Invertebrate Zoology: A Study Guide - T. A. Dauda 2014

Protozoans

General characteristics

The subkingdom of unicellular organisms comprises 5 phyla and over 30,000 species.

Morphologically, Protozoans consist of a single Cell, yet physiologically they function as independent organisms. They include both uninucleate (monoenergid) and multinucleate (polyenergid) forms. The Organization OF THE latter approaches that of Multicellular Organisms. Furthermore, colonial forms are quite common among protozoans. Thus, the term "protozoa" encompasses entities at various Levels of biological organization.

The body size of protozoans is typically microscopic. The smallest—measuring 2–4 µm in diameter—are parasitic flagellates and piroplasms. Average sizes range from 50 to 150 µm (0.05–0.15 mm). Some protozoa reach lengths of 1.5 mm or even 10 mm.

The body shape in lower Protozoa is variable due to a semi-fluid Cytoplasm and the absence of supportive structures. In complexly organized unicellular organisms, the body shape is fixed. It is maintained by a dense pellicle, cytoplasmic fibrils (microfilaments), and an external Skeleton, which may take the form of a shell (as in foraminiferans) or interconnected spicules. Common skeletal components include CaCO₃ or SiO₂. Many chlorophyll-bearing flagellates possess an outer Cellulose Cell wall, bringing them closer to plants. The body Structure of protozoans and their Symmetry shed light on the Phylogeny of the phylum. According to V. N. Beklemishev, the primary body form may have been asymmetrical, as observed in amoebas and amoeboid stages of other species (e.g., Plasmodium vivax, the malaria parasite). The next level of primitiveness is a spherical shape with radial symmetry (radiolarians, coccidians, resting stages). Interestingly, bilateral symmetry—the predominant type of symmetry in Multicellular animals—is very rare in protozoans; it is characteristic of crawling forms and leads to the differentiation of dorsal and ventral sides.

The Main Components of a protozoan cell are the cytoplasm and The Nucleus. The cytoplasm is enclosed by a membrane composed of Proteins and Lipids, which regulates the intake of substances into The Cell and provides mechanical strength. The cytoplasm typically consists of two layers: the ectoplasm, which is lighter and denser, and the endoplasm, containing various Organelles—Mitochondria, Ribosomes, the Golgi apparatus, The Endoplasmic reticulum, specialized organelles (vacuoles, supporting and contractile fibrils)—along with one or more nuclei.

The nucleus is bounded by a double-layer membrane pierced by numerous pores. Chromatin material and nucleoli are distributed throughout the nuclear sap (karyoplasm). Chromatin consists of Chromosomes (DNA and proteins), while nucleoli—one or several—are clusters of ultramicroscopic ribosomes (RNA and proteins). RNA Synthesis occurs within the nucleus, after which RNA passes into the cytoplasm to participate in METABOLISM/35.html">Protein Biosynthesis. Protozoan nuclei vary in size, cellular Location, number and distribution of nucleoli, and mode of division.

Habitat

Protozoans are exceptionally widespread in the biosphere. They inhabit aquatic environments—living on the bottom, in the Water Column, within marine foam, and as part of periphyton. Many species populate damp soil. Parasitism is also widespread among them, which accounts for their great practical significance in medicine, veterinary science, and agriculture.

Locomotion

Most protozoans (with the exception of parasites) possess The ability to move and migrate. Their modes of locomotion are diverse. The simplest and likely primordial form of movement is amoeboid locomotion, achieved by means of false feet, or pseudopodia (found in Sarcodina). More complex forms involve flagella and cilia.

Flagella are ultra-thin cellular outgrowths numbering from one to several hundred. Each flagellum originates from a blepharoplast—a basal granule in the cytoplasm—and extends outward through the pellicle. During movement, a flagellum most commonly describes a cone shape with its apex directed toward the attachment point, imparting a rotational motion to the Organism's body. The beating frequency of a flagellum ranges from 10 to 40 revolutions per minute. Maximum mechanical efficiency is achieved when the cone apex angle is 40–45°.

Alongside rotational movement, a flagellum can perform undulating or beating motions, propelling the cell forward. In multiflagellated organisms, The Mechanism of locomotion becomes more complex.

The cilia of Ciliates, which are numerous (ranging from several hundred to tens of thousands), perform beating strokes. Finally, certain highly organized protozoans achieve movement through body contraction (ciliates, radiolarians). To this end, contractile fibrils known as myonemes develop within the cytoplasm.

Ontogeny and Modes of Reproduction

The Cells of a multicellular organism, with the exception of Germ Cells, lack an independent life cycle. Their Functions are to maintain the integrity and viability of the tissue they comprise. Some cells have a short developmental lifespan and are replaced by new generations of identical cells (such as epithelium), whereas others function alongside the entire organism and die simultaneously with it (Nerve Cells).

Protozoans, by contrast, possess a life cycle. It consists of a sequence of successive stages that repeat with a regularity specific to each species. Most commonly, the cycle begins with a zygote stage, equivalent to the fertilized egg of multicellular animals. This is followed by single or multiple rounds of asexual reproduction via Cell Division. Finally, germ cells (Gametes) are formed, whose pairwise fusion yields a new zygote. Under adverse conditions, many Protozoa are capable of encystment: they round up, retract or shed their locomotory organelles, secrete a robust protective envelope on the cell surface, and survive in this dormant state through chilling or desiccation until favorable conditions return.

The simplest method of asexual reproduction in unicellular organisms is the division of the parent cell into two daughter cells via mitosis. A more complex division is schizogony, in which the nucleus divides multiple times first, forming a polyenergid cell. Subsequently, following the division of the cytoplasm, A large number of daughter cells are formed simultaneously. This mode of asexual reproduction is characteristic of Sporozoans and certain flagellates. In some ciliates, asexual reproduction may occur through budding.

Sexual processes in protozoans exist alongside asexual reproduction and are represented by copulation and conjugation. The former involves the complete fusion of two gametes, whereas the latter entails the close approach of two individuals followed by complex nuclear division and exchange. Conjugation represents the highest form of Sexual reproduction in protozoans and is exclusive to ciliates.

In some instances, asexual reproduction in protozoans is incomplete. The resulting daughter cells remain connected to one another, forming permanent or temporary colonies of linear, spherical, arborescent, or laminar shapes. Spherical colonies (such as Volvox) consist of specialized individuals and acquire the character of higher-order entities, in which the primary individuals play a subordinate role. Such forms are of particular interest regarding the evolutionary origin of multicellular organisms.

Practical Significance

Protozoans play a prominent role in the global biogeochemical cycling of matter. In aquatic ecosystems, they actively consume microorganisms—Bacteria and Algae—while serving as a food source for larger animals. Protozoans are a principal source of marine sediment formation (globigerina ooze, radiolarian ooze, etc.) and chalk deposits. The Study of fossil protozoa provides a key to understanding the geological laws governing the Earth's crust, dating strata, and prospecting for mineral resources. Analyzing species composition allows for the Assessment of the quality and Chemical composition of natural waters, making them vital indicators for Sanitary and hygienic monitoring of aquatic ecosystems. Furthermore, certain protozoans that parasitize humans and agricultural animals act as causative agents of severe diseases.

Systematics

The subkingdom of unicellular organisms comprises five phyla. This tutorial will examine three of them: Sarcomastigophora, Sporozoa, and Ciliophora.

QUESTIONS FOR SELF-Check and Review:

1. To which habitats are unicellular animals adapted? Where can they be found?

2. What main parts do the cells of unicellular organisms consist of?

3. What are the Similarities and differences between the cells of unicellular and multicellular animals?

4. Methods of reproduction in protozoa.