General Microbiology - Schlegel H. 1987

The Cell and Its Structure
Eukaryotic Cell (Eucyte)

The first insights into the general Organization and Fine Structure of The Cell were obtained using optical Methods. As optical instruments improved and Microscopy techniques advanced, our knowledge of cell micromorphology and its individual components expanded accordingly. Further developments in light microscopy, as well as ultraviolet and Electron microscopy, significantly enhanced the resolving power of optical instruments; dark-field and Phase-contrast microscopy greatly facilitated the observation of living Cells. To this day, microscopy—particularly electron microscopy—combined with sophisticated preliminary preparation of biological material, undoubtedly remains an essential research method. For deeper insights at THE MOLECULAR LEVEL, researchers must resort to indirect Physical and Chemical methods, which have made it possible to isolate and study individual cellular components.

Class="center">

Fig. 2.1. Combined diagram of The structure of a eukaryotic (plant) cell (after Sitte). Вак — vacuoles; Д — dictyosomes; КСт — Cell wall; Ли — lipid droplets; Мит — Mitochondria; Мтр — microtubules; П — pores with plasmodesmata; ПМ — Plasma Membrane; СП — secretory vesicles (exocytosis); Хл — METABOLISM/14.html">Chloroplasts; ЦПл — Cytoplasm; Я — Nucleus.

Using differential centrifugation of disrupted cells (homogenates), individual Organelles and fractions were isolated, enabling their study via Biochemical Methods. Thanks to a combination of optical and biochemical techniques, the STRUCTURE AND Functions of organelles and other cellular constituents were rapidly elucidated. These studies established that eukaryotes and prokaryotes differ significantly in many of their features.

Every cell consists of cytoplasm and nuclear material, enclosed externally by The Plasma Membrane. This protoplast may be further surrounded by a cell wall, which primarily performs mechanical functions; such a wall is present in PLANT CELLS AND the cells of most Bacteria.

We will briefly examine the Main Features of eukaryotic and Prokaryotic Cells (eucytes and protocytes). An embryonic plant cell serves as a typical representative of eucytes (Fig. 2.1).

The Cell Nucleus. The STRUCTURE OF THE nucleus and its mode of division are the most crucial and characteristic features distinguishing The Introduction/5.html">Eukaryotic Cell (Fig. 2.2) from the prokaryotic one. The (interphase) nucleus is surrounded by a nuclear envelope, which is a double-layered, perforated membrane. DNA carrying Genetic information is distributed among individual subunit Chromosomes, which become visible only during nuclear division. The Nucleus divides by mitosis (Fig. 2.2); mitosis ensures 1) the identical reduplication of genetic material (visibly manifested in the longitudinal splitting of chromosomes and the doubling of their number) and 2) the delivery of a complete set of chromosomes to each daughter nucleus. The exact mechanism of chromosome duplication is not yet fully understood. The distribution of chromosomes can be tracked using a Light Microscope and has therefore been known for a long time. While the interphase nucleus appears devoid of any distinct structure under a light microscope, during division the chromosomes shorten and become visible. They then align in a single plane, forming the equatorial plate. As the spindle fibers shorten, the halves of the longitudinally split chromosomes pull apart; the spindle disappears, the chromosomes become invisible, and the daughter nuclei are once again surrounded by nuclear envelopes.

Fig. 2.2. Diagram of mitotic division in a diploid cell. The nucleus contains two chromosomes of each type inherited from the parents (shown as red and black in the diagram). As a result of mitosis, chromosomes are distributed equally between both daughter cells. A. In prophase, the longitudinally split chromosomes become visible; the nuclear envelope breaks down. B. In metaphase, chromosomes align in the equatorial plane. C. In anaphase, the halves of the split chromosomes are pulled by spindle fibers toward opposite poles. D. In telophase, the newly split daughter chromosomes are surrounded by the nuclear membrane, after which the nucleus enters the interphase state.

In all Higher Plants and animals, sexual reproduction involves an alternation of nuclear phases. During Fertilization, sex cells (Gametes) and their nuclei fuse to form a zygote. The paternal and maternal nuclei contribute an equal number of chromosomes (n) during fertilization; thus, the zygote nucleus contains a double set of chromosomes (2n). In other words, gametes are haploid cells (i.e., cells with a single set of chromosomes), whereas somatic cells are diploid (with two sets). Consequently, during The formation of the next generation's gametes, the chromosome number in the cell (2n) must be halved (2n/2 = n). The set of processes leading to the reduction of the chromosome number is called Meiosis or reductional division (Fig. 2.3). Meiosis is a vital process in sexually reproducing organisms, leading to two main outcomes: 1) the recombination of paternal and maternal hereditary factors (genes) and 2) the reduction of the chromosome number. Meiosis begins with chromosome conjugation—each chromosome pairs with its corresponding (homologous) chromosome inherited from the other parent. During conjugation, fragments of equal length can be exchanged between homologous chromosomes through breakage and cross-joining (Crossing Over). This is followed by a twofold Separation of the paired, split chromosomes, resulting in four cells, each containing a haploid nucleus. Thus, meiosis not only "shuffles" maternal and paternal chromosomes but may also involve the exchange of segments between homologous chromosomes. Both processes lead to novel Gene combinations (recombination).

Fig. 2.3. Diagram of meiosis. Meiosis leads to the recombination of paternal and maternal genes and a twofold reduction in chromosome number. First, homologous chromosomes conjugate (A), and homologs exchange segments (crossing over); as a result of a twofold spindle formation (B and C), the chromosomes separate, during which homologous chromosomes also segregate (C); this yields four cells with haploid nuclei (D).

In many lower plants, including Algae, as well as in Protozoa, chromosome reduction occurs immediately after zygote formation, rendering the Organism haploid. In plants with heterophasic (antithetic) Morphology/12.html">ALTERNATION OF GENERATIONS (mosses, ferns), haploid generations alternate with diploid ones.

Eukaryotic chromosomes consist of DNA strands associated with numerous Proteins. Some of these proteins are Histones, which are basic proteins. DNA and histones appear to be associated in a highly ordered manner, forming nucleosomes—the structural subunits of chromosomes.

mRNA is synthesized on chromosomal DNA within the nucleus and transported into the cytoplasm through Pores in the nuclear envelope. A nucleolus is visible in the interphase nucleus; it contains nucleolar DNA, which holds The Genetic Code for ribosomal RNA and, likely, Transfer RNA. Both of these RNAs are synthesized in the nucleolus and also pass into the cytoplasm. Embryonic cells and egg cells contain several or even many nucleoli.

In the eukaryotic cell, the nucleus is the primary, but not the only, bearer of genetic information. Part of this information is contained within the DNA of Mitochondria and chloroplasts.

The Cytoplasm. The protoplast is enclosed externally by the plasma membrane. A eukaryotic cell is characterized by a pronounced subdivision of the cytoplasm into numerous isolated compartments. This compartmentalization is partly achieved through invaginations of the plasma membrane that form cisternae and vesicles; however, eucyte cytoplasm also contains mitochondria and (in plants) chloroplasts, which are entirely surrounded by membranes.

Invaginations of the plasma membrane give rise to The Endoplasmic reticulum (ER). Part of the ER forms the outer nuclear membrane, thereby surrounding the nucleus; the nuclear envelope features pores that ensure the unobstructed transport of Nucleic Acids, proteins, and metabolites between the nuclear space and the cytoplasm. Some of these membranes are studded with tiny granules called Ribosomes; this constitutes the so-called "rough" or granular ER. Protein Synthesis takes place on the ribosomes. Ribosomes freely suspended in the cytoplasm or attached to the ER belong to the 80S type.

A specialized membrane organelle in animal cells is the Golgi apparatus. Similar organelles in plant cells are called dictyosomes. They consist of stacks of flattened membrane vesicles known as cisternae. The Golgi apparatus and dictyosomes facilitate the secretion of various products, primarily Enzymes. Enzymes are synthesized on the cisternae and accumulate within them. Over time, such a vesicle detaches, migrates to the plasma membrane, fuses with it, and releases its contents outward. This process is known as exocytosis.

Mitochondria and Chloroplasts. Eukaryotic cells contain two additional Types of Membrane-bound organelles: mitochondria and chloroplasts. Mitochondria carry out cellular Respiration; these variable-shaped, lipid-rich structures feature a double membrane—an outer membrane and a heavily folded inner membrane (forming cristae or tubules). The inner membrane contains the Components of the Electron Transport Chain and ATP synthase. In the cells of algae and higher plants, chloroplasts are present alongside mitochondria. The internal membranes of chloroplasts (thylakoids) contain Photosynthetic Pigments and components of the photosynthetic electron transport chain.

Endocytosis. Eukaryotic cells are characterized by The ability to engulf nutrients in the form of dissolved substances or discrete solid particles. A well-known example of solid particle uptake is phagocytosis—the engulfment of particles by Blood Leukocytes or amoebas. The uptake of liquid nutrients is referred to as pinocytosis. Both modes of extracellular material uptake are collectively termed endocytosis.

The ability of eukaryotes to engulf discrete solid particles, including living cells, is of fundamental biological significance. Endocytosis provides both the prerequisite and the mechanism for THE ORIGIN OF endosymbiosis. Typically, solid particles engulfed via phagocytosis by an amoeba are digested and completely lysed. In some cases, however, the result can be intracellular Symbiosis. The best-known example of such endosymbiosis is the association of legume roots with bacteria of the genus Rhizobium within ROOT nodules (sec. 13.1). Endosymbionts of this kind are widespread among eukaryotes (sec. 17.2.1). The capacity of eukaryotic cells to acquire endosymbionts Supports The Theory of the symbiotic origin of chloroplasts and mitochondria. Prokaryotes are incapable of endocytosis.

The Endosymbiotic Hypothesis. Eukaryotic cell organelles share many fundamental features with prokaryotic cells. They contain circular DNA molecules, their ribosomes are of the 70S type, and their membranes contain components of The electron transport chain (flavins, Quinones, Fe-S proteins, Cytochromes) performing respiratory or photosynthetic energy conversion. According to the symbiotic hypothesis, mitochondria originated from color-free aerobic bacteria, and chloroplasts from cyanobacteria that became endosymbionts of primitive eukaryotic cells. Subsequent evolution involved extensive specialization, with ATP regeneration function being delegated to cell organelles. The outer membrane of a eukaryotic cell does not contain electron transport chain components. On the other hand, cellular organelles are not entirely autonomous either; while they possess their own DNA molecules, a significant portion of the information required for the synthesis of their proteins resides within the cell nucleus. An example is ribulose bisphosphate carboxylase, a key enzyme for autotrophic CO₂ fixation in green plants. It consists of 8 large and 8 small subunits. The genetic code for the large subunits is contained within the chloroplast DNA itself, whereas the Synthesis of the small subunits is directed by the nucleus. Consequently, it is impossible to sustain organelle development outside the cell, rendering the endosymbiotic hypothesis inaccessible to direct verification.

Motile Organelles. All eukaryotic cells possessing flagella or cilia (in protozoa, algae, spermatozoa, and ciliated epithelial cells) share an identical internal structure. A cross-section reveals nine peripheral double fibrils and two central single fibrils (the "9 + 2" structure). Externally, this entire system is coated with the plasma membrane. The Base of the flagellum is anchored in the outer layer of the cytoplasm by a basal body, or blepharoplast. The blepharoplast is a derivative of a self-replicating organelle (the centriole).