THE DEVELOPMENT OF AGRICULTURAL CHEMISTRY 15 



maintenance of life. Absorption, secretion, and excretion would not 

 be possible without water. 



From a biological standpoint it is important that water exhibits high 

 surface tension, that it forms hydrates with many compounds, and 

 that it possesses high specific heat. Water is a necessary reactant for 

 the hydrolytic splitting of carbohydrates, fats, and proteins. The plant 

 requires from 200 to 400 lb. of water to produce 1 lb. of dry matter. 

 In spite of this, the plant maintains a very accurate balance between 

 water and other normal cell constituents. Water exists in living tissue 

 as free water and as bound water, or water of hydration. Both forms 

 of water are of great biological importance. 



Water cannot pass freely in and out of living cells because its passage 

 is controlled by osmosis, hydration of colloids, and other factors. In 

 certain diseases of man the permeability of cell membranes is altered, 

 and normal water balance is upset. An example is the kidney disease 

 known as nephritis. In this disease, tissues swell to abnormal size 

 owing to excessive water uptake. The condition is commonly known as 

 "dropsy." Diabetes is associated with dehydration and acidity. Exces- 

 sive losses of water by sweating deplete the body tissues of essential 

 mineral salts to the point where salt depletion may cause prostration. 



As tissues age they lose their normal water-holding ability. As a 

 result tissues of young organisms are richer in water than those of 

 aged organisms. Mammals, such as dogs and man, can survive for a 

 month or more without food if water is available. However, death 

 will occur in but a few days if the body is deprived of water. 



When deuterium (heavy hydrogen) unites with oxygen, heavy water 

 is formed. Oxygen also exists in isotopic forms with atomic weights of 

 16, 17, and 18, respectively. Theoretically the two isotopic forms of 

 hydrogen and the three isotopic forms of oxygen are capable of uniting 

 to form 18 different kinds of water. Heavy water and isotopic forms 

 of hydrogen and oxygen serve as tools for the biologist and biological 

 chemist in their studies of cell metabolism. 



Inorganic Salts 



Mineral elements occur in living tissues as inorganic salts, as salts of 

 organic acids, and in combination with organic compounds. Although 

 sulfates, chlorides, phosphates, and carbonates of sodium, potassium, 

 calcium, and magnesium are usually considered most important, salts 

 of trace elements are often present in significant amounts. Examples 

 of such micronutrient elements are boron, copper, cobalt, manganese, 

 iodine, and zinc. 



Phosphorus and sulfur are unique in that they form vital combina- 



