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).
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.
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.
Fungi
1. What are the main cellular
features of fungi?
There are unicellular and pluricellular fungi. All fungi are eukaryotes and heterotrophs.
Fungi have cells with cell wall made of chitin, the same substance that
constitutes the exoskeleton of arthropods. Fungi, likewise animals, characterize for storing glucose in the form of its polymer glycogen.
2. Are there photosynthetic
fungi? How do fungi nourish
themselves?
All fungi are heterotrophs (so, they do not perform photosynthesis). Fungi are typical decomposers, they eat and degrade organic material.
3. Fungi are classified in their
own kingdom. Into which
phyla is the fungi kingdom
divided? Into which of those
phyla are mushrooms
classified?
The kingdom fungi is divided into four phyla: ascomycetes, basidiomycetes, zygomycetes and deuteromycetes. Mushrooms are basidiomycetes.
4. What are the hyphae and
the mycelium of pluricellular
fungi?
The main structures of pluricellular fungi are the hyphae (threadlike filaments made of contiguous uni or
multinucleated cells) and the mycelium (a set of hyphae).
5. What are the types of
reproduction that occur in
fungi?
In fungi there are asexual and sexual reproduction. Fungi reproduce asexually by fragmentation, gemmation and sporulation. Some species can
reproduce sexually by fusion of hyphae from different individuals, even with metagenesis (alternation of
generations).
6. What are the fruiting bodies
present in some fungi?
Fruiting bodies are structures made of hyphae that project radially from the superior portion of the peduncle of some fungi. These structures contain the reproductive cells of the individual. They form the umbrella-like cap in
mushrooms (basidiocarp) or the ascocarp in ascomycetes.
7. What is the ecological
importance of fungi?
Fungi are heterotrophs and
decomposers (they break down dead beings) and they actively participate in the recycling of organic material in ecosystems. Some fungi keep mutualist