البيئة البحرية

بيئة المياه البحرية

المياه الجارية