IO2 Practical Plant Biology. 



light passing through it. This surprising observation led Pasteur 

 to the discovery that the tartaric acid which he supplied to the 

 plant was a mixture of two substances resembling each other 

 chemically, both having the empirical formula (HCOH) 2 (COOH) 2 , 

 but differing from each other in the physical characteristic that 

 one rotates the plane of polarisation clockwise and the other 

 counter-clockwise. In the mixture these two effects counter- 

 balance each other and the polarised beam is not rotated. But 

 when Penicillium is in contact with the mixture it assimilates the 

 tartaric acid which rotates the plane to the right (^-tartaric acid) 

 while it practically leaves untouched that which rotates the plane 

 of polarisation in the opposite direction (/-tartaric acid). Hence, 

 it may be assumed that Penicillium possesses an enzyme which 

 can render df-tartaric acid suitable for its metabolism while this 

 enzyme is ineffective with /-tartaric acid. The discovery of the 

 two forms of tartaric acid revealed by the selective action of 

 Penicillium led to the further discovery of a large number of 

 other carbon compounds in the molecules of which the atoms and 

 atomic groups are capable of being arranged in several different 

 ways round certain of the carbon atoms. Distinctive physical 

 properties depend on these arrangements while the individual 

 atoms and atomic groups of these substances, which are called 

 stereo-isomers, remain the same. Thus it becomes very probable 

 that the means by which Penicillium can deal with dextro-tartaric 

 acid is ineffective with laevo -tartaric acid because it fits the atomic 

 arrangement of the one and does not fit that of the other. This 

 case and others of a similar nature have led to the " Lock and 

 Key" theory of enzyme action. According to it the specific 

 action of an enzyme on a certain substance depends on the fact 

 that the spatial arrangement of the atoms and atomic groups in the 

 enzyme-molecule fit in some way the arrangements of the atoms 

 in the substance acted upon. 



Comparatively simple nitrogen compounds form a suitable 

 supply of nitrogen for Penicillium. Thus it can utilise amido- 

 compounds and peptones. 



We have seen that fungi resemble animals in their nutrition. 

 They are both dependent on organic carbon compounds to supply 

 them with the carbon they require, and they are as a rule unable 

 to utilise inorganic nitrogen as a supply of that element. Hence, 

 they are classed together as holozoic organisms. Many fungi, 

 however, can utilise much simpler nitrogenous compounds than 

 animals. Thus yeast can utilise ammonium tartrate as a source 

 of nitrogen, while it is found that Penieillium thrives on ammonium 



