THE ALBUMINOIDS 167 



The stem and leaves expand into the air, where, through the agency of light, 

 ** chlorophyll " is produced, and they become green. Atmospheric carbonic 

 acid is absorbed by the leaves, and, through the agency of the chlorophyll, 

 in presence of sunlight and warmth, reacts with water absorbed by the 

 roots, carbohydrate being produced and oxygen set free into the air : — 



6CO2 + 5H2O = CgHioOs + 6O2. 



From the same materials, together with the inorganic nitrates and sulphates 

 absorbed from the soil, albuminoid is also produced. The carbohydrate 

 and albuminoid, being convertible into soluble substances by the soluble 

 ferments, acids, or salts also contained in the leaves, can be conveyed by 

 the sap to the growing part of the plant, or to the seed, stem, and root, 

 where storage of reserve material is required, and then may be deposited in 

 an insoluble form. Thus the complex organic constituents of plants are 

 products of redtiction from simple inorganic substances, sunlight and heat 

 being absorbed. 



2. Animals. — Animals have no power of elaborating complex organic 

 compounds from simple inorganic substances, and their food consists there- 

 fore, directly or indirectly, of vegetable products. These foods are usually 

 insoluble and require digestion, i.e. conversion into a form in which they 

 can pass through the membranes of the stomach and intestines into the blood. 

 The digestive fluids are — (i) the saliva, an alkaline fluid containing a 

 soluble ferment, ptyalin, which converts starch into malt sugar and grape 

 sugar ; (2) the gastric juice, a fluid, acid with hydrochloric acid, containing 

 a soluble ferment, pepsin, which converts coagulated albuminoids into 

 peptones ; (3) the bile, pancreatic juice, and intestinal juice, alkaline fluids, 

 also containing soluble ferments, which complete the conversion of starch 

 into sugar and albuminoids into peptones, and which also emulsify and 

 partially saponify the fats. 



The blood, into which all the digested food passes, contains a purplish red 

 compound, haemoglobin, which becomes oxidised in the lungs to bright red 

 oxyhgemoglobin. This compound is capable of oxidising the carbon and 

 hydrogen of assimilated food constituents to carbonic acid gas and water, 

 the oxyhaemoglobin being reduced to haemoglobin. This carbonic acid gas 

 remains dissolved in the blood till it reaches the lungs, when it is set free 

 and oxygen again absorbed. Thus respired air becomes rich in carbonic 

 acid gas and deficient in oxygen (see p. 23). The water produced is excreted 

 by the lungs, pores of the skin, or kidneys. 



The heat produced by the oxidation of the carbon and hydrogen provides 

 the power of doing work. When more carbohydrate and fat is digested 

 than is required for oxidation, they may be deposited as fat, and this will 

 serve as a store of fuel, and undergo oxidation when required. Work is 



