38 ORGANIC SUBSTANCES : CHEMICAL CHARACTERS 



generally present in the proportion of two to one, 

 as in water. 



The Sugars. 



The most important carbohydrates are the sugars, of 

 which there are many different kinds. Glucose (grape 

 sugar), CgH^^g^e' li^i^i^lose, CgHigOg, sucrose (cane sugar), 

 ^12^22^11' ^^^ maltose, ^x^^^O^^, are the only ones 

 we need mention here. It will be seen that the two 

 former have the same number of atoms in the molecule, 

 and so have the two latter. They differ, however, in 

 the way in which the atoms are grouped, and this 

 leads to differences in crystalline form, and to differences 

 in reaction with other compounds. 



The sugars are readily soluble in water ; moreover, they 

 easily move about the body by liquid diffusion, and their 

 molecules possess a large amount of potential (chemical) 

 energy, so that when the molecule is oxidised and broken 

 lip a great deal of energy is set free in the kinetic form. 

 The kinetic energy which appears in organisms as heat and 

 movement is mainly derived from the oxidation and breaking 

 up of sugars. For these reasons the sugars are extremely 

 important in the sum total of the chemical changes, 

 called metabolism, which are perpetually occurring in 

 the living protoplasm of plants and animals. 



Glucose (grape sugar), CgHjgOg, is the most prominent 

 of the sugars in the economy of organisms. While 

 glucose is a relatively stable substance in the test tube, 

 it takes up oxygen more easily than other sugars, and 

 thus acts as a moderately strong " reducing agent," 

 i.e. it can take oxygen from (or " reduce ") certain other 

 substances. An example of this action is the behaviour 

 of glucose solution when mixed with a cupric salt 

 (i.e. a salt of copper containing much oxygen) in alkaline 



