Form and Function of the Eucaryotic Cell: External Structures
The cell structures common to all eucaryotes are the cell membrane, a membrane-enclosed nucleus with nucleolus inside, vacuoles, mitochondria, endoplasmic reticulum, Golgi apparatus, and a cytoskeleton. Cell walls, chloroplasts, and locomotor organs are present in some eucaryote groups.

Microscopic eucaryotes utilize locomotor organs such as flagella or cilia for moving themselves or their food.

The glycocalyx is the outermost boundary of most eucaryotic cells. Its functions are protection, adherence, and reception of chemical signals from the environment and /or other organisms. The glycocalyx is supported by either a cell wall or a cell membrane.

The cytoplasmic membrane of eucaryotes is similar in function to that of procaryotes, but it differs in composition, possessing sterols as additional stabilizing agents.

Eucaryotic cells have membrane-bound organelles, which are never present in procaryotes. Such compartmentalization allows a wide variety of cellular functions to occur in specialized areas of the cell.

Form and Function of the Eucaryotic Cell: Internal Structures
The genome of eucaryotes is located in the nucleus, aspherical structure surrounded by a double membrane. The nucleus also contains the nucleolus, site of rRNA synthesis. Eucaryote DNA, or chromatin, is organized into chromosomes at cell division.

All eucaryotes cycle between a haploid state and a diploid state at some point in their life cycle. Most fungi, many algae, and some protozoa are haploid except in their zygote stage.

Plants and animals, as will as some protozoa and algae, are diploid for most of their life cycles, but their reproductive cells (gametes) are always haploid.

Microscopic eucaryotes reproduce asexually by mitosis and sexually by meiosis.

The endoplasmic reticulum is an internal network of membranous passageways extending throughout the cell and is involved in the transportation of materials throughout the cell.

The Golgi Complex is a packaging center which receives materials form the ER and forms vesicles around them for storage or transport to the cell membrane for secretion.

The mitochondria generate energy in the form of ATP for all cellular activities.

Chloroplasts are membrane-enclosed photosynthesis machines found in plants and algae.

The cytoskeleton maintains the shape of cells and produces movement of cytoplasm within the cell, movement of chromosomes at cell division, and in some groups, movement of the cell as a unit.

Ribosomes are present in both eucaryotes and procaryotes, but eucaryote ribosomes are larger (80S). Both types of ribosomes are necessary components of protein synthesis.

Survey of Eucaryotic Microorganisms
The eucaryote microorganisms include the Myceteae (Fungi), the Protista (algae and Protozoa), and the Helminths (Kingdom Animalia).

The Kingdom Myceteae (Fungi) is composed of non-photosynthetic haploid species with cell walls of chitin. The fungi are either saprobes or parasites.

The Kingdom Protista (algae and protozoa) is a kingdom of mostly unicellular or colonial eucaryotes that lack specialized tissues. It is composed of two major subgroups: the Subkingdom Algae and the Subkingdom Protozoa.

The Kingdom Animalia has only one group of microscopic organisms: the Helminths, or worms.

The Kingdom of the Fungi: Mycetae
Fungi can be either macroscopic or microscopic, unicellular or colonial. A very few species exhibit cellular specialization.

The two morphological types of fungi are yeasts (unicellular) and multicellular filaments called hyphae. Hyphae form macroscopic aggregates called a mycelium or mold.

Many fungi are important decomposers (saprobes ) and some are pathogens (parasites).

Fungi reproduce asexually by fragmentation and through formation of asexual spores. They reproduce sexually by the fusion of two compatible but genetically different hyphae and subsequent meiosis of the diploid zygote to form haploid sexual spores.

Sexual spores are used to classify into three phyla, the Zygomycota, the Ascomycota, and the Basidiomycota. Fungi with no discernible sexual spores are placed in a fourth phylum, the Deuteromycota (formerly the Fungi Imperfecta).

Medical mycologists subdivided the Kingdom Mycetae into two subgroups: The Myxomycota (slime molds) and the Eumycota (true fungi).

Very few fungi are pathogens.

The Kingdom Protista
The Subkingdom Algae contains all the microscopic algae, the only group of microorganisms which contains every eucaryotic organelle.

Algae are economically important as the basis of all aquatic food chains. They are also important commercial sources of agar, algin, and diatomaceous earth.

Algae are rarely disease agents. Dinoflaggellates produce a toxin which affects humans ingesting infected seafood. Protheca is the only pathogenic genus known.

The Subkingdom Protozoa contains nonphotosynthetic, motile, unicellular organisms which lack a cell wall. They are primarily free-living heterotrophs which require moist habitats.

Protozoans exist in two forms: the trophozoite (feeding) stage and the dormant cyst (resting) stage. The cyst form of pathogenic protozoa disperses readily, facilitating the spread of these pathogens.

There are seven taxonomic Phyla of protozoa, but only four medically important groups: the Mastigophora, the Sarcodina (amoebae), the Ciliphora, and the Sporozoa. The first three groups contain some pathogenic species, but all of the Sporozoa are parasitic.

There are four medically important groups of Protozoa which are organized by their method of locomotion. The Mastigophora move by flagella, the Ciliphora by cilia, the Sarcodina primarily by amoeboid movement, and the Sporozoa by lack of motility except in male gametes.

Protozoa are usually identified by direct observation rather than by culture. Key characteristics include the size and shape of cell, characteristics of locomotor organelles, number of nuclei, presence of specialized organelles and cysts.

Few Protozoa are pathogens, but those that are parasitic are transmitted to humans by animals, food, soil, and water.

The Kingdom Animalia
Most animals are macroscopic, but the Helminths, or worms, are a group of microscopic animals which cause disease. They are multicellular, with tissues, organs, and organ systems.

Helminths are included with microorganisms because their eggs and larvae are microscopic, although the adults are often large enough to be visible macroscopically.

Helminths are classified on the basis of shape, degree of organ specialization, type of reproduction, hosts, eggs, and larvae.