Food cycle is the continuous natural cycle that involves the synthesis of food from inorganic substances, the elaboration of this food into protoplasm, and the breaking down of protoplasm to these same inorganic substances.
The food cycle begins with the absorption by green plants of carbon dioxide from the atmosphere and of water from the soil. By means of photosynthesis green plants manufacture sugar from these inorganic substances and convert the radiant energy of sunlight into the chemical energy of the sugar. Green plants also absorb inorganic nitrates from the soil and elaborate proteins from them. This sugar and these proteins serve as food for the plants. The plants assimilate them and transform them into the living protoplasm that composes their tissues.
Another phase of the food cycle occurs if animals eat the plants. The animals assimilate the sugar and proteins of the plant tissues and transform them into the living protoplasm that composes their own tissues. After being assimilated into animal tissue the sugar reacts with oxygen and is broken down into carbon dioxide and water, the inorganic substances from which it was originally synthesized. Most important of all, the chemical energy stored in the sugar is released when the sugar reacts with oxygen. This energy, which originally existed as solar energy, can now be utilized by the animal in all of its vital activities.
The food cycle is completed when plants and animals die and their tissues decay. Bacteria cause the tissues to decay by changing chemically the proteins and sugar that compose them. The bacteria make nitrates from the nitrogen contained in the proteins. These inorganic nitrates are returned to the soil, but later they may be absorbed by other plants and again be elaborated into proteins. The sugar is decomposed into carbon dioxide, which is returned to the atmosphere, and into water, which is returned to the soil.
The food cycle is continuously being repeated and will continue to be repeated as long as life exists on earth.
