Composition of Intracellular and Extracellular Fluids

When we think of biological molecules, we normally think of all the special molecules that are unique to living organisms, such as proteins and nucleic acids: enzymes, DNA, RNA, and so on. These are the substances that allow life to occur and that give living things their special characteristics. Yet, if we were to dissociate a human body into its component molecules and sort them by type, we would find that these special molecules are only a small minority of the total. Of all the molecules in a human body, only about 0.25% fall within the category of these special biological molecules. Most of the molecules are far more ordinary. In fact, the most common molecule in the body is water. Excluding nonessential body fat, water makes up about 75% of the weight of a human body. Because water is a comparatively light molecule, especially when compared with massive protein molecules, this 75% of body weight translates into a staggering number of molecules of water. Thus, water molecules account for about 99% of all molecules in the body. The remaining 0.75% consists of other simple inorganic substances, mostly sodium, potassium, and chloride ions. In the first part of this book we will be concerned in large part with the mundane majority of molecules, the 99.75% made up of water and inorganic ions.

Why should we study these mundane molecules? Many enzymatic reactions involving the more glamorous organic molecules require the participation of inorganic cofactors, and most biochemical reactions within cells occur among substances that are dissolved in water. Nevertheless, most inorganic molecules in the body never participate in any biochemical reactions. In spite of this, a sufficient reason to study these inorganic substances is that cells could not exist and life as we know it would not be possible if cells did not possess mechanisms to control the distribution of water and ions across their membranes. The purpose of this chapter is to see why that is true and to understand the physical principles that underlie the ability of cells to maintain their integrity in a hostile physicochemical environment.

0 0

Post a comment