A textbook of biochemistry for students of medicine and science / by A.T. Cameron.

  • Cameron, A. T. (Alexander Thomas), 1882-1947.
Date:
1938
Catalogue details

Licence: In copyright

Credit: A textbook of biochemistry for students of medicine and science / by A.T. Cameron. Source: Wellcome Collection.

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    that of a large molecule such as starch or haemoglobin, so that the sum of the interfaces between haemoglobin and water in the red blood cell is obviously very great. (In general it can be shown (Fine, Biochem. J., 1931, 25, 647) that when any particle undergoes division into n equal sized smaller particles, the total surface of the smaller particles is 3y'n times the surface of the parent particle.) Each red cell contains, in round numbers, 300 million molecules of haemoglobin (cf. p. 13). The radius of the spherical molecule of haemoglobin has been found experimentally to be 2-44 mg. From this it can be calculated that the combined surface area of all the haemoglobin in a single red cell is somewhat more than 0 02 sq. mm. The volume of a red cell is 0-9 x 10-7 cubic mm. Its surface, were it a sphere, would be 0-97 X 10~4 sq. mm. Hence the combined surface of all the haemoglobin within it is more than 200 times that of its own. And it can similarly be calculated that the total combined surface area of all the haemoglobin in the blood of an adult man is not far from 400,000 times greater than the surface area of his body. Such large interfacial areas are of great importance in connection with the phenomenon termed adsorption. In a heterogeneous system of two phases it is often found that the concentration of a particular compound is either greater or less at the interface between the phases than it is in either phase itself. The presence of a second phase tends to upset the homogeneous distribution in the first. The difference of concentration at the phase boundary is termed adsorption (L. ad sorbere, to drink up). If the concentration of the compound in question is increased at the interface, then positive adsorption has occurred (with lessened concentration, negative adsorption, we are not especially concerned). The substance adsorbed (drunk up) stays at the surface ; it does not penetrate the second phase. Adsorption may be due to a variety of causes, chemical, cohesional, electrical, etc. A long known and outstanding example of adsorption is the power possessed by charcoal, with its relatively huge expanse of surface, to soak up carbon dioxide and other heavier gases from air, and to remove coloured compounds from solution. The property is essentially associated with large surface, such as is possessed by any solid when in a state of fine powder. As indicated above, in solutions of biochemical interest the large interfacial areas furnished by compounds with relatively huge molecules afford many opportunities for occurrence of phenomena dependent on adsorption. It is considered to play an important role in the actions of enzymes, the combination of toxins with antitoxins, the sensitising of leucocytes by opsonins with subsequent ingestion of bacteria by the sensitised leucocytes, etc., while possibly the distribution of a compound in the protoplasm of a cell is largely governed by its concentrations at different interfacial boundaries within the cell. Reaction Equilibria and the Law of Mass Action. When a chemical reaction takes place it seldom happens that it proceeds to completion and that some one of the initial reacting compounds entirely disappears, unless the products of the reaction are removed from the medium in which it is occurring. In presence of the products sooner or later an equilibrium is attained, at which the original reaction is balanced by one between the products, reversing the procedure. (This, however,