' 720 MAN'S PHYSICAL WELFARE and intensity of the light. Mushrooms and other plants that grow in the dark are merely saprophytes that feed upon the organic matter stored in the soil instead of creating their own foods. The temperature is very important in controlling the rate of photosynthesis, the most favorable temperatures lying between 30° and 40° C. Under natural conditions plants use about 1 per cent of the radiation which they receive from the sun. Photosynthesis Takes Place Only in the Presence of Certain Pigments. In 1779 Ingen-IIousz, a Dutch physician, discovered that green leaves, in the presence of sunlight, absorb carbon dioxide from the air and give off oxygen. In portions of the plant not exposed to the light, the reverse process of respiration takes place, but in the green leaves in sunlight photosynthesis predominates; the net result is the absorption of carbon dioxide and the evolution of oxygen. The over-all reaction that takes place may be given as follows : WCO2 + wHzO ^ (CH20)w + nOi oxygen carbon dioxide water organic products It is evident, then, that photosynthesis is the reaction between carbon dioxide and water, and it is only reasonable to suppose that the amount of photosynthesis would depend upon the amounts of carbon Fig. Dead Plant and Animal Residues Coal, Wood. Petroleum, Etc. 308. The carbon dioxide cycle in nature. dioxide and water supplied. Photosynthesis is carried out in the granules called chloroplasts, which are scattered through the leaf cells. These chloroplasts are small ovoid bodies containing a mixture of colorless protein material, chlorophyll and other pigments, starch, and droplets of oil. One or many chloroplasts occur in a single cell. A square centimeter of leaf surface may contain as many as 50,000,000 chloroplasts, each of which is capable of transforming about 40,000,000 molecules of carbon dioxide into organic compounds every second. A large pine » This is not a ti^e chemical equation, for it does not state how much carbon dioxide, water, or oxygen is involved, nor does it state which organic product is produced. The chief products produced are carbohydrates, having the general formula (CH2O),,.
I H I H C PHOTOSYNTHESIS STORES ENERGY 721 tree adds the amount of wood in an ordinary broomstick to its size every day. The active material in the chloroplasts is a mixture of a bluishgreen pigment, "chlorophyll a," and a green pigment, "chlorophyll b." These pigments are magnesium compounds of very complex structure. The empirical formula for "chlorophyll a" is C55H7205N4Mg. Chlorophyll is always associated with at least twelve other yellow and orange-red and sometimes red-brown pigments. In some way or other this mixture of pigments through the influence of light activates the carbon dioxide molecules with the result that they will react with water to form carbohydrates. All Living Matter Is Probably Dependent on Photosynthesis. All of the compounds in plants are believed to be produced indirectly by photosynthesis. Animals cannot synthesize their own foods but depend upon plants or other animals for sustenance. In the end every animal is directly or indirectly dependent on plants for its food. It is believed that a simple carbohydrate,^ such as glucose or fructose, is the first product formed in photosynthesis. This simple carbohydrate then polymerizes, i.e., several molecules unite to form large molecules. Starch and possibly such intermediate products as sucrose (cane sugar), dextrinS; and gums may then be formed by the polymerization of the simple sugars. Cellulose is a still more complex compound — or, rather, mixture of compounds — produced by still further polymerization. Animals cannot utilize carbon or the hydrocarbons because they cannot oxidize them. The reason for this probably lies in the equal sharing of electrons by the various atoms. Alcohols represent hydrocarbons in which at least one pair of electrons has been partially displaced from the carbon atoms and are therefore more readily oxidized. H—C— First stage in oxidation Second stage in oxidation H H O H H—C—O— H— II