In the animal body analytical processes are most conspicuous. Here complex organic mate rial is transformed and ultimately broken down with liberation of the energy stored up in the large molecules, and which came primarily from the sun. These analytical or destructive changes are of various kinds, such as hydrolysis, oxida tion, reduction, etc.
Examples of hydrolytic decomposition or change are perhaps best seen in connection with the digestive processes. Here the various classes of foodstuffs, under the influence of the diges tive juices, are gradually transformed into more or less soluble and diffusible products well fitted for absorption. Proteid. for example, is con verted into a row of soluble proteoses and pep tones: a transformation brought about by cer tain enzymes or ferments. such as pepsin and trypsin. The change, little understood. is termed catalytic. since it is assumed to occur through niere contact of the enzyme: but what really hap pens is the taking on by the proteid of one or more molecules of water (hydrolysis) with sub sequent splitting or cleavage of the molecule. and formation of a number of simpler product:. Ilence, the products are frequently termed hydro lytic cleavage products. _1 like transformation occurs in the digestion of starch with saliva or pancreatic juice, whereby soluble dextrins and sugars result. under the influence of the enzymes contained in the above secretions. Further, cane sugar. under the influence of the invert ferment secreted by the intestinal cells, is split into two molecules of a simpler sugar during digestion, as a result of simple hydration. The reaction which take; place may be represented as follows: c„n_p„ = Caro--suaar Dextrose Ilextrose Hydrolytic processes of the above type are very ,omnion methods by which •heinieal changes are effected in the animal body.
oxidative changes are especially conspicuous in animal tissues. Oxidation is the principal method by which the organic material of tho tis sues is broken down. Naturally, this oxidation is brought about in some measure by the oxygen coining to the tissues in the arterial blood, but there are present in many. if not in all. tissues. of the animal body, a, well a-. in vegetable tis sues, peculiar ferments such as the oxidases which are capable of inciting oxidation. Many of the oxidative processes going on in the ani mal body seem to be connectt.d with the life of the cell of the tissue or organ involved, but it is quite probable that in most, if not in all, such cases there are really oxidizing enzymes present, generated no doubt by the activity of the cell.
The best example of reduction in the animal body is found in the changes going on in the lower end of the small intestine and at the be ginning of the large intestine, where various reducing gases arc generated through the growth and activity of anaerobiotie bacteria. These mi •ro•organisins are ever present in the intestine, and, as products of their growth, hydrogen, ear buretted hydrogen, hydrogen sulphide, and car bonic acid are formed in varying amounts. These
reducing gases are quite able to reduce other sub stances brought in contact with them in the in testine. and it is in this way that the pigments of the urine and of the faeces are formed from the bilirubin of the bile.
The ultimate object of the various chemical processes oecurring in the animal body is the liberation of the energy necessary to keep up the vital processes. Upon this ability depends life and all forms of vital activity. The potential energy of the foodstuffs, the fuel of the animal body. is derived from the sun. As Binge has stated it, the potential energy of vegetable substances is converted sunlight." The food stuffs. whether animal or vegetable, are rendered capable of absorption by the several digestive processes. After passing into the blood or lymph, they are carried to the various organs and tis sues of the body, where they are assimilated. and, by processes of anabolism. are built up, in part. into the tissues of the body, or in part di rectly burned with liberation of their energy. This energy show: itself either in the form of heat or as wo•k—i.e. muscular movements, or organized movements by which we perform work. TIW sum of the work performed by an animal, and of the heat which it gives out. is the exact equivalent of the potential energy contained in the food taken in, or in the material of the tissues burned up. This energy is liberated as the result of oxidation. and the energy is essen tially the same in amount, whether the oxidation is carried on in the body or by combustion out side of the body. In other words, the anamnt of energy liberated is the same whether the food stuff or its equivalent is burned directly to ear honk acid and water, or whether it is broken clown gradually, step by step, until the final stage in the oxidation is reached. A man of average body weight, doing an average amount of work, must consume food material sufficient to yield 30110 kilogram-degree unit; of heat, or 3000 large calories, if lie is to keep himself in equilibrium. The amount of heat required to keep the body eontimmusly at 38' C., no matter what the temperature of the surrounding air, is by no means small, and. in addition, it is to lie remembered that all the involuntary innseular movements, such as the beating of the heart, the muscular movements involved in respiration, and the constant movements of the intestinal walls. involve a large expenditure of energy which is quite independent of the energy required for muscular contraction when sonu4 task is to be performed. Consequently, in the case of an ac tive mall, the amount of chemical in volved in the various processes incidental to the liberation of the required energy is large.