CHAPTER II 



CELLULAR REGULATION 



The complexity of the priming reactions and the reactions which follow, 

 and their interrelations, lead us to ask how the direction of the molecules 

 along the various metabolic paths is controlled. Not only do the directions of 

 these metabolic reactions, all of which are proceeding at the same time, have 

 to be controlled, but also the velocities of these reactions must be regulated. 

 For example, how is it that in a cell liberally provided with substrates and 

 oxygen, respiration during a given period is no greater than the production 

 of utilizable energy ? These are a few of the questions we have to answer 

 when considering the ways in which the metabolism of the cell is regulated, 



I. FACTORS WHICH DETERMINE THE VELOCITY 

 AND THE PATH OF ENZYMATIC REACTION CHAINS 



By "reaction chain" the biochemist means a process made up of a series 

 of chemical reactions joined together in a straight or in a branching manner. 

 The idea that in a chain of reactions it is the slowest reaction which deter- 

 mines the overall speed of reaction has often appeared to be self-evident. 

 Hinshelwood illustrates this concept by comparison with the transmission 

 of telegraph messages. The speed of transmission depends upon a certain 

 number of factors — the dexterity of the operator, the speed of the current 

 along the wires, and the rapidity with which the telegraph boy delivers the 

 message. It is undoubtedly, says Hinshelwood, the last factor which is 

 the dominating one. This is no doubt true, but a series of biochemical 

 reactions differs from the sending of a telegram. As pointed out by Burton, 

 the agility of the telegraph boy does not normally depend on the number of 

 telegrams waiting at the post office, whilst the velocity of biochemical 

 reactions is influenced by the concentrations of the reactants, according 

 to the law of mass action. 



The stationary state for a biochemical reaction chain may perhaps be 

 better compared. Burton points out, to that of the current in a stream. If an 

 obstacle is placed across the stream, there will be a temporary decrease in 

 the amount of water reaching the river mouth, but as soon as the water 

 level has reached the top of the dam, a new steady state will establish itself 

 and in the last analysis the amount of water arriving at the mouth of the 

 stream will depend upon only one factor, the amount of rain falling at the 



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