BIOLOGI SEL

Comparing Prokaryotic and Eukaryotic Cens

All cells have several basic features in common: They are all bounded by a selective barrier, called the plasma membrane. Enclosed by the membrane is a semifluid, jellylike substance called cytosol, in which organelles and other components are found. All cells contain chromosomes, which carry genes in the form ofONA. And all cells have ribosomes, tiny complexes that make proteins according to instructions from the genes.

A major difference betv·:een prokaryotic and eukaryotic cells is the 10000tion of their DNA, as reflected in their names. In a eukaryotic cell, most of the DNA is in an organelle called the nucleus, which is bounded by a double membrane (see Figure 6.9, on pp. 100--101). (The word eukaryotic is from the Greek eu, true, and kar)'on, kernel, here referring to the nucleus.) In a prokaryotic cell (from the Greek pro, before, and karyon), the DNA is concentrated in a region that is not membraneenclosed, called the nucleoid (Figure 6.6). The interior of a prokaryotic cell is called the cytoplasm; this term is also used for the region between the nucleus and the plasma membrane of a eukaryotic cell. Within the cytoplasm ofa eukaryotic cell, suspended in cytosol, are a variety of organelles of specialized form and function. These membrane-bounded structures are absent in prokaryotic cells. Thus, the presence or absence ofa true nucleus is just one example of the disparity in structural complexity between the two types of cells.

Eukaryotic cells are generally much larger than prokaryotic cells (see Figure 6.2). Size is a general aspect of cell structure that relates to function. The logistics of carrying out cellular metabolism sets limits on cell size. At the lower limit, the smallest cells known are bacteria called mycoplasmas, which have diameters bern'een 0.1 and 1.0 J.lm. These are perhaps the smallest packages with enough DNA to program metabolism and enough enzymes and other cellular equipment to carry out the activities necessary for a cell to sustain itself and reproduce. Typical bacteria are 1-5 J.lm in diameter, a dimension about ten times greater than that of mycoplasmas. Eukaryotic cells are typically 10-100 J.lrn in diameter.

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