Dissimilation

DISSIMILATION (dissimilatory process, catabolism), a phenomenon characteristic of every living organism, expressed in its continuously ongoing partial self-destruction; in this process, various organic substances of the body, with the participation of O₂, undergo breakdown into simple "end" products: CO₂, H₂O, and NH₃ (NH₃ is produced only from nitrogen-containing organic substances, e.g., from proteins). The end products are excreted from the organism as they accumulate, with ammonia being first converted through a certain chemical reaction into urea (NH₂.CO.NH₂). By the quantity of end products, one can judge the amount of organic substances that have undergone destruction, or in other words, the rate of dissimilation in the body. The significance of the dissimilatory process for the animal organism lies in the fact that organic substances, upon breaking down into end products, release their latent (chemical) energy, which is thereby transformed into kinetic forms of energy—mechanical, electrical, and thermal. Furthermore, the lowest rate of dissimilation occurs during complete rest, on an empty stomach, and under conditions where heat dissipation into the environment proceeds at the same rate as heat production arises (i.e., at average temperatures to which the organism is accustomed). Metabolism under such conditions is termed "basal." The factors that increase dissimilation are as follows: 1. A drop in ambient temperature, if it causes increased heat loss through radiation, i.e., when heat dissipation is not sufficiently inhibited by appropriate clothing; under these conditions, an intensified breakdown of the organic materials of the body (dissimilation) takes place, and heat is produced at the expense of the energy released from them, which compensates for the increased heat loss and ensures the maintenance of body temperature. 2. The appearance in the blood of food substances absorbed from the intestine, wherein it has been established that it is not the act of absorption itself, nor the digestive processes and the motor and glandular activity associated with them, that causes the rise in dissimilation during the first few hours after food intake, but rather the appearance in the organism during this time of such intermediate products (e.g., lower fatty acids) which, in the words of Carl Voit, "stimulate cellular metabolism." The intensification of dissimilation in this case is expressed by an increase in heat production (specific dynamic action of foodstuffs), reaching 10-12% of the basal metabolism. 3. Mechanical work; upon its execution, the organic substances in the body destroyed in the course of dissimilation release their latent energy, which is transformed 75-80% into heat and 20-25% into movement energy. Depending on the magnitude of the work, the dissimilation caused by it, when calculated per day, rises from a few percents (during eight hours of mental labor) to 150% and more (for persons engaged in heavy physical labor); during the hours of work itself, dissimilation increases extremely markedly compared to basal metabolism, yielding an increase of 1.5 times during light work, and 2.5 to 8 times during very heavy work.