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The Protozoa, Algae, and Slime Molds Help

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By — McGraw-Hill Professional
Updated on Aug 30, 2011

Introduction to The Protozoa, Algae, and Slime Molds

The Protozoa or "First Animals"

Among the protists, it is the protozoa which most resemble tiny microscopic animals. Reflecting this fact, the word protozoa means “first animals.” One of the main ways in which protozoa resemble animals is that they are heterotrophic. They must consume or “eat” other material as food. Typically, the protozoa ingest food (minute particles or other cells) via the process of phagocytosis.

The Rhizopoda Or Amoebas

One important phylum among the protozoa are the Rhizopoda (rye- ZAHP -oh-day) – technically, meaning the protozoa with “root” (rhiz) “feet” (poda). The amoebas are the major kind of rhizopods ( RYE -zah- pahds ). Amoebas get this name from their extensive use of pseudopodia ( SOO -doh- poh -dee-ah) or “false” (pseudo-) “feet” (pod). The word amoeba means “change.” The amoeba seems to have its cytoplasm in a nearly constant state of change or streaming action, using its pseudopodia like blunt feet or root feet to scoot its body along and engage in phagocytosis of encountered food.

The Algae: Plant-like Protists

The Fossil Record shows the first algae (literally, “seaweeds”) appearing about 1.5 billion years ago. Algae are sometimes considered the most primitive members of the Plant Kingdom, whereas other scientists classify them as plant-like protists. Algae are fundamentally different from most green plants, in that they lack any true leaves, roots, or stems. Primitive green algae are thought to be the forerunners of modern green plants. All algae contain chlorophyll, and therefore produce energy by photosynthesis. Most live in water, and many also have flagella to help them move. Some algae are tiny, unicellular, and microscopic, while others (such as certain giant seaweeds) exist as multicellular organisms many feet in length.

The Protists: “First of All” The Algae: Plant-like Protists

Fig. 7.3 The amoeba and its “false feet.”

Unicellular algae include the plankton ( PLANK -ton). They are free-floating algae that seem to be “wandering” (plankton) through the water of ponds or lakes or oceans. The plankton form huge floating masses that produce great amounts of oxygen. They also serve as an important food source for many aquatic (ah- QUAT -ic) or “water”-dwelling organisms.

An especially beautiful type of plankton are the diatoms ( DYE -ah-tahms). The diatoms are single-celled microscopic algae with hard walls or testas ( TES -tahs). These silicon-hardened testas must literally be “cut through” (diatom) to see the soft, tiny bodies housed within. Diatoms occur as about 10,000 different species, each species being covered by a unique and geometrically shaped testa. As evident from a quick scan of Figure 7.4, the diatoms show an extremely high degree of elegant geometric patterns and Biological Order at the microscopic level.

The Protists: “First of All” The Algae: Plant-like Protists

Fig. 7.4 The beautiful geometric order of the diatoms.

While microscopic, unicellular plankton (including diatoms) float in the waters of the sea, another kind of algae, the giant kelps, are huge, multicellular brown seaweeds that can grow to over 150 feet in length! These mammoth brown algae often form extensive underwater kelp forests, providing both food and shelter for fish, sea otters, and other aquatic organisms. Surprisingly enough, however, the giant kelp do not have true stems, roots, or leaves. As multicellular algae, the giant kelp look somewhat like plants, but still have distinct differences from them. The kelp weed has leaf-like blades, which capture the energy in sunlight by conducting photosynthesis. Kelp is anchored to the ocean floor by root-like holdfasts.

The Slime Molds: Fungus-like Protists

The final major subgroup of the protists are the slime molds. Whereas the protozoa are considered the animal-like protists, and the algae are the plant-like protists, the slime molds are often nicknamed the fungus-like protists. The slime molds are thin, living masses of slimy material that use pseudopodia to move over damp soil, decaying leaves, and dead logs. Some of the 65 known species of slime molds live in fresh water. They receive their nutrients by phagocytosing organic matter from decaying plants. This feeding on decomposed material makes them similar to fungi, even though they move more like slimy groups of amoebas!

Practice problems for these concepts can be found at:  The Protists: "First of All" Test

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