Biochemistry - The Chemical Reactions of Living Cells, Volume 1 - D. Metzler 1980

Setting the Stage
Eukaryotic Cells
Size and Structure of Eukaryotic Cells

The second major category of living organisms is eukaryotes, i.e., organisms whose Cells contain a true Nucleus. Eukaryotic cells are larger and structurally more complex than Prokaryotic Cells. The membrane-bound nucleus encloses most of the DNA, thereby separating it from the Cytoplasm. The cytoplasm contains various Organelles, each with a characteristic Structure, such as Cell/35.html">Mitochondria, Lysosomes, and centrioles. Eukaryotic cells vary so widely in size and shape and are so highly specialized that describing a "typical" cell is practically impossible. Nevertheless, in Figs. 1-3 we have attempted to depict a generalized "average" cell¹, part animal and part plant.

As can be seen from Table 1-2, which lists the diameter and volume of several roughly spherical cells, the range of cell sizes is enormous. Even so, it can be said that the typical diameter of both PLANT AND ANIMAL cells is 10—20 µm. As for large cells such as ovocytes, A large number of surrounding cells participate in synthesizing the nutrients that are subsequently transferred to the developing egg.

Many cells are far from spherical in shape. For example, human erythrocytes are disks measuring 8×8×(1—2) µm with a volume of 80 µm³. Plant fiber cells reach several millimeters in length. Animal Nerve Cells have long extensions, or axons, which in humans can reach 1 m in length.

¹ See the famous illustration by Brachet (J. Brächet, Sci. Am., 205, 55, Sep. 1961).

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FIG. 1-3. Schematic representation of an "average" Introduction/5.html">Eukaryotic Cell. The drawing shows the main organelles of both animal and plant cells (at approximately the same scale). Abbreviations: BM — basement membrane; ER — Endoplasmic reticulum (rough — with attached Ribosomes; smooth ER — near The Nucleus and on the right side of The Cell); FS — deep fold of The Plasma Membrane; GC — Glycogen granules; cleft — space ~10—20 nm wide between adjacent cells; M — mitochondria; MB — Microbodies; L — lysosome; D — desmosome; TJ — tight junction; Mv — microvilli; C — cilium; SG — secretory granules; GA — Golgi apparatus; V — vacuole; CW — Cell wall (in plants); Ch — centriole; Pl — plasmodesmata; N — nucleus; Nu — nucleolus; Chl — chloroplast; St — starch granules. (Based on a drawing by Michael Metzler.)

Table 1-2 Approximate sizes of certain cells

Table 1-3 lists The Genome sizes of several eukaryotic cells, alongside the Genome Size of E. coli for comparison.

As can be seen from the table, The amount of genetic material in yeast is roughly 5 times greater than in E. coli, and in humans (and mice) it is 600 times greater. It should be noted, however, that in higher organisms genes are frequently duplicated, and cells contain multiple repeating DNA sequences. The function of such repeats remains unknown (in some amphibians, the DNA content per cell is 25 times greater than in humans). Watson [10] suggested that the amount of actual genetic material in vertebrate cells is at least 20 to 50 times higher than in E. coli. Consequently, the number of genes in a human cell is on the order of 10⁵.