The Protozoans

Common Protozoa of Man

Introduction• The distinction between plant- like &

animal- like organisms is not always easily made, particularly among unicellular life forms. This difficulty reflects what is surely a distant but very real evolutionary relationship between plants & animals.

• Protozoans are clearly eukaryotes, since they harbor distinct nuclei & other membrane- bound organelles, but unlike members of the animal kingdom, protozoans never develop from a bastula embryonic stage.

• Protozoan classification has long been uncertain, however, nucleotide sequence data in particular are changing the scenarios of protozoan interrelationships at a rapid rate.

• Controversy abounds at the most fundamental levels of organization. Of particular interest, hundreds of unicellular species lack mitochondria. Whether these species represent a primitive condition before the advent of mitochondria, or whether they evolved from protozoan ancestors that had mitochondria is not clear.

General Characteristics

• It is difficult to generalize about protozoans; they absolutely defy tidy categorization.

• Structural Description of A Protozoa: The entire protozoan body is bounded

by a plasmalemma (cell membrane) that is structurally & chemically identical to that of multicellular organisms.

• The cytoplasm bounded by the plasmalemma resembles that of animal cells, except that it is often differentiated into a clear, gelatinous outer region, the ectoplasm, & an inner, more fluid region, the endoplasm.

• Contractile vacuoles are organelles involved in expelling water from the cytoplasm. Apparently, fluid is collected from the cytoplasm by a system of membranous vesicles & tubules called the spongiome.

Phylum Ciliophora

Defining Characteristics• Body externally ciliated in at least some life

cycle stages.• Individual cilia are connected below the

body surface through a complex cord of fibers (the infraciliature).

• Ciliates show the highest degree of subcellular specialization encountered among protozoans & thus are considerably advanced from the primitive protozoan condition.

• First among the features of this phylum is the presence of the external ciliation in at least some stage of the life cycle.

• The ultrastructure of cilia is remarkably uniform throughout the ciliophora & in fact, thourghout the animal (and plant) kingdom(s) as well, although some modifications to the basic pattern do occur sporadically.

Structure & Function of Cilia• Each cilium is cylindrical & arises from

a basal body (kinetosome). Within the cilium are a number of long rods called microtubules, composed of a protein known as tubulin.

• A cross section through the a cilium external to the body surface shows a ring of nine groups of microtubules, with only two microtubules per group.

• One pair of microtubule is located centrally within the cilium. These two microtubules form the central shaft the cilium, and they are often surrounded by a membrane constituting a central sheath.

• The entire microtubular complex, consisting of the nine doublet microtubules & the inner pair of single microtubules, is termed the axoneme.

Pattern of Ciliation• Individual cilia, seen external to the cell

body, are associated w/ each other through a complex infraciliature below the body surface.

• A striated fibril, called a kinetodesmos, extends from each kinetosome (basal body) in the direction of an adjacent cilium of the same row.

• Thus, running along the right side of each row of basal bodies is a cord of fibers, termed the kinetodesmata.

• In some species, groups of cilia are functionally associated in such a way to form discrete organelles. One such organelle is the so-called undulating membrane, a flattened sheet of cilia that moves as a single unit.

• A second commonly encountered ciliary organelle is termed a membranelle.

• In addition cilia may form a discrete bundle (cirrus), which tapers to a point toward the tip.

Other Morphological Features• One other morphological feature

particularly characteristic of ciliates is the covering of the body by an often complex series of membranes, forming a pellicle.

• The inner membranes, lying beneath the single plasmella enveloping the body, form a series of elongated, flattened vesicles called alveoli.

• Trichocysts are characteristically associated with the ciliate pellicle.

• Some species also maintain a permanent opening to the outside (a cytproct; procto = G: the anus) for expelling undigested wastes.

Reproductive Characteristics• In addition to the presence of cilia & an

infraciliature, a second unique characteristic of all ciliates is the possession by each individual of two types of nuclei; that is, the nuclei of every ciliate are dimorphic (of two kinds).

• The ciliates are thus heterokaryotic, while other cells, including other protozoans, are monomorphic or homokaryotic.

Ciliate Lifestyles• About 65% of ciliate species are free-living, &

most of these are motile. Some other species form temporary attachments to living or nonliving substrates for feeding purposes, while others are permanently attached & many may form colonies.

• Although all ciliates have a distinct pellicle, some sessile species also produce a rigid, protective encasement termed a test or lorica.

• Most free-living ciliates are holozoic, that is, they ingest particular foods. Some species may be raptorial; that is, they hunt & ingest living prey.