Principles of Biochemistry Volume 1 - A. Lehninger 1985

Biomolecules
Cells
Cells must be very small

Cells share another key structural feature: they are all relatively small (indeed, they could not be otherwise). Under laboratory conditions, Chemical Reactions are typically carried out in vessels with volumes of tens of milliliters or even liters. The contents of such reaction vessels must be constantly and thoroughly stirred so that the reaction rate is not limited by The rate of diffusion of the reacting molecules. In living cells, however, biochemical reactions take place in compartments of microscopically small volume. For example, the volume of an Escherichia coli bacterial Cell is only 2∙10^-12 milliliters (ml). To clearly visualize what

significance cell size has for the Chemical aspects of its cellular activity, we must first become familiar with the dimensions of Biomolecules and cells. As shown in Table 2-1, the nanometer (nm) and micrometer (μm) are currently used as units of length to define the sizes of cells and their components. Although older units, such as the angstrom or micron, are used less and less, they should also be known. To give the reader an approximate idea of cell sizes, Table 2-2 lists the dimensions of some of the most important biological structures, including small biomolecules (Alanine and glucose), macromolecules (three Proteins and a lipid), supramolecular systems (Ribosomes and Viruses), cellular Organelles (Mitochondria and METABOLISM/14.html">Chloroplasts), a bacterium, and a Liver cell. Many bacterial cells reach 2 μm in length, and most cells of higher animals are 20 or 30 μm.

Class="center">Table 2-1. International System of Units

Table 2-2. Sizes of some biological structures

One might ask^- why do living cells have these particular dimensions? Why are there no cells that are significantly smaller or significantly larger than those we know? As it turns out, there are compelling reasons for this. The smallest viable cell—the microorganism Mycoplasma—cannot be much smaller than it is, simply because the molecules from which it is constructed have a fixed size determined by the dimensions of carbon, hydrogen, oxygen, and nitrogen atoms. To sustain life, a cell must contain at least a minimum number of different biomolecules. Therefore, if cells were any smaller, they would have to be built from smaller atoms or molecules.

On the other hand, cells probably cannot be much larger than they are, simply because the rates of metabolic processes would then be limited by the rate of diffusion of nutrient molecules within The Cell, which would restrict the capacity for Metabolic Regulation. The maximum size of a cell therefore depends on the fundamental laws of physics that govern the rate of diffusion of molecules dissolved in an aqueous medium. Indeed, in larger cells, the Cytoplasm is partitioned into smaller compartments, or organelles, largely to facilitate rapid interactions between specific molecules by shortening the distance they must travel before colliding and reacting with one another. Clearly, one reason cells are small is that they must function without electrical or mechanical stirring devices. Another reason relates to the optimal surface-to-volume ratio of cells. Because a cell's surface area is relatively large compared to its volume, more nutrient molecules can enter the cell per unit of time. Simple calculations show that as the diameter of a sphere increases, its surface-area-to-volume ratio decreases sharply. (Try calculating the surface-to-volume ratios yourself for spheres with diameters of 1, 10, and 100 μm. The surface area of a sphere is 4πr², and its volume is 4/3πr³, where r is the radius and π is 3.14.)