SYNTHESIS, STORAGE AND BREAKDOWN 123 



Chemosynthesis — Certain bacteria are able to synthesise organic carbon- 

 compounds from carbon dioxide and water without the assistance either of 

 light or of chlorophyll by a process known as chemosynthesis. An example 

 is provided by the nitrifying bacteria (p. 452). The necessary energy for the 

 synthesis is obtained chemically by the oxidation of environmental materials. 

 Certain other bacteria contain a green pigment related to chlorophyll and 

 carry on a photosynthetic process resembling to some extent that of higher 

 plants. 



Carbohydrates. 



In the preceding section we have chiefly considered the synthesis of one 

 type of carbohydrate — the sugars. From these there arises within the plant 

 a whole range of more complex carbohydrates, by processes independent 

 of light or chlorophyll : these may occur in any part of the plant. Carbo- 

 hydrates figure very prominently in plant metabolism, and this is a convenient 

 point at which to review them. They will be considered in order of increasing 

 complexity. It will be noticed that carbohydrates are all compounds of 

 carbon, hydrogen and oxygen, in which there are twice as many hydrogen 

 atoms as there are 3.: oxygen. In treating of them frequent reference will 

 be made to Enzymes : a later section of the present Chapter will be devoted 

 to a special consideration of these highly important protoplasmic catalysts 

 (p. 128). The carbohydrates may be grouped as follows : 



(1) Monosaccharides . — Most of these have the molecular formula C 6 H 12 6 ; 

 and the most important examples in the plant are Glucose (Grape Sugar) 

 and Fructose. It has been mentioned that the initial product of photo- 

 synthesis consists probably of one or both of these sugars, and they are very- 

 common in plant-cells, both of the leaf and of other parts. The plant-cell 

 is able to convert one into the other, though no enzyme promoting the 

 conversion is known. 



(2) Disaccharides. — These have the molecular formula C 12 H 22 11 and like 

 the first class are sugars. The molecule of a disaccharide is built up by the 

 combination of two monosaccharide molecules with the elimination of a mole- 

 cule of water. The most important example, Sucrose (Cane Sugar), prepared 

 commercially from plant sources (Sugar Cane or Sugar Beet), is built up by 

 combination of glucose and fructose in the manner mentioned, a com- 

 bination which is probably accelerated in the plant by the enzyme Invertase. 

 It can easily be demonstrated that this enzyme at other times accelerates 

 the decomposition of sucrose into glucose and fructose. Sucrose and these 

 two monosaccharides are together the commonest sugars of the plant. Another 

 disaccharide, Maltose (Malt Sugar), has also been detected in plants in small 

 quantity. It is produced by the action of the enzyme Diastase on starch, 

 but in the plant the maltose is quickly converted by the enzyme Maltase 

 into glucose, from which maltose is built up. 



(3) Polysaccharides. 



These are complex substances built up by the combination of many mole- 

 cules of monosaccharide sugars, again with elimination of water ; the empirical 

 formula for most of them is (C 6 H 10 O 5 ) n . The molecular weights of the poly- 

 saccharides are not definitely known. Unlike the sugars, the polysaccharides 

 are tasteless, colloidal substances. 



