1.4Prokaryotes and Eukaryotes
Although the general cell structure could be observed in the optical microscope era (see Appendix), finer cell structures were not well visualized. Although they could not be accurately distinguished, the following structures could be observed using a range of staining techniques: chloroplasts (easy to observe), mitochondria, Golgi apparatus, centrosome, the nucleus (pl. nuclei), and chromosomes. However, it was still very difficult to distinguish the intracellular structure of small cells using the optical microscope. Using the electron microscope (see Appendix) in the 1960s, it was found that cells of organisms are composed of many membranes. In the era of optical microscopy, it was possible to distinguish organisms in which the nucleus could be observed using staining from those in which this was not possible. The presence or absence of the nuclear membrane could be determined for the first time using the electron microscope, which allowed for close observation of membranes. As a result, all organisms could be broadly classified as eukaryotes or prokaryotes.
Figure 1-2 is an extremely simple schematic diagram of the cell. In addition to nuclear membrane-bound DNA, eukaryotic cells contain many membrane-bound organelles. New organelles were discovered in addition to the well-known chloroplasts, mitochondria, and Golgi apparatus, and each was found to perform specific functions. In contrast, DNA in prokaryotes is not enclosed in a nuclear membrane, and prokaryotes do not contain organelles. Although an established intracellular membrane system is present in photosynthesizing cyanobacteria, it is called the internal membrane system to differentiate it from other organelles in eukaryotes. While the differences between prokaryotes and eukaryotes may not yet be clear from previous descriptions, there are other extremely important differences that have been outlined below. Some of these differences include the mechanisms for gene expression, flow of substances, and sexual reproduction, which is a basic eukaryotic characteristic.