Originally, there were two subsections in the Protist (a/k/a Protoctista) kingdom: the Phylum_Protozoa (the animal-like protists) and the Division_Algae (the plant-like protists). Both groups were previously members of the animal and plant kingdoms, respectively. When you study zoology and botany, these groups will be included in the review of the animal and plant kingdoms. Since the early days of the Protist kingdom in the 1800’s a few more groups have been added to the kingdom, including the fungus-like (slime_molds and water_molds) protists.
The Protozoa ("first animals") are the animal-like (heterotrophic, motile) Protists and are classified (in part) based upon their method of motility (such as cilia, pseudopods, and flagella). Representatives of this group are primarily unicellular or colonial.
Use pages 18-23 and 32-38 in the Photo Atlas as a reference for your review of the Protists. Recall that the authors of your Photo Atlas do not use the same exact method of classification as the authors of your textbook. Hence, the water molds and most algae are placed in the Protist group by your text authors, but not the Photo Atlas authors.
The plant-like Protists are called algae. The main characteristics of algae include that they are aquatic, they range in size from unicellular to multicellular and they are photosynthetic. These organisms account for at least half of all photosynthetic organisms on Earth. One of the reasons they are not considered true plants is because they lack a waxy_cuticle. Another reason is that the algae have chlorophyll-A like the true plants, but differ in terms of other pigments. These organisms are ecologically important not just because of photosynthesis, but also because they are used either as food or in the preparation of some forms of food. The basis of their classification includes (1) type of pigment(s) and (2) type of storage product(s).
Most diatoms are unicellular and photosynthetic and are part of floating plankton. Their primary pigment is fucoxanthin: a carotenoid. Their bodies consist of two silica-reinforced cellulose plates called frustules that resemble a Petri_dish. Frustules are also called valves (silica_valves). The raphe is a fusion line between two valves. There are two basic body shapes for frustules:
(1) Radial, which is wheel-shaped (a/k/a centric) and
(2) Bilateral, which is needle-shaped (a/k/a pennate).
Diatoms reproduce asexually by lid separation in which each lid becomes the top of a new diatom. Therefore, body size decreases for half of the organisms in each generation. When a critically small size is reached, a sexual phase produces gametes that fuse, feed and mature to become a full-sized "covered-dish" type of structure. Their food reserves are stored as oils or carbohydrates.