Fundamentals of Molecular Biology. Part 1: Cell Molecular Biology - A. N. Ogurtsov 2011

Fundamentals of Cell Theory
Cellular Organization

Cell sizes range from 0.1–0.25 µm (for instance, Cells of the bacterium Escherichia coli are rod-shaped, 2 µm high and 0.8 µm in diameter) to 155 mm (a shelled ostrich egg). The average diameter is 10–20 µm for animal cells and 30–40 µm for plant cells. Such dimensions are the result of certain physical constraints.

On the one hand, to sustain its own metabolic processes, a cell must house a complete set of essential macromolecules; therefore, cells cannot be excessively small.

On the other hand, an increase in cell size is limited by diffusion, which mediates the exchange of substances with the environment and directly constrains The rate of biochemical reactions within The Cell. Consequently, cells cannot be indefinitely large.

The diverse Functions of the cell are carried out by specialized structures known as Organelles (or organoids) (Figure 17).

Universal organelles of Eukaryotic cells include:

✵ in The Nucleus - Chromosomes,

✵ in the Cytoplasm - Ribosomes, Mitochondria, Endoplasmic reticulum, Golgi apparatus, Lysosomes, and The cell membrane.

Many Cells also contain structures that help maintain cell shape, such as microtubules, microfibrils, and various inclusions.

The most critical Chemical Components of the cell—Proteins, including Enzymes—are found both inside the cell and in the body's fluid media, but they are synthesized exclusively within the cell.

A characteristic feature of the cell is compartmentalization, which is the Spatial Organization of biochemical processes. For example, cellular Respiration in eukaryotes occurs exclusively on mitochondrial membranes, while Protein Synthesis takes place on ribosomes.

The concentration of enzymes and their orderly arrangement within cellular structures (1) accelerate biochemical reactions, (2) coordinate metabolic pathways, (3) establish molecular assembly lines, and (4) segregate distinct processes.

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Figure 17 - Combined diagram of The Structure of a Introduction/5.html">Eukaryotic Cell: a - animal cell; b - plant cell: 1 - nucleus with Chromatin and nucleolus; 2 - cell (plasma) membrane; 3 - Cell wall; 4 - plasmodesmata; 5 - rough endoplasmic reticulum; 6 - smooth endoplasmic reticulum; 7 - pinocytotic vacuole; 8 - Golgi apparatus; 9 - lysosome; 10 - lipid inclusions in the smooth endoplasmic reticulum; 11 - centriole and centrosphere microtubules; 12 - mitochondria; 13 - cytoplasmic polyribosomes; 14 - vacuoles; 15 - chloroplast

The microheterogeneity inherent in cell architecture makes it possible to synthesize various products from the same starting Materials simultaneously and within a remarkably compact, miniature total volume.

THE PRINCIPLE OF compactness, characteristic of all cellular METABOLISM, is particularly pronounced in DNA Structure: 6x10-¹² g of human egg cell DNA encodes The properties of all its proteins.

Eukaryotic compartmentalization mitigates diffusion constraints on cell size by facilitating rapid interactions between specific molecules through shorter diffusion pathways before they encounter and react with one another. Consequently, Eukaryotic cells are larger than Prokaryotic Cells.

Plant cells are larger than animal cells, firstly due to the presence of a large central vacuole, which acts as a chemically inert compartment, and secondly due to cyclosis—a continuous, active (i.e., energy-dependent) cytoplasmic streaming at a speed of 0.2–0.4 mm/min that helps overcome diffusion limitations.

Cell size can significantly exceed diffusion limits if the cell serves as a reservoir for nutrient storage, such as egg cells (bird eggs, or the porbeagle shark egg cell with a diameter of 22 cm) or fruit pulp cells (up to 10 mm in citrus fruits).

Another common scenario is an increase in cell size achieved by the repeated multiplication of internal structural elements.

For instance, multinucleation increases the concentration of Messenger RNA (mRNA) molecules in the cytoplasm, allowing multinucleated cells to grow larger than mononuclear ones by alleviating the constraints associated with RNA diffusion from the nucleus.

The repetition of internal structural elements often leads to an increase in cell length while maintaining a microscopic diameter (for example, animal Muscle cells can be up to several centimeters long, Nerve Cells with their processes can reach up to 1 m in length, and plant phloem cells can grow up to 5 mm long).

A specific intracellular ion concentration is continuously maintained, differing markedly from the concentration in the extracellular environment.

Cells are capable of pinocytosis and phagocytosis through The formation of cell membrane invaginations, which subsequently pinch off and are internalized as vesicles.

Pinocytosis is the uptake of fluid droplets containing large molecules, including proteins, from the environment.

Phagocytosis is the engulfment of large particles, Viruses, and small cells.

The number of cells in Multicellular Organisms varies significantly. In primitive invertebrates, it ranges from a few hundred (for instance, exactly 959 cells in the nematode Caenorhabditis elegans) to tens of thousands, whereas The Human Body contains approximately 10¹⁴ cells (one hundred trillion). A tree has a comparable number of cells, while an elephant has about 6.5 times more cells than a human. Originating from the division of a single initial cell—the zygote—the cells of a multicellular Organism diversify in Structure and function. For example, among the 10¹⁴ cells of the human body, over 1,000 distinct cell types are recognized.

The process whereby cells acquire stable internal differences associated with performing specialized functions is known as Cell Differentiation. Differentiation is invariably accompanied by the Synthesis of specific proteins. For instance, epidermal Skin cells synthesize the structural protein Collagen, muscle cells produce the contractile protein Myosin, and retinal cells synthesize the visual pigment opsin, among others.

Every cell in a multicellular organism possesses an identical and complete set of genetic material along with the potential to express this Genetic information—a property known as cellular totipotency.

In 1958, F. Steward successfully grew a complete, normal plant—with roots, leaves, stems, and flowers—from a single phloem cell isolated from an adult carrot.

In 1968, J. B. Gurdon successfully transplanted nuclei from tadpole intestinal cells into enucleated frog eggs and raised normal, fertile male and female frogs capable of reproduction.

Thus, during differentiation, eukaryotic totipotency-capable cells do not lose their genetic information; rather, different genes are simply

"switched on" or, conversely, "switched off" by specialized activator proteins (or repressor proteins, respectively), which are in turn synthesized from regulatory genes.

It is estimated that in the cells of highly organized organisms, 90% of all genes perform such "administrative" regulatory functions, while only 10% are actually responsible for The Biosynthesis of enzymes and structural proteins.