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40 FRINCIFLES OF ANIMAL BIOLOGY 



its capacity to dissolve many ditferent substances, and the free movement 

 which it permits in the ions of salts dissolved in it. These features of 

 water enter into so many of the living processes that life without water, 

 if it could exist at all, would have to be of a very different sort from any 

 that is known. 



Dissolved in this water of protoplasm are the salts. The commonest 

 ones have sodium and calcium as their positive ions, but potassium, mag- 

 nesium, iron, and manganese are also present in this positively charged 

 state. The negative ions are the chloride ion and the radicals known as 

 carbonate, nitrate, sulfate, and phosphate. These ions of salts dissolved 

 in water give protoplasm certain electrical properties. Inorganic salt 

 make up about 1 per cent of average protoplasm. 



The Organic Compounds. — There are three principal classes of 

 organic compoimds, the carbohydrates, lipids, and proteins. The carbo- 

 hydrates are the sugars, starches, celluloses of plant walls, glycogens or 

 animal starches, and some others. They constitute less than 1 per cent 

 of most protoplasm but are important out of proportion to their quantity. 

 They are composed of onty three elements: carbon, hydrogen, and 

 oxygen. The hydrogen and oxygen are always in the ratio of 2:1; that 

 is, there are twice as many atoms of the former as of the latter, just as 

 in water. In most carbohydrates the carbon atoms are in multiples of 

 six. A simple sugar has only six carbon atoms and is known as a mono- 

 saccharide. Glucose, one of the most common of them, is present in 

 nearly all cells. Other simple sugars are fructose (fruit sugar) and 

 galactose. The formula of all these simple sugars is CeHigOe, but there 

 are differences between them in internal ai-rangement. When two 

 molecules of a monosaccharide are combined into one (with loss of 

 water) the combination is a disaccharide. Sucrose (CioHooOh), the 

 ordinary cane or beet sugar of table use, maltose (malt sugar), and 

 lactose (milk sugar) are of this type. When many molecules of simple 

 sugar are combined (with more loss of water), a polysaccharide is pro- 

 duced. The starches (of plants), glycogens (animal starches), and 

 celluloses (of cell walls) are of this kind. The polysaccharides are prac- 

 tically insoluble in water, so that the starches and glycogens are excellent 

 food-storage forms. None of the carbohydrates forms ions when dis- 

 solved; hence they play no role in electrical i)hen()mena. They contain 

 a gi'eat deal of potential energy, which may be released by oxidation. 



either reservoirs of stored energy are the lipids. The physical 

 properties of these substances are very characteristic, including the non- 

 evaporating grease spots which they make and their insolubility in 

 water. This insolubility is what makes them good storage products. 

 The lipids constitute about 3 per cent of oi-dinary protoplasm, though 

 stored lipids may be many times that fra(;tion of an animal's body. 



