PROTOPLASM 25 



Carbohydrates. The simplest of the organic substances are the car- 

 bohydrates—the sugars, starches and celluloses— which contain carbon, 

 hydrogen and oxygen in a ratio ot 1 C : 2 H : 1 O. Carbohydrates are 

 found in all living cells, usually in relatively small amounts, and are 

 important as readily available sources of energy. Both glucose (also 

 known as dextrose) and fructose (also called levulose) are simple sugars 

 with the formula CoHjoOg. However, the arrangement of the atoms 

 within the two molecules is different and the two sugars have somewhat 

 different chemical properties and quite different physiologic roles. Such 

 differences in the molecular configurations of substances with the same 

 chemical formula are frequently found in organic chemistry. Chemists 

 indicate the molecular configuration of a substance by a structural for- 

 mula in which the atoms are represented by their symbols— C, H, O, N, 

 etc.— and the chemical bonds or forces which hold the atoms together are 

 indicated by lines. Hydrogen has one such bond; oxygen, two; nitrogen, 

 three; and carbon, four. The structural formulas of glucose and fructose 

 are compared in Figure 2.4. Note that the lower four carbon atoms have 

 identical groups in the two sugars; only the upper two show differences. 



Glucose is the only simple sugar which occurs in any quantity in the 

 cells and body fluids of both vertebrates and invertebrates. The other 

 carbohydrates eaten by vertebrates are converted to glucose in the liver. 

 Glucose is an indispensable component of mammalian blood, and is 

 normally present in a concentration of about 0.1 per cent. No par- 

 ticular harm results from a simple increase in the concentration of 

 glucose in the body fluids, but when the concentration is reduced to 

 0.04 per cent or less, the brain cells become hyperirritable. They dis- 

 charge nerve impulses which result in muscular twitches, convulsions, 

 and finally unconsciousness and death. Brain cells use glucose as their 

 prime metabolic fuel, and a certain minimum concentration of glucose 

 in the blood is required to supply this. A complex physiologic control 

 mechanism, which ojjerates like the "feed-back" controls of electronic 

 devices, and which involves the liver, pancreas, pituitary and adrenal 

 glands, maintains the proper concentration of glucose in the blood. 



The double sugars, with the formula C12H22O11, consist of two 

 molecules of simple sugar joined by the removal of a molecule of water. 



H 



I 

 H— C— O— H 



c=o 



I 

 HO— C— H 



I 

 H— C— O— H 



I 

 H— C— O— H 



H— C— O— H 



1 



H 

 Fructose 



Figure 2.4. Structural formulas of two simple sugars. 



