Laboratory Notes for BIO 1003
© 30 August 1999, John H. Wahlert & Mary Jean Holland
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.
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 propelleras 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.
The phylum Mastigophora includes free-living, flagellated protistans important in both marine and freshwater environments, and some parasites.
Examples of parasites: Trypanosoma brucei, responsible for African sleeping sickness, and Trypanosoma cruzi, which causes Chagas disease.
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.
Amoebas: Amoeba proteus lives in fresh water; Entamoeba histolytica is a parasite that causes amoebic dysentery. Use a dropper to remove some Amoeba specimens from the bottom of the sample jar (do not stir the water). Put a drop of culture on a slide; since Amoeba is a large protistan, do not use a cover slip and do not use high power. Observe the way that the cell membrane extends and the cell contents flow. The extensions are called pseudopodia (false feet) because what looks like a foot may disappear, as the cell moves into it, and another part of the membrane extend.
Foraminifera: (no high power) mostly marine. Hardened shells of calcium carbonate (CaCO3) have holes through which pseudopodia extend.
Radiolaria: marine. A skeleton of silica glass (Si02) supports the cell.
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.
Paramecium caudatum is a large-celled, free-living organism that is entirely covered with cilia. Collect some organisms from the bottom of the sample jar; if there is a food pellet, they will be near it. Mix them on a slide with a drop of methyl cellulose and gently float a cover slip on the fluid.
Vorticella, another ciliate, attaches to a surface by a thread-like extension that is capable of coiling suddenly; a band of cilia sweeps water and contained food to the feeding area of the cell.
Acineta, a ciliate with tentacles from our aquarium:
These are water molds; a familiar example is the cottony colonies (mycelia) on aquarium fish.
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