The Protista are the most complex single cells that you will see in this course. Each organism is just one cell, and it has to perform all the functions of the many organs of a multicellular organism. List some of these functions. All are aquatic: some primarily marine and some primarily fresh water; some live in the tissue fluids of other organisms. Thousands of species have been described, but the actual number is much greater since the diversity of tropical members is not well studied. Size of unicellular organisms is limited. When a cell triples in volume, its need for nutrients and oxygen also triple, but membrane surface area, which these molecular substances must cross, does not even double.

Phylum Euglenophyta

Euglena lives in freshwater. Mix a drop of Euglena culture with a drop of methyl cellulose on a microscope slide and float a cover slip onto the fluid. Methyl cellulose is a soluble thickening agent that slows the organism down so you have time to examine it. Elongated cells contain chloroplasts; there is an orange light-sensitive spot near the flagellum. Examine live Euglena and consider whether the flagellum is at the front end and pulls the cell through the water (position like an airplane propeller) or is at the back end and pushes the cell through the water (position like a boat propeller—as in mammalian sperm).

Euglena is covered with flexible, helical strips of proteinaceous material; together with the plasma membrane, this is called a pellicle. The contortions that the cell can make are called euglenoid motion. If Euglena gracilis is kept in the dark, the chloroplasts regress, and the organisms turn white and become entirely dependent on external food suplies for growth; reillumination turns them green again within a few hours.

Phylum Chrysophyta, diatoms

Diatoms live in marine and freshwater. The cell walls of diatoms are composed primarily of opaline silica (SiO2 and associated water). The wall is like a box with one side fitting into the other, which is larger; the shells have holes that facilitate molecular transport. Some shells have slits through which part of the cell extends. Diatoms are photosynthetic; yellow pigments (xanthophylls and beta carotene) mask the green color of the chlorophyll. Diatoms are important as a food source and as oxygen producers.

Examples of diatoms.

Phylum Mastigophora

The phylum Mastigophora includes free-living, flagellated protistans important in both marine and freshwater environments, and some parasites.

Phylum Sarcodina

The Sarcodina are amoeboid protistans that live in freshwater, marine water and soil. Adults move and capture prey with transitory extensions of the cell called pseudopodia.

Left: live Amoeba proteus (100 x). Right: fixed, stained Amoeba with diatoms, nucleus (stained dark pink).

Phylum Ciliophora

These ciliated protistans are freshwater and marine. Cilia are structurally similar to flagella but are short and numerous; their movement is synchronized to row a cell through the medium, or to move the medium over the cell, as in feeding.

Two views of live Paramecium multimicronucleatum.

Fixed, stained slide of Paramecium.

Phylum Oomycota

These are water molds; a familiar example is the cottony colonies (mycelia) on aquarium fish.

Return to index.