The products of living of different types of organisms are alike in some respects and unlike in others. Complex products cast off by one organism as of no further use for it are broken down into simpler forms by some other organism, and by another are built up anew into a newly combined complex substance usable by still others. Waste products of living are usually broken down. What is consumed is built up into something new or used up in something done as work.

In their functions, that is, what they do in nature through their living, organisms of different types are less alike even than in their products of living. Their functions are therefore very important. If one fails in what it alone can do, others or all are hindered and delayed, or may even be destroyed.

Bacteria carry nitrogen from the complex forms of nitrogenous waste products of animal life to the simpler compounds serving plants as food.

Plants build nitrogen into protein - the building food of animal life - by combining it with other elements. Plants decompose the carbon dioxid of expired air and so unite the carbon with hydrogen and oxygen as to construct sugar, starch, cellulose, oil, - carbohydrates and fats, - the heat-energy foods of animal life. Plants also carry the salts of minerals into combination with organic substances, which is the only form in which they are thoroughly assimilable by animal organisms. Mineral salts are the regulating food-elements for animal life; they build somewhat, too.

Animals make no really new kind of food-substance. They do, however, so transform some substances as to make them more readily or fully digested by humankind. Vegetable protein is incased in cellulose (woody fiber). This makes it less fully available for use than animal protein.