372 CARBOHYDRATE METABOLISM 



The few analyses of plant tissues which have been made for the pentoses 

 indicate that the}' never represent more than i per cent of the dry weight of 

 plant tissues. Arabinose and xylose form pentosans upon condensation (see 

 later) while ribose is a hydrolytic product of nucleic acid (Chap. XXVI). 



Pentoses are also hydrolytic products of the gums, mucilages, pectic com- 

 pounds, and hemicelluloses. They are therefore important building blocks 

 in the synthesis of certain more complex plant carbohydrates. The origin of 

 the pentoses in plants is unknown. It is possible that they are synthesized 

 directly in photosynthesis; an alternative possibility is that they are formed 

 from the hexose sugars. 



The hexoses are six carbon atom sugars represented by the molecular for- 

 mula C6H12O6. Sixteen stereoisomeric aldoses and eight stereoisomeric ke- 

 toses having this formula are known to be possible. Of all these, only two, 

 <^-glucose (an aldose) and ^-fructose (a ketose), are commonly found in 

 plants in the free state, ^-mannose and ^/-galactose, both aldoses, and ^-sor- 

 bose, a ketose, are hydrolytic products of a number of the more complex 

 carbohydrates. 



^-glucose (also called dextrose, blood sugar, corn sugar, or grape sugar) 

 is the most familiar of all the hexoses, and is of widespread occurrence in 

 plants. It is apparently present in practically every living plant cell, ^-glu- 

 cose is dextrorotatory (hence the name "dextrose"); its specific rotatory 

 power at 20° C. being + 52.7°. Apparently transformation of glucose to 

 fructose, as well as the reverse reaction, occurs readily in plant cells. Glu- 

 cose is frequently assumed to be the first product of photosynthesis, and while 

 there is little direct evidence that this is true, there can be little doubt that 

 it is one of the first products. Glucose is probably the sugar most frequently 

 oxidized in respiration, and is probably also a common translocation form of 

 carbohydrate. Its condensation products include starch, dextrins, cellulose, 

 and glycogen. It is also a hydrolytic product of certain di-, tri-, and 

 tetrasaccharides. 



^-mannose and ^-galactose are both found in plants in the free state only 

 in traces, and are evidently only transitory products in the metabolism of 

 plants. Their condensation products are mannosans and galactosans respec- 

 tively, both discussed later. Galactose is a fundamental constituent of the 

 pectic compounds and is also one of the sugars produced upon the hydrolysis 

 of lactose, rafHnose, stachyose, gums and mucilages. 



^-fructose (also called levulose, or fruit sugar), like glucose, is nearly 

 always present in the cells of the higher plants. The straight-chain struc- 

 tural formula for ^-fructose is as follows: 



