Biochemical Engineering Fundamentals, Part 1 - Bailey J., Ollis D. 1989

Introduction to Microbiology
Cell Structure
Eukaryotic Cells

The second major Cell type is The Introduction/5.html">Eukaryotic Cell. Eukaryotic Cells are defined as those whose Nucleus is enclosed within a membrane. As a rule, a eukaryotic cell is 1,000 to 10,000 times larger in volume than a prokaryotic one. All cells of higher organisms belong to this type. Eukaryotic cells exhibit A wide variety of shapes, which is necessary, in particular, to perform various specialized Functions. Within higher organisms, these cells coexist and interact with each other in various ways, and therefore do not require the biochemical flexibility and adaptability so essential for prokaryotes. Eukaryotes also include many important species of microorganisms. In the next section, we will provide several Examples of Unicellular Eukaryotes.

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FIG. 1.3. A typical eukaryotic cell. Such an idealized cell with the indicated generalized Structure does not exist in nature; in reality, eukaryotes vary significantly in their Organization. Nevertheless, certain common features and structural elements are characteristic of many eukaryotes, making METABOLISM/2.html">THE CONCEPT OF a typical eukaryote convenient and useful in many cases.

As shown in Figs. 1.3 and 1.4, eukaryotes far exceed Prokaryotic Cells in The complexity of their internal structure. Eukaryotes are characterized by a high degree of spatial organization and differentiation of individual elements of the cellular structure. The internal volume of The Cell is divided into A number of clearly defined structural components, which we will discuss in more detail later; each of these components has its own Structure and function necessary for the normal life of the entire cell. Here, we will discuss only the most basic structural details of eukaryotic cells.

The cell is surrounded by a Plasma Membrane, similar to that of prokaryotes. Externally, this membrane may be protected by a Cell wall or envelope. The Nature of other outer cell structures depends on the cell type. Thus, the cells of higher animals are typically surrounded by a thin coat, whose specific adhesive properties are essential for cell-to-cell binding and the subsequent formation of specialized Tissues and Organs (e.g., the Liver). Plant cells, by contrast, are usually surrounded by a very thick and rigid wall. The walls of dead tree cells constitute the primary component of wood.

FIG. 1.4. Electron micrograph of a rat liver cell (x11,000). (Photograph courtesy of G. E. Palade, Yale University.)

Intracellular membranes play a major role in the specialization of various Structural elements of eukaryotes. An intricate membrane system, called The Endoplasmic reticulum, extends from The cell membrane into the interior of the cell. Eukaryotic nuclei are surrounded by porous membranes. Ribosomes—the centers of Protein Synthesis, as already mentioned in the section on prokaryotes—are attached to The surface of most elements of the endoplasmic reticulum. Prokaryotic ribosomes, however, are somewhat smaller than those of eukaryotes.

The primary function of The eukaryotic nucleus is to control and regulate the catalytic activity of ribosomes, with chemical messengers released by The Nucleus (messenger and other RNAs) not only regulating reaction rates but also determining The sequence of amino acid assembly during protein synthesis.

FIG. 1.5. Electron micrograph of the eukaryotic alga Chlamydomonas reinhardtii (x13,000). The photograph shows Chloroplasts (c), wall (w), nucleus (n) and nucleolus (no), vacuoles (v), and Golgi complex (g). [Reproduced from: Goodenough U. W., Porter K. R., J. Cell Biol., 38, 403 (1968)].

The nucleus is one of the membrane-bound structural elements of the cell. These specialized, membrane-enclosed structures are generally called Organelles. Mitochondria are organelles with an extremely specialized and highly ordered structure; they catalyze reactions that serve as the primary source of cellular energy. They are found in all eukaryotic cells that consume oxygen during energy generation. In phototrophic cells, which use light as their primary energy source, other organelles—chloroplasts—act as the main energy generators (Fig. 1.5). In addition to providing cells with energy, chloroplasts and mitochondria perform many other biochemically important functions.

Figures 1.3–1.5 also depict other organelles—the Golgi complex (Golgi apparatus), Lysosomes, and vacuoles. In the most general terms, their functions are to carry out certain Chemical Reactions and to compartmentalize (i.e., restrict to specific areas of the cell) various compounds, ensuring their isolation from the rest of the Cytoplasm. Compartmentalization processes are important both for reaction efficiency and for preventing undesirable interactions between the Contents of the organelles and other cellular components.

The Discovery of the above-described types of organelles in a wide variety of eukaryotes has provided a new perspective on the fundamental advantages of the Cell Theory. Now, various aspects of cellular activity can be viewed as the sum of processes occurring within organelles, each of which, in turn, can be studied individually. Organelles of the same type are believed to perform similar operations and functions regardless of the Nature of the cells to which they belong; so far, no exceptions to this rule have been found.

Thus, the primary approach to studying the cell lies in determining The chemical composition, structure, and biochemical activity of its organelles. Most of the currently available data on cell biochemistry has been obtained in this manner. Therefore, in the next section, we will briefly review centrifugation Methods, which are widely used to isolate cellular components.