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Protists Review from
Biology Questions and Answers
Learn Protozoans and Algae - Easy Review
1. Which are the groups of living beings that form the protist kingdom?
The protist kingdom includes protozoans and algae. (Two groups of fungi with similar characteristics to protozoans, myxomycetes and oomycetes, have been classified as protists.)
Unicellular protozoans and algae are unicellular eukaryotes. The pluricellular algae are eukaryotes of simple structure too. It is believed that protists are phylogenetic ancestors of living beings of the other eukariotic kingdoms (fungi, animals and plants).
Protists Review - Image Diversity: protozoan algae
2. What is the fundamental difference between protozoans and algae?
The basic difference between protozoans and algae is the fact that protozoans are heterotrophs while algae are photosynthetic autotrophs.
3. What are the characteristics of protozoans that make them resemble animals?
Protozoans are unicellular beings with some similar characteristics to animal cells.
In comparison to pluricellular organisms protozoans are more proximal to the animal kingdom than to plants: they are heterotrophs, they have a rudimentary locomotion system (amoeboid movements, cilia, flagella), they do not have cell wall, some species present structures that resemble structures of a primitive digestive system, with cytostome (mouth) and cytopyge (anus), specialized in digestion and excretion.
The evolutionary hypothesis that animal cells have come from differentiation of protozoans is strong.
4. What is the basic morphology of a protozoan cell?
Protozoans are eukaryotic cells so they have organelles and structures common to this kind of cell: endoplasmic reticula, Golgi apparatus, digestive vesicles, ribosomes, mitochondria, nucleus with genetic material, karyotheca, etc. All these elements are found dispersed throughout the cytoplasm. Protozoans do not have cell walls.
Protozoans from the mastigophora group (like trichomonas) have flagella and others, others from the ciliated group (like paramecium) have cilia.
5. Do protozoans have a cellular nucleus?
All protozoans, as eukaryotes, have nucleus. Some species, like the paramecium, have two nuclei: the macronucleus and the micronucleus.
Protists Review - Image Diversity: paramecium
6. What are the respective functions of the macronucleus and of the micronucleus in the paramecium?
The macronucleus is properly the cell nucleus, it has DNA and RNA and acts as the center of the cellular control and regulation. The micronucleus has reproductive functions and it is related to the conjugation process (sexual reproduction).
7. What do protozoans “eat”? Do they move in search for food?
Protozoans are heterotroph beings, i.e., they do not make their own food and thus they need to search for it in the environment. Protozans have developed several locomotion mechanisms and they actively move towards food.
8. How do amoebae, paramecia and trichomonas respectively move?
Amoebae move by amoeboid movements, small projections and invaginations of their plasma membrane (pseudopods) that alter the external morphology of the cell making it move on surfaces. Paramecia have the outer face of their plasma membrane covered by cilia that flap helping the cell to move. Trichomonas are flagellated protozoans, i.e., they have relatively long filaments outside the cell that beat and make possible active swimming in fluid environments.
Protists Review - Image Diversity: amoeboid movement
9. How is digestion performed in protozoans?
Digestion in protozoans is intracellular digestion: organic material is internalized and degraded inside the cell.
Protozoans get food by phagocytosis and then the food is digested when phagosomes fuse with lisosomes within the cell, forming digestive vacuoles. The digestive vacuoles give origin to residual bodies that are eliminated from the cell by exocytosis.
In the paramecium the entrance of food into the cell and the excretion of digestive residuals occur at specialized regions of the plasma membrane, the cytostom and the cytopyge, respectively.
10. Are protozoans presenting contractile, or pulsatile, vacuoles easily found in fresh or in salt water?
Fresh water is the less concentrated of solutes than sea water and it (fresh water) tends to be less concentrated than the intracellular environment making cells to swell. Sea water, on the other hand, since it is very concentrated tends to dehydrate the cell.
The vacuoles of protozoans are internal structures specialized in water storage that when necessary liberate water to the cytoplasm. Vacuoles thus can dilute the cytoplasm for it to enter into osmotic equilibrium with the environment. Protozoans of fresh water then need vacuoles more since their intracellular is hypertonic in relation to the exterior. Without the dilution mechanism provided by the vacuoles, protozoans of fresh water would absorb too much water and would die.
11. Do protozoans have sexual or asexual reproduction?
In protozoans reproduction is sexual or asexual. The most frequent form of sexual reproduction is binary division, or scissiparity, in which the cell divides itself by mitosis originating two daughter cells. Some species, like the plasmodium, agent of malaria, reproduce asexually by schizogony (multiple fission); in this form of reproduction the cell becomes multinucleated, generally inside a host cell, and each nucleus is expelled out together with cytoplasm portions giving rise to new protozoans.
The sexual reproduction in protozoans can happen by conjugation, with incorporation of genetic material from one cell into another, or by gametes that fecundate others and form zygotes. In the plasmodium sexual reproduction happens in the mosquito, the definitive host, and the zygote undergoes mitosis (sporogony) creating many sporozoites.
12. Which is the form of protozoan reproduction that generates more variability?
Sexual reproduction always generates more genetic variability than asexual reproduction. That is because in sexual reproduction the fusion of genetic material from different individuals occurs and so the offspring is not genetically identical to the parent cell.
If the hypothesis that protozoans originated multicellular animals is strong, other hypotheses may be even stronger: that these protozoans were able to reproduce sexually, since only genetic variation can produce biological differentiation to the point of creating new types of living beings.
13. What are the four groups of protozoans?
The four main groups of protozoans are the sarcodines (that form pseudopods, like amoebae), the mastigophores (flagellated, like the trypanosome that causes Chagas’ disease), the ciliated (like paramecia) and the sporozoans (spore-forming, like plasmodia).
Protists Review - Image Diversity: sarcodines mastigophores ciliated protozoans sporozoans
14. Why are euglenas involved in polemics related to their taxonomic classification?
Euglenas are involved in taxonomic polemics because they tend to be classified sometimes as protozoans and sometimes as algae. Although they have chloroplasts and they are photosynthetic autotrophic beings, euglenas do not have a cell wall and they can survive by “eating” substances from the environment when light is not available for photosynthesis. Curiously euglenas also have a photosensitive structure called stigma that orients the movement of the cell towards light. Nowadays euglenas are classified as algae, but it is suspected that they are common ancestors of algae and protozoans.
Protists Review - Image Diversity: euglena
15. Do algae reproduce sexually or asexually?
There are algae that reproduce sexually and there are algae that reproduce asexually.
In unicellular algae reproduction is generally asexual by binary division.
In pluricellular algae asexual reproduction can occur by fragmentation or by sporulation.
In sexual reproduction of algae, uni or pluricellular, there is fusion of gametes (syngamy). There are algae in which all cells can become gametes and there are algae in which only some cells can play that role. Some species may present alternation of generations, forming gametophytes and sporophytes with different ploidies.
16. What is the commercial importance of algae?
Many algae have high nutritional value and are commercialized and consumed as human food, they are very popular food in the oriental world. Jelly compounds are extracted from some algae, like glues and pastes for industrial and commercial use.
The agar-agar, used as a medium for biological culture in laboratories and in medicines, and the substance known as carrageenin, a component of tooth pastes, cosmetics, paint and hygienic products, are extracted from rhodophyte algae. Diatom algae deposited on the bottom of the sea form diatomites, used in the production of filters, refractories, thermal isolation and cement. Some algae are used as agricultural fertilizers.
17. What is the phenomenon known as “red tide”? Which ambiental harms can it cause?
Red tide is a phenomenon that occurs when dinoflagellates (algae from the pyrrophyte group) proliferate excessively in the ocean. These algae liberate toxins that affect the nervous system and can cause death when ingested by marine animals and by humans that eat contaminated animals.
Protists Review - Image Diversity: red tide
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