Eucaryotic vs. Procaryotic Cells
EUCARYOTIC CELLS
How complex cells are was not truly appreciated until the advent of the electron microscope and modern biochemical techniques, which combined to change our whole picture of the cell. Three or four dec¬ades ago, biology books still regularly included a diagram of a so-called typical cell that showed only five or six simple internal components as seen under compound binocular microscopes. No simple diagram of this sort can be given today. In the first place, there is no such thing as a typical cell, not even a typical eucaryotic cell. Not only do plant and animal cells differ in many important ways, but also the various cells of the body of any one plant or animal are often strikingly different from one another in shape, size, and func¬tion as seen under compound binocular microscopes. This much, of course, has been known for a long time. But now that the number of known cellular components has grown so large and their great variability has been so well demonstrated, it becomes even more obvious that no single diagram, or even series of diagrams, can really portray a “typical” cell.
Procaryotic Cells
Procaryotic cells, when viewed under compound binocular microscopes, lack most of the cytoplasmic organelles present in eucaryotic cells. They have no nuclear membrane; they also lack other membranous structures, such as an endoplasmic reticulum, a Golgi apparatus, lysosomes, and mitochon¬dria (many of the functions of mitochondria are carried out by the inner surface of the plasma membrane). There is an exception, how¬ever, to the general rule that procaryotic cells lack intracellular mem¬branous structures. All Cyanobacteria and the few photosynthetic bacteria contain chlorophyll, which is associated with membranous vesicles or lamellae, but the lamellae are not contained in membrane bounded plastids when this tissue was viewed under compound binocular microscopes.
For a long time, before the advent of the electron microscopes, it was thought that procaryotic cells had no chro¬mosomes. With the advent of the electron microscope, however, it became possible to detect in each procaryotic cell a nuclear region, or nucleoid, containing a single large DNA molecule, which, though not tightly associated with proteins as DNA is in eucaryotic cells, may nonetheless be considered a chromosome. Unlike eucaryotic chromosomes, which are usually linear, the procaryotic chromosome is ordinarily circular,
Like eucaryotic chromosomes, the procaryotic chromosome bears, in linear array, the genes that control both the hereditary traits of the cell and its ordinary activities. The genes determine the sequence in which amino acids will be incorporated into proteins on the ribosomes. Ribosomes, when discovered under a microscope, do occur in both eucaryotic and procaryotic cells. Those of procaryotic cells, however, aree structurally different from those of eucaryotic cells, and they are considerably smaller.
Some bacterial cells possess hair, which can be seen under compound binocular microscopes, like organelles used in swimming, and these have traditionally been called flagella. But since these organelles do not have microtubules, which do not exist in most pro¬caryotic cells, their internal structure is completely different from that of eucaryotic flagella and the mechanism of their movement is also different.
