Microcheck 121

Algae are a diverse group of eukaryotic organisms that all contain chlorophyll a and carry out photosynthesis. Algae are found in both fresh and salt water and are a significant part of the phytoplankton. Algae do not cause disease directly, but they can produce toxins that are harmful when ingested by humans.

■ What are the primary characteristics used to distinguish algae from other organisms?

■ What harmful effect can algae have on humans?

■ Why should single cells have a large absorptive surface relative to their surface area?

■ Would organisms that reproduce by binary fission necessarily be genetically identical? Why or why not?

12.2 Protozoa



- Zygomycetes -Ascomycetes -Basidiomycetes


-Chlorophytes (green algae)

■ Cryptomonads

Rhodophyta (red algae)


■ Chrysophyta (golden brown algae and diatoms - Phaeophyta (brown algae) . Oomycetes (water molds)

Pyrrophyta - dinoflagellates


■ Cellular slime molds Acellular slime molds










Figure 12.7 A Phylogenetic Scheme of Eukaryotes Based on rRNA Sequence Comparisons The protozoan groups are highlighted in red.


Along with the algae, the protozoa constitute another group of eukaryotic organisms that traditionally have been considered part of the microbial world. Protozoa are microscopic, unicellular organisms that lack photosynthetic capability, usually are motile at least at some stage in their life cycle, and reproduce most often by asexual fission.

Classification of Protozoa

As with algae, classification of protozoa according to rRNA and ultrastructure shows that they are not a unified group, but appear along the evolutionary continuum (figure 12.7). The primary reason that they are lumped together in a field known as protozoology is because they are all single-celled eukaryotic organisms that lack chlorophyll. Some members of this group cause disease. We will concentrate on these organisms.

Protozoa have traditionally been divided into groups primarily based on their mode of locomotion. Table 12.2 and figure 12.7 show where each group fits in these schemes.

The phylum Sarcomastigophora includes two subphyla in which most of the human disease-causing protozoa are found. The subphylum Mastigophora includes the flagellated protozoa. They are mostly unicellular and have one or more flagella at some time in their life cycle. These flagella are used for locomotion and food gathering as well as sensory receptors. Some of the organisms included within the Mastigophora are among the ancestors of some of the oldest organisms on earth. The most important disease-causing Mastigophora are Giardia lamblia, LLeishmanii species, Trichomonas vaginalis, Trypanosoma brucei rhodesiense and Trypanosoma bruceigambi-ense (see table 12.2). Each of these diseases will be discussed later in this book. ■ flagella, p. 74

Members of the subphylum Sarcodina move by means of pseudopodia. The Sarcodina change shape as they move. Entamoeba histolytica infects humans, causing diarrhea ranging from mild asymptomatic disease to severe dysentery. ■ diarrhea, p. 610

Table 12.2 Protozoa of Medical Importance

Traditional Classification

18s rRNA

Genus of


Mode of

Mode of

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