Characteristics Of Algae

Many plantlike protists are known as algae. Unlike animal-like protists, which are heterotrophic, algae (singular, alga) are autotrophic protists. Algae have chloroplasts and produce their own carbohydrates by photosynthesis, as plants do. Older classification systems placed algae in kingdom Plantae. However, algae lack tissue differentiation and thus have no true roots, stems, or leaves. The reproductive structures of algae also differ from those of plants. Algae form gametes in single-celled gamete chambers called gametangia (GAM-uh-TAN-jee-uh). By contrast, plants form gametes in multicellular gametangia.

Despite their diversity, algae share several features. Algae have the photosynthetic pigment chlorophyll a. Most algae are aquatic and have flagella at some point in their life cycle. Algal cells often contain pyrenoids (pie-REE-NOYDZ), structures associated with algal chloroplasts that synthesize and store starch.

Algae have four basic body forms. Unicellular algae consist of a single cell. Most unicellular algae are free-living aquatic organisms and together are known as phytoplankton. Phytoplankton form the base of nearly all marine and freshwater food chains. Colonial algae, such as Volvox, consist of groups of individual cells acting in a coordinated manner. Some of the cells may become specialized, allowing for a division of labor. Filamentous algae, such as Spirogyra, are multicellular algae that are slender, rod shaped, and composed of cells joined end to end. Some filamentous algae have structures that anchor the organism to the ocean bottom. Multicellular algae, such as the kelp shown in Figure 25-8, are usually large and complex and often appear plantlike. Many large mul-ticellular algae are also known as seaweeds. The plantlike body portion of a seaweed is called a thallus (THAL-uhs), and its cells are usually haploid.

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