chapter 3 eukaryotic microorganisms

第3章 真核微生物

Eukaryote cell

A type of cell found in many organisms including single-celled protozoom and multicellular fungi, plants, and animals; characterized by a membrane-bounded nucleus and other membraneous organelles

eukaryotic cells construct

Appendages (cilia, flagella), glycocalyx, cell wall, cytoplasmic (or cell) membrane, ribosomes, organelles(nucleus, nucleolus, endoplasmic reticulum, Golgi apparatus, mitochondria, chloroplasts, cytoskeleton, microfilaments.

Main characterstic of eukarytic cells

1.      The nucleus, which contains DNA in the form of chromosomes, is the most characteristic eukaryotic organelle.

2.      The nuclear membrane is connected to a system of parallel membranes in the cytoplasm, called the endoplasmic reticulum.

3.      The endoplasmic reticulum provides a surface for chemical reactions, serves as a transporting network, and stores synthesized molecules.

4.      805 ribosomes are found in the cytoplasm or attached to the endoplasmic reticulum.

5.      The Golgi complex consists of cisternae. It functions in secretion, carbohydrate synthesis, and glycoprotein for mation.

6.      Mitochondria are the primary sites of ATP production.They contain small 70S ribosomes and DNA, and they multiply by fission.

7.      Chloroplasts contain chlorophyll and enzymes for photosynthesis. Like mitochondria, they contain 70S ribosomes and DNA and multiply by fission.

8.      Lysosomes are formed from Golgi complexes. They store powerful digestive enzymes.

9.      A pair of cylindrical structures called centrioles that are involved in cell division are near the nucleus.

10.Both flagella and cilia contain a "nine pairs + two pairs"arrangement of microtubules.

Major Groups of eukaryotic microorganisms

eucaryotic microorganisms include the Fungi ,algae and protozoom..

Fungi

Fungi is composed of non-photosynthetic species Fungi include yeasts (unicellular budding cells) and molds (filamentous cells called hyphae,菌絲) and some macroscopic fungi(mushrooms).

Major Groups of fungi: The four important divisions among the terrestrial fungi, given with sexual spore type, are Zygomycota（接合菌綱） (zygospores，接合孢子), Ascomycota（子囊菌綱） (ascospores，子囊孢子), Basidiomycota（擔子菌綱） (basidiospores，擔孢子), and Deuteromycota（半知菌綱） (no sexual spores).

Yeast

1.      Yeasts are single cells that form buds（芽體） and sometimes short chains called pseudohyphae（假菌絲）.

2.      Yeasts are 5~20μm in length and 1~5μm in diamater.

3.      The majority are harmless saprobes living off organic substrates such as glucose. A few are parasites, living on the tissues of other organisms, but none are obligate.

4.      Yeast cell walls consist of glucan and mannan.

5.      plasma membranes contain carbohydrate sattached to the proteins and sterols not found in prokaryotic cells.

6.      most of yeast cell have large vacuolars(液泡).

7.      budding is the main reproduction style of yeast.

8.     In sexual reproduction, spores are formed through fusion of male and female strains and the formation of a sexual called ascospore.

Molds

hyphae are characteristic of the filamentous fungi called molds. some fungi (mushrooms) produce multicellular structures such as fleshy fruiting bodies.

Importance of molds

Essential decomposers of plant and animal detritis in the environment with return of valuable nutrients to the ecosystem. Economically beneficial as sources of antibiotics; used in making foods and in genetic studies. Harmful plant pathogens; decompose fruits and vegetables; several fungi cause infections, or mycoses; some produce substances that are toxic if eaten.

Hyphae of molds

Hyphae can also be classified according to their particular function. Vegetative hyphae(營養菌絲) (mycelia) are responsible for the visible mass of growth that appears on the surface of a substrate and penetrates it to digest and absorb nutrients. During the development of a fungal colony, the vegetative hyphae give rise to structures called reproductive, or aerial hyphae（繁殖菌絲）., which orient vertically from the vegetative mycelium. These hyphae are responsible for the production of fungal reproductive bodies called spores.

Cell wall of molds

The main material of fungal cell walls is chitin.

Reproduction of molds

Primarily through spores formed on special reproductive hyphae .Their primary means of reproduction involves asexual and sexual spores.

Asexual Spore

1.   Sporangiospores（孢囊孢子） are formed by successive cleavages within a saclike head called a sporangium,which is attached to a stalk, the sporangiophore. These spores are initially enclosed but are released when the sporangium ruptures.

2. Conidia（分生孢子）(conidiospores) are free spores not enclosed by aspore-bearing sac. They develop either by the pinching off of the tip of a special fertile hypha or by the segmentation of a preexisting vegetative hypha. Conidia are the most common asexual spores

3. arthmspore (節孢子) A rectangular spore formed when a septate hypha fragments at the cross walls.

4. chlamydospore（厚垣孢子） (klams-ih'-doh-spor) A spherical conidium formed by the thickening of a hyphal cell. It is released when the surrounding hypha fractures, and it serves as a survival or resting cell.

sexual spores

The majority of fungi produce sexual spores at some point. The nature of this process varies from the simple fusion of fertile hyphae of two different strains to a complex union of differentiated male and female structures and the development of special fruiting structures. Four different types of sexual spores have been identified, but we will consider the three most common: zygospores（接合孢子）, ascospores（子囊孢子）,oospore）卵孢子）， and basidiospores（擔孢子）.

Zygospores* are sturdy diploid spores formed when hyphae of two opposite strains (called the plus and minus strains) fuse and create a diploid zygote that swells and becomes covered by strong, spiny walls. When its wall is disrupted, and moisture and nutrient conditions are suitable, the zygospore germinates and forms a sporangium. Meiosis of diploid cells of the sporangium results in haploid nuclei that develop into sporangiospores. Both the sporangia and the sporangiospores that arise from sexual processes arc outwardly identical to the asexual type, but because the spores arose dom the union of Iwo separate fungal parents, they are not genetically identical.

In general, haploid spores called ascospores are created inside a special fungal sac. or ascus.Although details can vary among types of fungi, the ascus and ascospores are formed when two different strains or sexes join together to produce offspring. In many species, the male sexual organ fuses with the female sexual organ. The end result is a number of terminal cells, each containing a diploid nucleus. Through differentiation, each of these cells enlarges to form an ascus, and its diploid nucleus undergoes meiosis (often followed by mitosis) to form four to eight haploid nuclei that will mature into ascospores. A rise ascus breaks open and releases the ascospores. Some species form an elaborate fruiting body to hold the asci.

Basidiospores: are haploid sexual spores formed on the outside of a club-shaped cell called a basidium)