20.2 Prokaryotes
How are prokaryotes classified?- How do prokaryotes vary in their structure and function?
- What roles do prokaryotes play in the living world?
prokaryote
bacillus
coccus
spirillum
binary fission
endospore
conjugation
Preview Visuals Look at Figure 20–9. Describe in your own words the three shapes of prokaryotes shown.
For more on the diversity of Bacteria and Archaea, go to the Visual Guide. 
DOL•6–DOL•9.
THINK ABOUT IT Imagine living all your life as a member of what you believe is the only family on your street. Then, one morning, you open the front door and discover houses all around you. You see neighbors tending their gardens and children walking to school. Where did all the people come from? What if the answer turned out to be that they had always been there—you just hadn't seen them? How would your view of the world change? The sudden appearance of the new neighbors would be quite a shock.
When the microscope was first invented, we humans had just such a shock. Suddenly, the street was very crowded! Far from being alone, we share every corner of our world with microorganisms. Even something that seems clean, like a toothbrush, may be covered with a film of bacteria.
Classifying Prokaryotes
How are prokaryotes classified?
Microscopic life covers nearly every square centimeter of Earth. The smallest and most abundant of these microorganisms are prokaryotes (pro KAR ee ohts)—unicellular organisms that lack a nucleus. Prokaryotes have DNA, like all other cells, but their DNA is not found in a membrane-bound nuclear envelope as it is in eukaryotes. Prokaryote DNA is located in the cytoplasm. For many years, most prokaryotes were simply called “bacteria.” We now know, however, that the classification of prokaryotes is more complex.
Until recently, all prokaryotes were placed in a single kingdom. More recently, however, biologists have divided prokaryotes into two very distinct groups: Bacteria and Archaea. These groups are as different from each other as both are from eukaryotes. Therefore, biologists now consider each group of prokaryotes as a separate domain. Prokaryotes are classified as Bacteria or Archaea—two of the three domains of life. Eukarya is the third domain. The domain Bacteria corresponds to the kingdom Eubacteria. The domain Archaea corresponds to the kingdom Archaebacteria.
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