26 GENERAL CONCEPTS 



Sucrose, or table sugar, is a combination of glucose and fructose. Other 

 common double sugars are maltose, composed of two molecules of glu- 

 cose, and lactose, composed of glucose and galactose. Lactose, found in 

 the milk of all mammals, is an miportant item in the diet of the young 

 of these forms. Fructose, the sweetest of the common sugars, is more than 

 ten times sweeter than lactose; sucrose is intermediate. 



Most animal cells contain some glycogen or animal starch, the 

 molecules of which are made of a very large number— thousands— of 

 of molecides of glucose joined together by the removal of an H from 

 one and an OH from the next. Glycogen is the form in which animal 

 cells store carbohydrate for use as an energy source in cell metabolism. 

 The glycogen molecules within a living cell are constantly being built 

 up and broken down. Glucose and other simple sugars are not a suit- 

 able storage form of carbohydrate for, being soluble, they readily pass 

 out of the cells. The molecules of glycogen, which are much larger and 

 less soluble, cannot pass through the plasma membrane. Glycogen is 

 typically stored within cytoplasm as microscopic granules, which can be 

 made visible by special stains. Glycogen is readily converted into small 

 molecides such as glucose-phosphate (p. 72) to be metabolized within the 

 cell. 



Cellulose, also composed of hundreds of molecules of glucose, is an 

 insoluble carbohydrate which is a major constituent of the tough outer 

 wall of plant cells. 



Glucosamine and galactosamine are nitrogen-containing derivatives 

 of the sugars glucose and galactose and are important constituents of 

 supporting substances such as connective tissue fibers, cartilage and 

 chitin, a constituent of the hard outer shell of insects, spiders and crabs. 



Fats. The term fat, or lipid, refers to a heterogeneous group of 

 compounds which share the property of being soluble in chloroform, 

 ether or benzene, but are only very sparingly soluble in water. True fats 

 are composed of carbon, hydrogen and oxygen, but have much less 

 oxygen than carbon. Each molecule of a true fat contains one molecule 

 of glycerol (C3H5(OH)3) and three molecules of some fatty acid, joined 

 together by the removal of three molecules of water. The fats differ in 

 the kinds of fatty acids present. Oleic acid, C17H33COOH, is a common 

 fatty acid, and triolein, the fat containing three molecules of oleic acid, 

 has the formula Cr,7Hi04O6. Fats have a greasy or oily consistency; some, 

 such as beef tallow or bacon fat, are solid at ordinary temperatures, 

 others such as whale oil or cod liver oil are liquid. 



Fats are important in protoplasm both as fuels and as structural 

 constituents. They yield more than twice as much energy per gram than 

 do carbohydrates and thus are a more economical form for the storage 

 of food reserves. Carbohydrates can be metabolized to release energy very 

 quickly and thus serve as short-term storage forms. Fats provide for a 

 longer-term storage of food reserves. Carbohydrates are readily con- 

 verted by cells into fats and may be stored in this form. This, of course, 

 is the explanation for the observation that sugars and starches are "fat- 

 tening." The reverse process may also occur, but to a lesser extent. 

 Experiments with fats labeled with radioactive carbon atoms have 



