MANUFACTURE AND UTILIZATION OF CARBOHYDRATES 167 



Carbohydrates. — What carbohydrate is the first product of carbon 

 assimilation, is not known. Assuming it to be glucose, the reaction may 

 be written as follows: 6CO2 + 6H2O + light + chlorophyll = CeHiaOe 

 + 6O0. 



Simple, naturally occurring carbohydrates may contain five or six 

 carbon atoms and are called accordingly pentoses or hexoses. There are 

 two pentoses of common occurrence, arabinose and xylose; neither of these 

 has been shown to be formed directly by assimilation. Four naturally 

 occurring hexoses are known: glucose, fructose, mannose and galactose. 



Besides these simple sugars, there are compound sugars made up of 

 two or more molecules of the simple, less one or more molecules of water. 

 The disaccharides yield two molecules of simple sugars on hydrolysis. 

 The two most common disaccharides are sucrose (cane sugar) which 

 yields one molecule of glucose and one of fructose when hydrolyzed by 

 dilute acids or inverted by an enzyme and maltose (malt sugar) which 

 yields two molecules of glucose. 



In addition to the sugars there are complex carbohydrates, called 

 polysaccharides; these yield an indefinite number of molecules of simple 

 sugars on hydrolysis. They are for the most part less soluble in water 

 than the sugars. One kind of sugar or a mixture of different kinds may 

 be formed on hydrolysis. If the predominant sugar produced is a hexose, 

 they are called hexosans; if a pentose, pentosans. 



Hexosans are classified according to the nature of the predominating 

 sugar produced on hydrolysis. Thus there are glucosans which include 

 starch, soluble starch, dextrin and cellulose; fructosans such as inulin; 

 mannans, a constituent found in the wood and leaves of the hme, apple 

 and chestnut, and galactans such as agar-agar. Pentosans include gums, 

 mucilages and pectins, on which the jelling properties of fruit depend. 

 The relationships of the carbohydrates are shown diagrammatically in 

 Fig. 17. 



Daihj and Seasonal Fluctuation in Leaves. — Though no reliable data 

 are available on which to base a detailed picture of the carbohydrate 

 changes in the leaf, the following statements may be made.^"^ Hexose 

 sugars and sucrose increase during the day, reach a maximum about mid- 

 day, after which the quantity present decreases; these changes closely 

 parallel the temperature variations and probably the variations in light 

 intensity. There is no diurnal fluctuation in the amount of pentoses or 

 of pentosans. As a result of the accumulation of sugars, starch is formed; 

 the process occurs only in cell plastids, either in chloroplasts which are 

 green or in leucoplasts which are colorless. Species vary greatly in their 

 capacity to form starch. Many plants — the onion, for instance — form 

 none at all. Starch and sucrose formation in the leaf are only temporary. 

 The carbohydrates are continuously conducted from the leaf as hexoses, 

 which occur in greater amounts than other sugars in the conducting 



