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Cell divides into two cells and the DNA is partitioned into each future daughter cell "

Cell divides into two cells and the DNA is partitioned into each future daughter cell "

Daughter cells

Figure 4.3 Binary Fission The chromosomal DNA is attached to the cytoplasmic membrane. As the membrane increases in length, the DNA is replicated and then partitioned into each of the two daughter cells.

Daughter cells

Figure 4.3 Binary Fission The chromosomal DNA is attached to the cytoplasmic membrane. As the membrane increases in length, the DNA is replicated and then partitioned into each of the two daughter cells.

fortable. These are the organisms we associate with disease and food spoilage. Some prokaryotes live in harsh environments that would kill most other organisms. Most of these, called extremophiles, are members of the Domain Archaea.

Recognizing environmental factors that influence micro-bial growth—such as temperature, amount of oxygen, pH, and water availability—helps scientists study microorganisms in the laboratory and aids in understanding their role in the complex ecology of the planet. The major environmental conditions that influence the growth of microorganisms are summarized in table 4.1.

Temperature Requirements

Each species of prokaryote has a well-defined upper and lower temperature limit within which it grows and outside of which growth stops. The temperature span between these limits is usually about 25°C. Within this range lies the optimum growth temperature, the temperature at which the organism multiplies most rapidly. As a general rule, this optimum temperature is close to the upper limit of the organism's range. This is because the speed of enzymatic reactions in the cell approximately doubles for each 10°C rise in temperature. At a critical point, however, the temperature becomes too high and enzymes required for growth are denatured and can no longer function. As a result, the cells die.

Prokaryotes are commonly divided into five groups based on their optimum growth temperatures (figure 4.4). Note, however, that this merely represents a convenient organization scheme. In reality, there is no sharp dividing line between each group. Furthermore, not every organism in a group can grow in the entire temperature range typical for its group.

■ Psychrophiles have their optimum between -5°C and

15°C. These organisms are usually found in such

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