Overview Cells and Cellular Compartments

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1.1— Overview: Cells and Cellular Compartments
Over three billion years ago, under conditions not entirely clear and in a time span difficult to comprehend, elements such as carbon, hydrogen, oxygen, nitrogen, sulfur, and phosphorus formed simple chemical compounds. They combined, dispersed, and recombined to form a variety of larger molecules until a combination was achieved that was capable of replicating itself. These macromolecules consisted of simpler molecules linked together by chemical bonds. With continued evolution and formation of ever more complex molecules, the water environment around some of these self­replicating molecules became enclosed by a membrane. This development gave these primordial structures the ability to control their own environment to some extent. A form of life had evolved and a unit of three­dimensional space—a cell—had been established. With the passing of time a diversity of cells evolved, and their chemistry and structure became more complex. They could extract nutrients from the environment, chemically converting these nutrients to sources of energy or to complex molecules, control chemical processes that they catalyzed, and carry out cellular replication. Thus the vast diversity of life observed today began. The cell is the basic unit of life in all forms of living organisms, from the smallest bacterium to the most complex animal.
The limiting outer membrane of cells, the plasma membrane, delineates the space occupied by a cell and separates the variable and potentially hostile environment outside from the relatively constant milieu within. It is the communication link between the cell and its surroundings.
On the basis of microscopic and biochemical differences, living cells are divided into two major classes: prokaryotes, which include bacteria, blue­green algae, and rickettsiae, and eukaryotes, which include yeasts, fungi, and plant and animal cells. Prokaryotes have a variety of shapes and sizes, in most cases being 1/1000 to 1/10,000 the size of eukaryotic cells. They lack intracellular membrane­bound structures that can be visualized by a microscope (Figure 1.1). The deoxyribonucleic acid (DNA) of prokaryotes is often segregated into a discrete mass, the nucleoid region, that is not surrounded by a membrane or envelope. The plasma membrane is often invaginated. In contrast, eukaryotic cells have a well­defined membrane surrounding a central nucleus and a variety of intracellular structures and organelles (Figure 1.1b). Intracellular membrane systems establish distinct subcellular compartments, as described in Section 1.4, that permit a unique degree of subcellular specialization. By compartmentalization different chemical reactions that require different environments can occur simultaneously. Many reactions occur in or on specific membranes, thus creating an additional environment for the diverse functions of cells.
Besides these structural variations between prokaryotic and eukaryotic cells (Figures 1a and 1b), there are differences in chemical composition and biochemical activities. Prokaryotes lack histones, a class of proteins that complex with DNA in eukaryotes. There are major structural differences in the ribonucleic acid–protein complexes involved in biosynthesis of proteins between the cell types, as well as differences in transport mechanisms across the plasma membrane and in enzyme content. The many similarities, however, are equally striking. The emphasis throughout this book is on the chemistry of eukaryotes, particularly mammalian cells, but much of our knowledge of the biochemistry of living cells has come from studies of prokaryotic and nonmammalian eukaryotic cells. The basic chemical components and fundamental chemical reactions of all living cells are very similar. Availability of certain cell populations, for example, bacteria in contrast to human liver, has led to much of our knowledge about some cells; in some areas our knowledge is derived nearly exclusively from studies of prokaryotes. The universality of many biochemical phenomena, however, permits many extrapolations from bacteria to humans.
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Figure 1.1 Cellular organization of prokaryotic and eukaryotic cells. (a) Electron micrograph of Escherichia coli, a representative prokaryote; approximate magnification ×30,000. There is little apparent intracellular organization and no cytoplasmic organelles. Chromatin is condensed in a nuclear zone but not surrounded by a membrane. Prokaryotic cells are much smaller than eukaryotic cells. (b) Electron micrograph of a thin section of a liver cell (rat hepatocyte), a representative eukaryotic cell; approximate magnification ×7500. Note the distinct nuclear membrane, different membrane­bound organelles or vesicles, and extensive membrane systems. Various membranes create a variety of intracellular compartments. Photograph (a) generously supplied by Dr. M. E. Bayer, Fox Chase Cancer Institute, Philadelphia, PA; photograph (b) reprinted with permission of Dr. K. R. Porter, from Porter, K. R., and Bonneville, M. A. In: Fine Structure of Cells and Tissues. Philadelphia: Lea & Febiger, 1972.