Protists = All Single-celled Eukaryotes Help

Introduction to Protists

Figure 7.1 reviews the fact that unicellular organisms belong to one of either two major kingdoms–Kingdom Protista or Kingdom Monera.

The protists include all of the single-celled organisms having a nucleus. This is a huge category, of course! And this category is distinguished from the bacteria, you may remember, which are the main single-celled organisms not having a nucleus. Recollect that the primitive amoeba is a representative member of the Kingdom Protista.

Fig. 7.1 The two kingdoms of unicellular organisms.

“Where Did The First Cell Organelles Come From?”

Somewhere along the vast timescale of evolutionary history, a group of prokaryote cells without nuclei eventually developed into eukaryote cells containing nuclei. Recall that the first eukaryote cells appeared about 2.1 billion years ago. These original eukaryote cells started what we loosely call the protists, literally the “very first” organisms with nucleus-containing cells. Speculation about just how these nucleated protists first appeared has lead to the Theory of Endosymbiosis ( EN -doh- sim -be- OH -sis). The concept of symbiosis ( sim -be- OH -sis), in general, involves a “condition of” (-osis) “living” (bi) “together” (sym-). The term endosymbiosis, then, means a condition of living together and “within” (endo-). In a condition of endosymbiosis, two organisms of different species live together, with the smaller organism living inside the cells of the larger organism, which act as hosts.

Study suggestion: Picture a band of gypsies living together in a group of tents surrounded by a fence. The gypsies used to live independently, but for their safety and well-being, their band has come to live together with other members of a circus. The circus compound itself, including the fenced-off gypsy area, is surrounded by a tall wall. In return for food and protection, the gypsies live inside the circus compound and entertain their hosts (circus owners) and read their fortunes! The contained gypsy band and the larger host circus compound therefore live in a condition of endosymbiosis, where each group benefits.

According to the Theory of Endosymbiosis, small prokaryote cells that used to live independently, moved into larger host prokaryote cells and achieved greater safety and well-being. (Examine Figure 7.2.) After living together in a state of endosymbiosis for a long time, the smaller prokaryote cells completely lost their ability to live independently. Instead, they evolved into nuclei and other organelles. Consider the possible origins of the mitochondrion, flagellum, and chloroplast. Mitochondria may have evolved from tiny, free-living, aerobic bacteria that were engulfed via phagocytosis by a larger anaerobic cell. Eventually, the bacteria mutated into mitochondria and became permanent residents and organelles of their larger host cell. By this means, the once anaerobic host cell became an aerobic (O ₂ -using) one containing mitochondria. Suppose that the now-aerobic cell then phagocytosed ( fag -oh- SIGH -toesd) a small, fast-moving prokaryote with a whip-like flagellum. The entire small flagellated cell eventually evolved into a new organelle for the larger host cell – a flagellum. The host cell benefited by gaining increased mobility.

Fig. 7.2 Endosymbiosis and the origin of cell organelles.

Similarly, the chloroplast and its capacity for photosynthesis may have evolved by endosymbiosis. Recall (Chapter 3) that tiny, bluish-green, chlorophyll-containing bacteria, shaped like threads or filaments, may have been the most ancient of living things. These bacteria were prokaryotes, lacking a nucleus. Many biologists speculate that early aerobic cells may have phagocytosed such bluish-green bacteria, which eventually evolved into slender chloroplast organelles. The large host cells would benefit by obtaining the capacity to produce energy via photosynthesis.

The ultimate result of all this endosymbiosis going on over long periods of time may well be such modern protists as Euglena (yew- GLEN -ah). This modern protist contains both mitochondria and chloroplasts as organelles, and it moves about rapidly by means of its flagellum. Commonly found in ponds, Euglena living in sunny water develop chloroplasts and become autotrophic, producing their own energy via photosynthesis. They use their flagella to scoot towards the light. In dark pond water, however, Euglena becomes an aerobic heterotroph, absorbing nutrients from the rich pond water and using its mitochondria to produce energy aerobically. Such dark-dwelling Euglena may even lose their chloroplasts!

So, is Euglena a plant (living anaerobically via photosynthesis), or is it an animal (living aerobically and using oxygen)? The answer is, “Neither!” For you see, Euglena is a protist!