The changes which the above-mentioned types of food material undergo in the various anabolic and katabolic processes _within the cell_ are as yet but very slightly known. Nevertheless there are a number of reactions brought about by bacteria acting on various food materials, partly within _but largely without the cell_ which are usually described as “physiological activities” or “biochemical reactions.” Some of these changes are to be ascribed to the utilization of certain of the elements and compounds in these materials as tissue builders, some as energy-yielding reactions and still others as giving rise to substances that are of direct benefit to the organism concerned in its competition with other organisms. Though all of the twelve elements already mentioned are essential for the growth of every bacterium, two of them are of especial importance for the reason that most of the “physiological activities” to be described in the next chapters are centered around their acquisition and utilization. These elements are _carbon_ and _nitrogen_. Some few of the special activities of certain groups have to do with one or the other of the remaining nine, as will be shown later. But generally speaking _when a bacterium under natural conditions secures an adequate supply of carbon and nitrogen, the other elements are readily available in sufficient amount_. Carbon is necessary not only because it is an essential constituent of protoplasm but because its oxidation is the chief source of the energy necessary for the internal life of the cell, though nitrogen and sulphur replace it in this function with a few forms. This latter use of carbon constitutes what may be called its _respiratory function_. Bacteria like other organisms in their respiration utilize oxygen and give off carbon dioxide. The amount of the latter given off from the cell in this way is very small as compared with that which is frequently produced as an accompaniment of other reactions (see Fermentation, next chapter). But there is no doubt of its formation and it has been determined by a few investigators. On account of this use of carbon, bacteria require relatively large amounts of this element. One group of bacteria concerned in the spontaneous heating of coal seems to be able to use free carbon from this material. Another group is said to be able to oxidize marsh gas, CH₄, and use this as its source of carbon. The nitrite, nitrate and sulphur bacteria mentioned later utilize carbon dioxide and carbonates as their carbon supply, and one kind has been described which uses carbon monoxide. With these few exceptions bacteria are dependent on _organic compounds_ for their carbon and cannot use CO₂ as green plants do.