CARBOHYDRATE METABOLISM 



451 



METABOLIC INTERRELATIONSHIPS BETWEEN 

 CARBOHYDRATE, FAT, AND PROTEIN 



Connections between the metabolic systems for carbohydrates, fats, 

 and proteins have been mentioned in a number of earlier sections. 

 These linkages involve aspartic and glutamic acids, alanine, glycerol, 

 and acetyl coenzyme A. They are represented schematically in Figure 

 18-2. As shown here, the arrows indicate the direction of catabolism, 

 the breakdown of foods, and intermediates for the energy they con- 

 tain. Anabolism is the opposite aspect of metabolism and refers to 

 the synthesis of the cellular components used for other purposes 

 than solely as sources of energy. 



pentospphosphatc 

 pathway 



Glycerol 



+ 



fatty acids 



Polysaccharides 

 digestion 

 sugars 



phosphorylation 

 sugar phosphates 

 glycolysis 

 pyruvate 



-^- acetyl coenzyme A -^ 



Proteins 



digestion 

 amino acids 



digestion 



tricarboxylic 

 ^^^ acid cycle 



-Oo 



CO2 + H2O + energy 



FIGURE 18-2. The relationships between the metabolisms of carbohydrates, proteins, 

 and lipides. In most cases each arrow represents a complex system of reactions. 

 These processes are outlined individually elsewhere in the book. Energy is actually 

 recovered from several individual steps not shown in the figure and is not limited to 



the bottom stage indicated. 



