Principles of Biochemistry, Volume 1 - A. Lehninger 1985

Biomolecules
Cells
Eukaryotic cells are larger and more complex than prokaryotic cells

Typical Eukaryotic Cells are significantly larger than prokaryotes. For instance, hepatocytes—the primary Liver cells of higher animals—have a diameter of 20–30 µm, whereas bacterial diameters generally do not exceed 1–2 µm. However, the critical factor is not just the linear dimensions of a Cell, but its volume, which in most eukaryotic cells is 1,000 to 10,000 times greater than in Bacteria. To roughly visualize the relative volumes of eukaryotic and Prokaryotic Cells, one can use the formula for the volume of a sphere (Section 2.2). Some eukaryotic cells, such as an unfertilized chicken egg, are much larger than the dimensions given above, although almost the entire volume of such a cell is filled with nutrients required for embryo growth. There are also exceptionally long eukaryotic cells: for example, the length of individual human motor Neurons can exceed 1 m. Nevertheless, the most defining feature of eukaryotic cells is the presence of a well-defined, double-membrane-bound Nucleus with a complex internal Structure. Like prokaryotes, eukaryotic cells can also divide asexually, but this occurs via a significantly more complex process known as mitosis. The sex cells (Gametes) of eukaryotic organisms are capable of sophisticated sexual conjugation accompanied by Gene exchange.

Another vital distinction between eukaryotes and prokaryotes is that eukaryotic cells, In addition to an organized nucleus, contain a variety of other membrane-bound intracellular Organelles, such as Mitochondria, The Endoplasmic reticulum, and Golgi bodies. Each of these organelles performs specific Functions in cellular METABOLISM and physiology. Figure 2-7 illustrates a typical Introduction/5.html">Eukaryotic Cell—a rat liver cell—which exhibits an exceptionally complex internal architecture with a high degree of compartmentalization. As we will see later, eukaryotic cells are characterized by a finer division of labor among their numerous structural elements, each playing a specific role in cellular life.

All higher animals, plants, and Fungi consist exclusively of eukaryotic cells. Eukaryotes also include many unicellular organisms—various species of Protozoa, diatoms, euglenids, Yeasts, and slime Molds.

Because Eukaryotic cells contain significantly more genetic material and frequently undergo sexual conjugation involving gene exchange, eukaryotic life forms possess a much greater capacity for differentiation and specialization compared to prokaryotes. This explains why there are millions of distinct species of eukaryotic organisms, whereas the number of prokaryotic species does not exceed a few thousand. On the other hand, prokaryotic organisms are far more resilient to environmental changes and reproduce substantially faster, enabling them to survive in extremely harsh conditions.

Let us now examine the Structure and function of various components of eukaryotic cells in greater detail.

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Fig. 2-7. Transmission electron micrograph of a rat hepatocyte (the predominant type of liver cell). The Plasma Membrane, highlighted in red, features a large surface area and forms numerous folds. These finger-like projections of The Cell membrane, which substantially increase its surface area, are more clearly visible in the scanning electron micrograph of an isolated hepatocyte shown in Figure 2-20.