50 



PRINCIPLES OF ANIMAL BIOLOGY 



blue is ineffective. The production of carbohydrates in this manner is 

 called photosynthesis, literally, construction by means of light. 



The first substance that can be readily identified in the process of 

 photosynthesis is glucose, CeH^Oe. Various suggestions have been 

 offered for the intermediate steps which occur in the formation of glu- 

 cose. One of these is that the carbon dioxide and water unite to form 

 carbonic acid which is in some way reduced, perhaps to formic acid, and 

 the formic acid is then reduced to formaldehyde. In this process oxygen 

 is set free. The molecules of formaldehyde then unite to form glucose. 



The formulas for these reactions may be written 



thus: 



CO2 + HoO^^iOH.COOH (carbonic acid) (1) 



OH.COOH - O2 = H.CHO (formaldehyde) (2) 



By a rearrangement 6(H.CH0) becomes 

 C6H12O6 or glucose. By the condensation of n 

 molecules of glucose and the elimination of n — 1 

 molecules of water, the glucose is changed into 

 starch. Thus, approximately n (CeH^Oe) — (n — 

 1) (H2O) = (CeHioOs) n, the formula for starch. 

 Glucose may be changed into other sugars, espe- 

 cially maltose (C12H22OU), and the latter may be 

 changed into starch. The su^rs are soluble and 

 in some plants may not be transformed into starch. 

 Starch is insoluble and is temporarily stored in the 

 form of starch grains in the chloroplasts. It is 

 readily reconverted into a soluble carbohydrate 

 (glucose) and in this form may be transported to 

 other parts of the plant where again it may be 

 converted into starch and stored in leucoplasts, or 

 the glucose may be converted into cellulose, a form of carbohydrate 

 used in cell walls. 



The oxygen released in the formation of carbohydrates, as in equation 

 (2) above, may be readily recovered and tested, particularly if water 

 plants are used for the experiment. In such an experiment the cut ends 

 of a water plant, as Elodea, are inserted in a test tube filled with water, 

 the plant and tube are immersed in water and then the tube is inverted 

 (Fig. 25). When the plants are placed in sunlight bubbles of gas escape 

 from their cut ends and collect in the tube. The usual tests show the 

 gas to be oxygen. 



Protein Synthesis. — Proteins are formed by the linkage of molecules 

 of various substances known as amino-acids. These acids all possess 

 pne or more carboxyl groups of atoms (COOH) which give the acids their 



Fig. 25. — Method of 

 collecting oxygen pro- 

 duced by the aquatic 

 plant Elodea during 

 photosynthesis. The 



oxygen rises from the 

 plant into the closed end 

 of the test-tube. 



