80 - The Cell 



H — C=0 



I 

 H— C— OH 



I 

 HO— C— H 

 I 

 H— C-OH 



I 

 H— C— OH 



I 

 H— C— OH 



I 

 H 



glucose 



H 

 I 

 H— C— OH 



c=o 



I 

 HO— C— H 



I 

 H— C— OH 



I 

 H— C— OH 



I 

 H— C— OH 



H 



fructose 



H— C=0 



I 

 H— C— OH 



I 

 H— C— OH 



I 

 H— C— OH 



I 

 H— C— OH 



I 

 H 



H 



c=o 



I 

 H— C— H 



H— C— OH 



I 

 H— C— OH 



H— C— OH 



I 

 H 



two hexose sugars 



ribose deoxyribose 



two pentose sugars 



Fig. 4-8. Molecular structure of four important sugars. Note the slight differences between glucose and fruc- 

 tose, although both are 6-carbon sugars (hexoses) with the lame gross formula (C f; H r ,0 | .). Also note the differ- 

 ence between the two 5-carbon (pentose) sugars, which are extremely important constituents of the nucleic acids 

 (see p. 88). As the name implies, deoxyribose has one less oxygen (O) atom than ribose. 



erates the corresponding monosaccharides, 

 when hydrolysis occurs. 



Polysaccharides. The polysaccharides are 

 important substances that include the 

 starches, glycogens, and celluloses. All these 

 compounds are derived from glucose by mul- 

 tiple dehydration synthesis. The starches and 

 glycogens possess huge molecules, each repre- 

 senting a complex of almost a thousand glu- 

 cose molecules, united chemically into a 

 single unit; and the cellulose molecule is 

 even larger. The exact number and arrange- 

 ment of the glucose units differ somewhat in 

 the different starches, glycogens, and cellu- 

 loses found in different cells. 



All polysaccharides are relatively insolu- 

 ble and tasteless. The starches (in plant cells) 

 and glycogens (in animal cells) tend to be 

 deposited in the form of definite grains, 



which can be seen in the cytoplasm (Fig. 4-9). 

 Cellulose is deposited on the surfaces of plant 

 cells, as a thin but visible sheet — the cell 

 wall. 



Cellulose has great importance as the chief 

 component of cell walls and of woody mate- 

 rials generally. Starch and glycogen are also 

 important, as reserve protoplasmic fuels. The 

 cell can hydrolyze glycogen or starch, obtain- 

 ing glucose for use at an)' moment. Mean- 

 while, the deposit of fuel remains safely in 

 reserve. Due to the size and insolubility of 

 polysaccharide molecules, neither starch nor 

 glycogen can escape from the cell; and the 

 chemical inertness of these polysaccharides 

 safeguards them from deterioration. 



Lipids. All fats and fatlike substances are 

 included among the lipids. Physically, the 

 lipids are "greasy" substances, relatively in- 



Table 4-6— Chemical Nature of Some Familiar Carbohydrates 



Class General Formula Specific Karnes 



Monosaccharides C c H 12 Ou Glucose, fructose, lactose 



Disaccharides C'l-H-.-C'ii Maltose (glucose-glucose), sucrose (glucose- 



fructose), lactose (glucose-galactose) 

 Polysaccharides (C f> H ir |0.-,).v Starches, glycogens, celluloses 



