BIOLOGICAL ORDER 



the bacterium is selective permeability. A bacterium is not perme- 

 able to everything. And it was discovered that the penetration inside 

 the bacterium of many small molecules is mediated by a given 

 specific enzyme called permease. There is one specific permease 

 for the amino acid tryptophan, another for the amino acid methio- 

 nine, another for the sugar glucose, another one for lactose, etc. 

 (Figure 13). 



lactose -^♦glucose + galactose 



— n o D — cr 



, •^gal • 



I I I > O 



1+ z+ y+ ^ 



o • • ' 



v^ O O O 



permease 



^ /3- galactosidase 



Figure 13. The Utilization of Lactose. 



Schematic representation of a bacterium. The two structural genes y and z 

 control the synthesis of the /3-galactoside permease and the ^-galactosidase, 

 respectively. 



The permease is responsible for the specific transfer of lactose and other 

 /3-galactosides across the membrane; the /3-galactosidase splits lactose into 

 glucose and galactose. The gene i controls inductivity versus constitutivity. 



Thus, the metabolism of lactose is controlled by a specific per- 

 mease. Its role is just to take the molecules of lactose in the environ- 

 ment and to transfer them across the membrane, a reaction in which 

 an acetylation is perhaps involved. Once lactose has been pumped 

 into the cytoplasm, it is split into glucose and galactose by the 

 enzyme ^-galactosidase. Our original model bacterium possesses 

 both permease and /?-galactosidase. Both enzymes are synthesized 

 only when lactose is present. Lactose is an inducer. Accordingly, 

 the enzymes are said to be inductive. Inductive means here, as in 

 logic, "based upon or using induction." 



[40] 



