A text-book of human physiology / by Dr. Robert Tigerstedt ... tr. from the 3d German ed. and edited by John R. Murlin ... with an introduction to the English ed., by Professor Graham Lusk.

  • Robert Tigerstedt
Date:
1906
Catalogue details

Licence: Public Domain Mark

Credit: A text-book of human physiology / by Dr. Robert Tigerstedt ... tr. from the 3d German ed. and edited by John R. Murlin ... with an introduction to the English ed., by Professor Graham Lusk. Source: Wellcome Collection.

Provider: This material has been provided by the Augustus C. Long Health Sciences Library at Columbia University and Columbia University Libraries/Information Services, through the Medical Heritage Library. The original may be consulted at the the Augustus C. Long Health Sciences Library at Columbia University and Columbia University.

    74/794 page 38
    C. DIGESTION J Tlie solids taken up in this way by tlic cells must unders^o various change.^ in order that they may be of use to the cell. Oltcii this is true also with the dissolved foods. All such processes l)y which the foods arc chanj^ed so that they may be assimilated or further elal)oratcd by the cell are included under the term digedion. The digestion accomplished by the cell is either extra- or iutracclhihir. In the former case digestion takes place under the influeuce of special sub- stances, the enzymes, formed by the cell. These substances, like catalytic agents, have the property when present in very small quantity of producing chemical changes in great quantities of complex molecules, thereby splitting them into simpler compounds, which in their turn reduce the activity of the enzymes. By the action of enzymes proteid is split into albumoses and ])ep- tones, starch into sugar, and fat into glycerin and free fatty acids (cf. C'hajiter VII). It should be noted further that every special enzyme acts only upon a definite compound or group of compounds. The proteid-splitting en- zyme therefore does not act upon starch, nor the starch-splitting enzyme on fats. etc. It is quite possible that the enzymes can produce their effects inside the cell just as well as in the surrounding medium, and it is very doubtful whether an intracellular digestion takes place anywhere without the help of enzymes, though such participation cannot be definitely asserted. Enzymes which have the property of dissolving proteids just like the corresponding enzymes in the digestive fluids have been ol)tained from finely minced organs—spleen, lymph glands, kidneys, liver, heart, etc. It is assumed that they are present in the living cells, though this has not been finally proved. Nothing definite can be said at this time with regard to the impor- tance of such enzymes in the normal processes of the body. It is possible that in starvation they may effect the solution of the tissue proteids. also that the autolytic processes which take place after death are initiated and carried on by such enzymes. The enzymes are products of cell activity, but once they are formed they act entirely without the help of the cell and are nonliving sul)stances. In general it is supposed that they are proteid in nature, and in fact Pekelbaring has prepared from the stomach of the dog a very pure enzyme (pepsin) which was free of phosphorus and had the constitution of proteid. But it would be premature to draw any general conclusions from a single observation. Enzymes occur in the cells for the most part in the form of precursors, known as zymogens. Often the zymogen is changed into the active enzyme in the act of secretion: or its activation may be brought about under the influence of another enzyme. Enzymes are only slightly diffusible, but they pass through a porcelain filter and can be separated in this way from the cell fragments of an extract or juice. In the dissolved state they withstand heating up to 70° C; in the dry state many enzymes are not destroyed by a temperature of over 100° C. In general they are most powerfully active at 35° to 45° C.; at a lower tem-
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