Plasma proteins molecular dimensions

The concentration of total protein in human plasma is approximately 7.0-7.5 g/dL and comprises the major part of the solids of the plasma. The proteins of the plasma are actually a complex mixture that includes not only simple proteins but also conjugated proteins such as glycoproteins and various types of lipoproteins. Thousands of antibodies are present in human plasma, though the amount of any one antibody is usually quite low under normal circumstances. The relative dimensions and molecular masses of some of the most important plasma proteins are shown in Figure 50-1. 

Ans. Blood is not a solution but a suspension because it contains components, e.g., red blood cells, platelets, which can be discerned as separate from the liquid medium. However, those visible components are immersed in a complex solution, called plasma, consisting of a mixture of molecules of very different sizes and masses. Along with substances of molecular dimensions comparable to the solvent (e.g., NaCl, glucose) are substances of much greater sizes and masses, that is, proteins of molecular mass up to 250,000. Therefore, plasma is classified as a colloidal solution. 

Now, while it is possible, by lowering the surface tension of the liquid medium to 50 dyn/cm by means of solutes of low molecular weight, to dissociate antigen-antibody precipitates (of the pure VDW type) (6), whole mammalian serum (with a surface tension < 50 dyn/cm) definitely has no such dissociating power. Upon some reflection this is not as anomalous as it might seem. Table I shows that it is mainly because of the presence of proteins (of molecular weight > 20,000) that blood (or plasma) has a surface tension y < 50 (and not y 70), and it cannot be expected that molecules with dimensions of the order of 100 A will effect a separation by purely physical means between other proteins of the same approximate size that are only about 2 A apart at their site of interaction (13). 

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