The Chemical and Physical Structure of Protoplasm - 79 



are essentially aggregates, formed by the 

 chemical union of a greater or lesser number 

 of glucose molecules. 



Dehydration Synthesis. One of the com- 

 monest methods by which cells synthesize 

 larger molecules from smaller units is by the 

 process of dehydration synthesis. This type 

 of reaction is shown by the synthesis of malt 

 sugar (maltose), which occurs in many plant 

 cells (Fig. 4-7). Each molecule of maltose is 

 formed from two molecules of glucose, and 

 in the course of the reaction, one molecule of 

 water is formed as a by-product. 



H — C 6 H 10 O 5 — C 6 H, O s 

 1 mol maltose 



synthesis 



OH + HOH 



1 mol 

 water 



hydrolysis 



H- C 6 H 10 O 5 -OH + H- C 6 H, O 5 -OH 

 2 mol glucose 



Fig. 4-7. Building one molecule of maltose from two 

 molecules of glucose, by dehydration synthesis. Note 

 that a molecule of water is formed as a by-product. 

 Opposite reaction, termed hydrolysis, uses a mole- 

 cule of water to split one maltose into two glucose 

 molecules. 



The synthesis of maltose may be taken as 

 the prototype for dehydration synthesis gen- 

 erally. In fact, any union between smaller 

 molecules that involves the formation of 

 water as a by-product is called a dehydration 

 synthesis. In a dehydration synthesis, water 

 is always formed as a by-product, because one 

 molecule of water is eliminated at each point 

 where union occurs between two smaller 

 molecules, as is shown in Figure 4-7. 



In the cell, many large molecules, not only 

 of carbohydrates, but also of proteins and 

 other substances, are built by a series of de- 

 hydration syntheses, which keep adding to 

 the length of the carbon skeleton. 



Hydrolysis. Hydrolysis is the opposite of 

 dehydration synthesis, as is also shown in 



Figure 4-7. Hydrolysis occurs when a larger 

 molecule combines with water and fragments 

 into smaller molecules. 



Hydrolysis and dehydration synthesis occur 

 very frequently in metabolism. Constructive 

 metabolism involves the building of many 

 complex substances from simpler chemical 

 units, and frequently this involves a series of 

 dehydration syntheses. Conversely, destruc- 

 tive metabolism often involves the hydrolytic 

 splitting of larger molecules into simpler 

 components. Hydrolysis reactions generally 

 are exothermic, whereas dehydration synthe- 

 ses are endothermic; but, with certain im- 

 portant exceptions (see p. 143), the quantity 

 of energy involved is rather small. 



Other Carbohydrates. Monosaccharides. 

 The formula G,;H 12 0<j is not specific for 

 glucose. This formula designates a whole 

 group of simple sugars, the hexose mono- 

 saccharides, among which glucose, fructose, 

 and galactose are the most important 

 (Table 4-6). These monosaccharide sugars dif- 

 fer from one another very slightly as regards 

 the degree of sweetness, solubility, chemical 

 reactivity, and so forth. In the molecules of 

 each, the same numbers and kinds of atoms 

 are represented, but the arrangement of the 

 atoms is not quite the same (Fig. 4-8). Most 

 of the monosaccharides are 6-carbon com- 

 pounds, but a few (see p. 80) have only 5 

 carbon atoms. 



Disaccharides. From glucose and other 

 monosaccharides the cell synthesizes more 

 complex carbohydrates as these are needed. 

 Two monosaccharide molecules, chemically 

 united, constitute a disaccharide, for which 

 the general formula is C 1 ^H 2 ^O n . The three 

 most important disaccharides (Table 4-6) are 

 maltose (malt sugar), sucrose (table sugar), 

 and lactose (milk sugar). Each maltose mole- 

 cule represents a union between two mole- 

 cules of glucose; sucrose is formed from one 

 glucose and one fructose; and lactose is con- 

 stituted of glucose and galactose. Accord- 

 ingly, all the disaccharides are formed by 

 dehydration synthesis from the monosaccha- 

 rides; and conversely, each disaccharide lib- 



