Most protists are aquatic. Some live in marine environments, others in fresh water, and still others in the body fluids of other organisms. The slime molds inhabit damp soil and the moist, decaying bark of rotting trees. Many other protists also live in soil water, some of them contributing to the global nitrogen cycle by preying on soil bacteria and recycling their nitrogen compounds into nitrates. Most protists are unicellular, but some are multicellular, and a few are very large.
Protists are strikingly diverse in their structure, but not so diverse in their metabolism as the prokaryotes—which is not surprising, since some of the eukaryotes' most important metabolic pathways were "borrowed" from bacteria through endosymbiosis. However, protists do display a number of nutritional modes. Some are photosynthetic autotrophs, some are heterotrophs, and some switch with ease between the autotrophic and heterotrophic modes of nutrition.
Some protists, formerly classified as animals, are sometimes referred to as protozoans, although biologists increasingly regard this term as inappropriate because it lumps together protist groups that are phylogenetically distant from one another. Most protozoans are ingestive heterotrophs. Similarly, there are several kinds of photosynthetic protists that some biologists still refer to as algae (singular, alga). Although these two terms are useful in some contexts, they do not correspond with natural phylogeny, and we generally avoid them in this book except as parts of descriptive names such as "brown algae." Let's next consider some of the other ways in which protists differ from one another.
Although a few protist groups consist entirely of nonmotile organisms, most groups include cells that move, either by amoeboid motion, by ciliary action, or by means of flagella.
In amoeboid motion, the cell forms pseudopods ("false feet") that are extensions of its constantly changing body mass. Cells such as the amoeba in Figure 28.4 simply extend a pseudopod and then flow into it. Cilia are tiny, hairlike organelles that beat in a coordinated fashion to move the cell forward or backward (see Figure 4.23). A eukaryotic flagellum moves like a whip; some flagella push the cell forward, others pull the cell forward. Cilia and eukaryotic flagella are identical in cross section; they differ only in length.
Unicellular organisms tend to be of microscopic size. As we noted above, an important reason that cells are small is that they need enough membrane surface area in relation to their volume to support the exchange of materials required for their existence. Many relatively large unicellular protists minimize this problem by having membrane-enclosed vesicles of various types that increase their effective surface area.
As we saw in Chapter 5, organisms living in fresh water are hypertonic to their environment. Many freshwater pro-
28.4 An Amoeba The flowing pseudopods are constantly changing shape as the amoeba moves and feeds.
tists address this problem by means of specialized vesicles that excrete the excess water they constantly take in by osmosis. Members of several protist groups have such contractile vacuoles. The excess water collects in the contractile vacuole, which then expels the water from the cell (Figure 28.5).
Asecond important type of vesicle found in many protists is the food vacuole. Protists such as Paramecium engulf solid food by endocytosis, forming a food vacuole within which the food is digested (Figure 28.6). Smaller vesicles containing
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This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.