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ANIMAL BIOCHEMISTRY 



pression of a coil spring as shown in Figure 13-1. Other workers have 

 suggested that a combination of positive and negative groups in the 

 protein is involved in the contraction and that AIT functions to 

 prevent the interaction within the molecule through the contribution 

 of the three negatively charged groups. 



The sliding filament theory is based on the observation that actin 

 and myosin must be bridged or combined in order to form contractile 

 threads. By selective treatment it is possible to remove the myosin 

 and the actin portions separately, and by use of the electron micro- 

 scope to determine the dimensions of each unit in the myofibril. 

 Studies of electron micrographs show no evidence of significant de- 

 crease in length of myosin or actin during contraction. If actin and 

 myosin slide by each other forming new points of attachment (Figure 

 13-2), muscle contraction or expansion can occur without assumption 

 of helix expansion and contraction. ATP again is presumed to func- 

 tion in the formation of the oscillating bridge. All the theories 

 advanced agree that hydrolysis of ATP into ADP plus inorganic 

 phosphate provides the energy required for muscular contraction, but 

 little progress has been made toward explaining the mechanism of 

 the conversion of about 8,000 cal./mole of chemical energy into 

 mechanical energy. 



The ATP required for muscle contraction is derived from aerobic 

 oxidation of glycogen or other food reserves. In some muscles, flight 

 muscle of blowfly, for example, the myofibrils are separated by layers 

 or rows of mitochondria for rapid oxidation of food reserves. Under 

 anaerobic conditions the soluble glycolysis enzymes metabolize carbo- 

 hydrates to the lactic acid stage. This acid accumulates until aerobic 

 conditions retian, when lactic acid is metabolized completely. 



During muscle contraction there is a movement of potassium ion 

 from the muscle cells to the extracellular fluid (Figure 13-1). On 



ATP 



K" 



K" 



K" 



FIGURE 13-1. Schematic drawing of muscular contraction through helix or "coiled 

 spring" compression. The nature of the forces binding the potassium ions to the 

 extended muscle is unknown, but some reorientation of the polar groups on the 



molecule is necessary to release them. 



