BIOCHEMISTRY FOR TEACHERS - F.F. BOYECHKO - 1985

THE FORMATION OF LIVING SYSTEMS AND THEIR STRUCTURAL ORGANIZATION

THE CELL — THE FOUNDATION OF THE STRUCTURE OF LIVING SYSTEMS

The Cell is the fundamental Structural and functional unit of all living systems—ranging from the simplest unicellular organisms to highly organized Multicellular animals and plants.

There is no single cell in nature that can be considered typical of all living systems. The Cells that make up animal and plant organisms vary in size, shape, origin, degree of Organization, and function, although they all share a common structural plan. This shared blueprint is undeniable proof of the common ancestry of All living organisms inhabiting our planet.

Unicellular organisms, as well as cells that comprise various Organs and systems in Multicellular Organisms, are striking in their morphological diversity. They can be spherical, elongated, cylindrical, cubic, prismatic, disc-shaped, or stellate. The shape of each cell is determined by the Genetic information encoded in its DNA and, for unicellular organisms, it is often the primary criterion for their Classification. Unicellular desmid Algae are particularly notable for the wide variety and beauty of their forms.

Cells of varying shapes are also found in multicellular organisms, where The Unity of form and function is especially pronounced. Cells of the Skin, Epithelial Tissues, and nerve and Muscle fibers exhibit distinct shapes tailored to their specific Functions. Muscle and nerve fiber cells are predominantly elongated, intestinal epithelial cells are cylindrical, and Blood Cells are rounded and disc-shaped.

The shape of plant cells depends on their Location and function. For instance, the cells of stems and leaves differ significantly from one another. Cell sizes vary widely, both within a single Organism and across different species. For example, human ovum cells, measuring 0.1 mm, are more than a million times larger than spermatozoa. Ostrich eggs are 1·10⁶—1·10⁷ times larger than pneumococci. Cells of integumentary and epithelial tissues are characterized by their small size—around 30 µm in diameter—while small Blood Leukocytes are even smaller at 3—4 µm. Conversely, the dimensions of nerve fiber cells are quite substantial, with neuronal axons occasionally reaching 1 m in length. Among unicellular organisms, the smallest are cells of the microorganism Mycoplasma, which are comparable in size to Viruses, yet similar to Bacteria in their structural organization. The most common cell size ranges from 10 to 100 µm, while 1—10 µm cells are less frequent (such as the pulp cells of watermelons and citrus fruits, or the glandular cells of Mollusks).

It is believed that a certain correlation exists between Cell size and The rate of metabolic processes occurring within them. The smaller the cell, the more intensive its metabolic reactions. Such a relationship also holds true for multicellular organisms. In hummingbirds, bees, and flies, the rate of metabolic processes is significantly higher than in humans and vertebrate animals. If the metabolic intensity in The Human Body were as high as that of a hummingbird, a person would quickly perish due to excessive heat generation.

The number of cells in multicellular organisms also varies greatly. In invertebrates, their number averages 1·10²—1·10⁴, whereas in highly organized vertebrates it reaches 1·10¹⁵—1·10¹⁷. Despite this diversity, All cells share common features: a uniformity in Chemical Composition and the chemical processes occurring within them, The ability to capture and transform energy, and the Synthesis of specific macromolecules. The structural Building Blocks of various cells exhibit similarities across atomic and molecular levels. All cells contain biogenic elements—carbon, hydrogen, oxygen, nitrogen, and sulfur—along with a series of Macronutrients (K, Na, Ca, P, Fe, etc.) and micronutrients (Zn, Mn, Cu, Co, I, etc.). Regardless of their level of organization, all cells contain such vital Biopolymers as Proteins, Nucleic Acids, and Enzymes, as well as Introduction/36.html">CARBOHYDRATES and Lipids. The vital activity of any cell depends on two interacting systems: one responsible for cell growth, reproduction, and development, and an energy-supply system that fuels the various metabolic processes within the cell. Both systems function in close coordination. Living cells possess the ability to absorb Water and nutrients from their environment to sustain their life processes, and to excrete Metabolic waste products—in other words, they are characterized by active METABOLISM with their surroundings.

Class="center">Table 1. Structure/83.html">Structural elements of the cell

Depending on their degree of organization, all cells are divided into prokaryotic and eukaryotic. The former are typical predominantly of unicellular organisms—bacteria and blue-green algae—while the latter characterize all other unicellular and multicellular organisms. In addition to shared features, prokaryotic and Eukaryotic cells possess A number of structural and functional differences that set them apart. Table 1 outlines the main structural elements of prokaryotic and eukaryotic cells.

General characteristics of Prokaryotic Cells

Prokaryotic cells are the simplest of all known living cells. They are evidently the first cells to have emerged 3–3.5 billion years ago. The vast majority of them are small in size, with an average dimension of about 5 µm; however, some prokaryotes measure 0.1—0.25 µm, making them the smallest of all known cells. Bacterial cells and blue-green algae cells are slightly larger.

Prokaryotic cells feature an outer plasma membrane, which in bacteria and blue-green algae is covered by a Cell wall and a gelatinous mucous capsule or sheath. The composition of this envelope varies among prokaryotes and likely determines the pathogenic properties of certain strains. Specifically, bacterial Cell walls contain lipids, carbohydrates, and complex proteins, whereas Cellulose is the primary component in blue-green algae.

The internal contents of a Prokaryotic Cell consist of a nuclear region with low electron density and a very dense Cytoplasm. The cell lacks a distinct Nucleus and any membrane-bound organelles, save for invaginations of the cytoplasmic membrane. Instead of a true nucleus, prokaryotes contain its genetic equivalent (nucleoid), which is diffusely dispersed in the cytoplasm or formed by intertwined spirals of DNA molecules that, in some bacteria, take the form of tiny Plasmids resembling the extranuclear DNA of eukaryotes. Blue-green algae and certain bacteria contain a small number of layered membranes formed by the infolding of The Plasma Membrane. Some of these—thylakoids—function analogously to eukaryotic Plastids in photosynthetic prokaryotic cells of certain algae and bacteria.

The cytoplasm of prokaryotic cells contains small round bodies called ribosomes, as well as various non-membrane-bound paraplasmic inclusions (granules) that perform diverse functions. Some of these granules participate in the storage of reserve substances. These include volutin grains, or polyphosphate granules, which accumulate phosphates, and cyanophycin granules—a reserve polypeptide composed of Arginine and aspartic acid residues. Lipid and Glycogen granules are also present. Secretory granules, which facilitate the release and uptake of various substances, play an important role in prokaryotic cells as well. There are no other inclusions in the prokaryotic cytoplasm. Despite their relatively simple internal structure, prokaryotic cells are capable of carrying out complex biochemical reactions that ensure their independent survival in their respective environments.

Under unfavorable conditions, most prokaryotes are capable of sporulation through The formation of a tough protective coat around the nuclear region. In this state, prokaryotes exhibit high resistance to various environmental factors and may show no signs of metabolic activity for extended periods. As spores, they can withstand high temperatures of 100—120 °C. Under normal conditions, blue-green algae thrive quite well at temperatures up to 70 °C. Indeed, they create the distinct color palettes and hues characteristic of hot springs and geysers.

General characteristics of eukaryotic cells

The Structural organization of eukaryotic cells is much more complex than that of prokaryotic cells. They are larger in size (~15 µm). Externally, the cell is enclosed by a plasma membrane, and in plant cells, it is additionally encased in a rigid cell wall composed of cellulose fibers.

The interior of the cell contains cytoplasm with an intricate system of internal membranes that partition it into distinct compartments. Among the membranous systems, the Endoplasmic reticulum and the Golgi apparatus occupy a special place, participating in the synthesis and packaging of substances necessary for the construction and functioning of both the cells and the organism as a whole. Furthermore, the cytoplasm of eukaryotic cells contains various organelles—such as The Nucleus, mitochondria, plastids, vacuoles, lysosomes, Peroxisomes, and ribosomes—that perform diverse functions within the cell.

Table 2. Types of eukaryotic cells

All cell organelles are embedded in the matrix—the portion of the cytoplasm that appears homogeneous even under an Electron microscope. Three types of organelles—the nucleus, mitochondria, and plastids—are separated from the cytoplasm by double membranes. Membranes are also present in other cell organelles, with the exception of chromosomes, ribosomes, microtubules, and microfilaments. Each type of organelle performs a specific functional role.

Among eukaryotic cells, a distinction is made between plant, animal, and fungal cells, which differ in their structure (Table 2). As can be seen, plant cells differ from animal cells in having a thick cellulose cell wall and certain types of organelles. The main structural components are universal and characteristic of both PLANT AND ANIMAL cells.