Quiz 11: Unicellular Eukaryotes


The structural complexity of unicellular eukaryotes is well explained by the endosymbiotic theory. According to this, the two lineages of ancient prokaryotes gave rise to a common ancestor of eukaryotes by the process of symbiogenesis. In this process, cells of one prokaryotic lineage engulfed the cells of other prokaryotic lineage without digesting the cells. The cell that was engulfed eventually turned into an organelle inside the cell of the host. The two products of symbiogenesis in eukaryotes are mitochondria and chloroplast.' The mitochondria originated when an anaerobic bacteria engulfed an aerobic bacteria that was capable of deriving energy from carbon compounds using environmental oxygen. Plastids in eukaryotes originated when a photosynthetic bacterium was engulfed and was gradually reduced into a eukaryotic organelle. This process is often known as primary endosymbiosis. Hence, symbiogenesis is an explanation of structural complexity of unicellular eukaryotes.

The typical features of the 4 different phyla are: Euglenozoa - • It possesses longitudinal microtubules • Flagella often with paraxial rod • mitochondria with discoid cristae • nucleoli persist during mitosis Apicomplexa - • All are endoparasites • All contain a Characteristic set or organelles called the apical complex • cilia and flagella absent except for flagellated microgametes in some groups • rhoptries and micronemes help in penetrating the host of the tissue Ciliophora - • Cilia or ciliary organelles in at least one stage of life cycle • Two types of nuclei, with rare exception • Trichocysts and Toxicysts present • Cilia, kinetosomes and other fibrils form a kinety Dinoflagellata - • Typically with two flagella, one transverse and one longitudinal; • Chromoplasts usually yellow or dark brown, occasionally green or blue-green, bearing chlorophylls a and c; • Nucleus unique among eukaryotes in having chromosomes that lack or have low levels of histones;

Under the light microscope, when the chromosomal material in the nucleus shows clumps irregularly, leaving some relatively clear areas within the nucleus, the nucleus is known as vesicular nucleus. This is characteristic of protozoan nuclei. The Macronucleus of ciliates is described as compact because the chromatin material is more finely dispersed and clear areas cannot be observed with the light microscope.

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