82 - The Cell 



Glycerol is a viscous, colorless, water-solu- 

 ble liquid, possessing a formula of C 3 H 8 3 , 

 and the following chemical structure: 



H 



I 

 H— C— OH 



I 

 H— C— OH 



I 

 H— C— OH 



I 



H 



1 mol glycerol 



The fatty acids are less familiar than glyc- 

 erol, although fatty acids are widely used, in 

 the form of a water emulsion, as the "brush- 

 less" type of shaving cream. Dry crstalline 

 fatty acids are white, waxy-feeling solids, 

 which are not very soluble in water, although 

 considerably more so than the fats from 

 which they are derived. The most familiar 

 fatty acid is stearic acid, CH :1 (CH.) 1( ;COOH, 

 which is obtained by hydrolyzing beef fat. 



The formula for stearic acid shows that 

 one end (called the head) of the fatty acid 

 molecule is constituted by a special grouping 

 of atoms ( — COOH), which is the carboxyl 

 radical. The carboxyl radical is written as 

 — COOH, although its true structure is bet- 

 ter shown by an expanded formula: 



— C— OH = —COOH 



II 



o 



The carboxyl radical is important because 

 all compounds possessing it are organic acids. 

 The hydrogen of the carboxyl is "loosely 

 connected," and this hydrogen dissociates 

 as hydrogen ion (H+) when an organic acid 

 is dissolved in water. Thus organic acids 

 may be specified by the general formula 

 R — COOH, where the R stands for the body 

 of the molecule, which varies in different 

 compounds. The dissociation of an organic 

 acid may be written: 



RCOOH?^RCOO-+ H- 



organic 

 acid 



organic 

 anion 



hydrogen 

 ion 



In the fatty acids, the "body" (R) of the 

 molecule represents a straight carbon chain 

 in which most if not all of the available com- 

 bining points are occupied by hydrogen. For 

 example, the specific formula of palmitic 

 acid (from coconut oil) is CH 3 (CH L >) ]4 - 

 COOH. Most natural fatty acids possess 

 rather "long chain" molecules, although a 

 few, such as acetic acid (CH 3 COOH), are 

 much simpler. Moreover, all naturally occur- 

 ring fatty acids are constituted by an even 

 number of carbon atoms (see above), since 

 the metabolic build-up and breakdown of 

 these molecules involves the adding or sub- 

 tracting of 2-carbon pieces to the carbon 

 chains of the molecular skeletons (Fig. 8-5). 



Plant and animal cells can synthesize fat 

 by triple dehydration synthesis, provided 

 glycerol and fatty acids are present in the 

 protoplasm, as is shown in Figure 4-11. 



When a cell uses fat as fuel, it first hydro- 

 lyzes the fat into glycerol and fatty acids, and 

 then gradually oxidizes the products. Since 

 C, H, and O are the only constituents of a 

 fatty acid, the only end products formed 

 when a fatty acid is oxidized completely are 

 CO L , and H^O. Disregarding the complex 

 intermediary stages, the total oxidation of a 

 fatty acid may be written: 



C 18 H 3 eO g + 260 2 



1 mol 26 mol 



stearic acid oxygen 



stepwise 



' oxidation 



{see Fia. 8-5) 



18C0 2 +18HoO-f- energy 



18 mol 18 mol approx. 



carbon wafer 9 Cal 



dioxide per g 



Phospholipids and Steroids. The chemical 

 complexity of phospholipids and steroids 

 precludes an adequate brief description. One 

 very common phospholipid is lecithin, a fat- 



