Introduction
Protists are eukaryotes and include unicellular (e.g., amoeba) and multicellular forms (e.g., algae). The word protista (Gr. protos, very first, ktistos, to establish) implies great antiquity. Protista represent a polyphyletic group. Two interesting scenarios regarding the history of life on earth emerged during the evolution of protists: the origin of the eukaryotic cell and the subsequent emergence of multicellular eukaryotes.
An organism whose cells contain a nucleus (organelles) enclosed within membrans.
The Emergence of the Eukaryotic Cell
The small size and simpler construction of the prokaryotic cell has many advantages but also imposes a number of limitations:
- Number of metabolic activities that can occur at any one time is smaller.
- Smaller size of the prokaryotic genome limits the number of genes which code for enzymes controlling activities.
Natural selection resulted in increasing complexity in some groups of prokaryotes; two major trends were apparent:
1. Toward multicellular forms such as cyanobacteria; different cell types with specialized functions .
2. The compartmentalization of different functions within cells; the first eukaryotes resulted from this solution.
Introduction to Protozoan Protists
Protozoans (Gr. proto = first; zoa = animal) are the single-celled animal-like members of the kingdom Protista. They are clearly eukaryotes, e,g., distinct nuclei, membrane bound organelles, etc.; unlike animals, never develop from a blastula. Remarkably diverse in terms of size, morphology, mode of nutrition, locomotory mechanism, and reproductive biology. Protozoans are regarded as being a polyphyletic group.
General Characteristics of Protozoan Protists
Entire organism is bounded by the plasmalemma (cell membrane). The cytoplasm is often differentiated into a clear, outer gelatinous region, the ectoplasm, and an inner, more fluid region fluid or sol state, the endoplasm. Many organelles are typical of most multicellular metazoan cells. However, many protozoans contain organelles not generally found among the metazoa, e.g., contractile vacuoles and trichocysts.
Cilia and Flagella
Locomotor appendages that protrude from the protozoan cell. Cilia are shorter and more numerous, whereas, flagella are longer and less less numerous. Cilia and flagella are similar structurally; microtubules are arranged in a ring of 9 microtubule doublets surrounding a central pair of microtubles (9+2 arrangement); microtubules are covered by an extension of the plasma membrane; they are anchored to the cell by a basal body. Cilia and flagella differ in their beating patterns.
Pseudopodia
When organisms like amoeba are feeding and moving, they form temporary cell extensions called pseudopodia. The most familiar form are called lobopodia: contain ectoplasm and endoplasm; used for locomotion and engulfing food. When a lobopodium forms, an extension of the ectoplasm called the hyaline cap appears and endoplasm flows into this cap. As the endoplasm moves into the cap it fountains out and it changes from the fluid state to the gel state (endoplasm to ectoplasm). Pseudopodium anchors to the substrate and the cell is drawn forward.
Amoeboid movement involves endoplasm and ectoplasm. Endoplasm is more fluid than ectoplasm which is gel-like. When a pseudopodium forms, an extension of ectoplasm (the hyaline cap) appears and endoplasm flows into it and fountains to the periphery where it becomes ectoplasm. Thus, a tube of ectoplasm forms that the endoplasm flows through. The pseudopodium anchors to the substrate and the organism moves forward.
Nutrition and Digestion
Ingested food particles generally become surrounded by a membrane, forming a distinct food vacuole; digestion is entirely intracellular. Vacuoles move about in the fluid cytoplasm and the contents are broken down by enzymes. The contents of the vacuoles can change, e.g., go from acidic to basic. This is important because digestion for these organisms requires exposing the food to a series of enzymes, each of which has a specific role that operates under a narrow range of pH. Controlled changes of pH that occur within the food vacuoles allow for the sequential disassembly of foods. Once solubilized, nutrients move across the vacuole wall and into the endoplasm of the cell. Indigestible solid wastes are commonly discharged to the outside through an opening in the plasma membrane.
How Amoebas Feeding
Feeding in amoebas involves using pseudpodia to surround and engulf a particle in the process of phagocytosis. The particle is surrounded and a food vacuole forms into which digestive enzymes are poured and the digested remains are absorbed across the cell membrane.
Excretion and Osmoregulation
Contractile vacuoles are organelles involved in expelling water from the cytoplasm. Fluid is collected from the cytoplasm by a system of membranous vesicles and tubules called spongiome tubules. The collected fluid is transferred to a contractile vacuole and is subsequently discharged to the outside through a pore in the cell membrane. Vacuoles are most commonly found among freshwater species.
Reproduction
Asexual reproduction is commonly encountered among protozoans. Some reproduce asexually through fission, a controlled mitotic replication of chromosomes and splitting of the parent into two or more parts. Binary fission - protozoan splits into two individuals, multiple fission many nuclear divisions precede the rapid differentiation of the cytoplasm into many distinct individuals. Budding a portion of the parent breaks off and differentiates into a new individual.
Many protozoans possess the capacity for regeneration. For example, encystment and excystment exhibited by freshwater and parasitic species. During encystment, substantial dedifferentiation of the organism occurs, forming a cyst: compact, expels excess water, forms a gelatinous covering is secreted. The cyst can withstand long periods of exposure to what would otherwise be intolerable conditions of acidity, thermal stress, dryness, etc. Once conditions improve excystment ensues with the regeneration of all former internal and external structures.
All protozoa reproduce asexually, but sex is widespread in the protozoa too. In ciliates such as Paramecium, a type of sexual reproduction called conjugation takes place in which two paramecia join together and exchange genetic material.
Classification
Phylum Sarcomastigophora
Move by means of flagella and/or pseudopodia; possess a single type of nucleus.
1. Subphylum Mastigophora
Locomotion is by means of one of more flagella :
a. Class Phytomastigophorea or phytoflagellates; autotrophic
forms containing chlorophyll; one or two flagella,
e.g., dinoflagellates; Euglena; Volvox.
b. Class Zoomastigophorea or zooflagellates: heterotrophic forms,
e.g., trypanosmes that parasitize humans and cause “sleeping
sickness”; tsetse flies serve as vectors.
2. Subphylum Sarcodina
Mostly marine, but some inhabit freshwater and soil; some are parasitic. Use pseudopodia for feeding and locomotion. Feed by a process known as phagocytosis. A number of species of sarcodines possess a protective outer shell or test, e.g. the radiolarians (silica) and foraminiferans (calcium carbonate). Both the radiolarians and the forams feed by extending their pseupodia through openings in the shell.
Phylum Ciliophora
Exclusive to freshwater. Cilia or ciliary organelles present in at least one stage of the life cycle. The ciliates are unique in that they possess 2 kinds of nuclei: a large macronucleus and one or more smaller micronuclei. The macronucleus controls the normal metabolism of the cell, while the micronuclei are concerned with sexual reproduction.
Ciliophoran Reproduction
Asexually via binary fission; sexually via conjugation. 2 individual align and partially fuse; all but one micronucleus in each cell disintegrates. The partners swap one micronucleus; this micronucleus then fuses to another micronucleus, forming a diploid organism with genetic material from the 2 individuals.
Phylum Apicomplexa
This is an exclusively parasitic group of protozoans that lack locomotory organelles, except during certain reproductive stages. They possess a characteristic set or organelles called the apical complex, which aids in penetrating host cells. Includes parasites that cause malaria (e.g., Plasmodium) to humans; mosquitoes serve as vectors.
