Protists
Biologists are not certain how to classify Heterophrys, the freshwater protist shown in this micrograph. It harbors symbiotic photosynthetic algae called zoochorellae. Heterophrys is one of many protists called “heliozoans” (literally, “sun animals”) because of the thin pseudopods extending from its surface, giving it a sun-like appearance.
KEY CHARACTERISTICS
A protist is a eukaryote, generally single-celled, that does not fit into any of the other major taxonomic groups. The protists do not make up a true kingdom.
Organization Great diversity of cell organelles and organization: some have cell walls, some have chloroplasts, most have mitochondria or organelles related to mitochondria; those that are multicellular have relatively little differentiation into tissues.
Movement Some move by cilia or flagella.
Reproduction Most reproduce by cell division; many have sexual phases to their life cycle; some exchange genetic material by conjugation.
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The Kingdom That Isn't
The Challenges of Classifying Protists
Biologists traditionally classified protists by splitting them into funguslike, plantlike, and animal-like groups. This seemed to work for a while, but when they studied protists more carefully with new research tools, including genome-level molecular analysis, this traditional system simply fell apart.
Biologists now think that protists shouldn't be classified as a kingdom at all. In fact, when scientists look for the deepest and most fundamental divisions among eukaryotes, they find that all of those divisions are within the protists themselves, not between protists and other eukaryotes. Starting over, biologists could simply use those divisions to define newer, more accurate “kingdoms,” but that might cause new problems. For one thing, it would lump two of the traditional kingdoms (animals and fungi) together, and it would leave a handful of kingdoms that contain only unicellular organisms. There is no perfect solution to this problem. Here, “protists” are considered a kingdom for the sake of convenience, but keep in mind that their differences are really too great for any single kingdom to contain.
Table of Contents
- Formulas and Equations
- Applying Formulas and Equations
- Mean, Median, and Mode
- Estimation
- Using Measurements in Calculations
- Effects of Measurement Errors
- Accuracy
- Precision
- Comparing Accuracy and Precision
- Significant Figures
- Calculating With Significant Figures
- Scientific Notation
- Calculating With Scientific Notation
- Dimensional Analysis
- Applying Dimensional Analysis