130 PLANTS AND MAN 



of the three foods, carbohydrates are essential for the existence 

 of the others. 



We have already seen that the first carbohydrate manu- 

 factured by choroplasts is glucose, and that this glucose can be 

 synthesized only by plants (see p. 16). Since all of the other 

 carbohydrates are derived from glucose, the world's entire food 

 supply is dependent upon this one process of photosynthesis, and 

 this one chemical compound. Glucose (G6H12O6) is also known 

 as dextrose, grape sugar or corn sugar. It is found in abundance 

 in fruit juices and honey. It is commercially produced from corn 

 starch, to form the familiar corn syrup. Glucose is only 60% as 

 sweet as cane sugar, and is much easier to digest; the latter 

 (sucrose) must be converted into glucose by digestive enzymes 

 before being absorbed into the circulatory system. Thus invalids 

 and persons with poor digestions are fed glucose-sweetened foods 

 rather than those with the usual sugars. 



The Carbohydrate Foods 



From glucose, the simple carbohydrate resulting from photo- 

 synthesis, plants elaborate numerous other carbohydrate foods 

 (fig. 82). Garbohydrates make up the bulk of the dry weight of a 

 plant, and appear to be more important in plant functions than 

 among animals. The glucose may be utilized directly by the plant 

 as a source of energy, or may be converted into numerous other 

 compounds used for specific purposes by various parts of the 

 plant. These compounds include other sugars, starch, cellulose, 

 dextrins, gums and pectins. 



There are three other sugars which originate from glucose. 

 Fructose or fruit sugar has the same formula as glucose; it is 

 the sweetest of all the sugars. Fructose occurs in honey and in 

 fruit juices, but is commercially secured from the Jerusalem 

 artichoke. Maltose or malt sugar (G12H22O11) is rarely found in 

 great quantities in plants; the usual source for commercial pur- 

 poses is in germinating barley grains. Gane sugar or sucrose 

 (also G12H22O11) is the most generously produced sugar, secured 

 by man from the storage root of the sugar beet or the sap of the 

 sugar cane stem. 



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