The act or process, by which living tissues or cells
   take up and convert into their own proper substance the nutritive
   material brought to them by the blood, or by which they transform their
   cell protoplasm into simpler substances, which are fitted either for
   excretion or for some special purpose, as in the manufacture of the
   digestive ferments. Hence, metabolism may be either constructive
   (anabolism), or destructive (katabolism).