PETER M. RAY 



University of Michigan 



Problems in the Biophysics of Cell Growth 



It has become accepted that growth of plant cells by enlargement 

 must depend upon phenomena occurring in the primary cell wall 

 which result in stretching under the force imposed upon it by tur- 

 gor pressure, and consequently, that the stimulation of growth by 

 auxin must involve an effect on the properties of the cell wall. It is 

 usual to refer to this effect as a softening or plasticizing of the cell 

 wall and to think of growth as a plastic stretching of the cell. It 

 must be remembered that this need not be a direct effect; it may be 

 exerted via metabolic processes in the protoplasm. 



The present remarks are made to help clarify our thinking about 

 cell wall growth by pointing out that at least two fundamentally dif- 

 ferent mechanisms of growth may be confused under the idea of plas- 

 tic stretching. It is convenient to illustrate these mechanisms with the 

 pectate theory of cell wall growth proposed by Bennet-Clark (1) and 

 Ordin, Cleland, and Bonner (5, 6). This does not imply a belief on 

 the author's part that the pectate theory is necessarily a factual de- 

 scription of the important events involved in cell wall growth; it 

 serves well, however, to illustrate some molecular principles which 

 appear to be generally applicable whatever the actual chemistry of 

 the growth process. 



It must be assumed that the resistance of the cell wall to stretch- 

 ing by the force of turgor is due to some type of bonding between 

 the molecular units of the cell wall, the polysaccharides composing 

 the microfibrillar material and the matrix in which this is embedded. 

 Growth, then, must involve the breaking of certain of these bonds 

 in some manner. According to the pectate theory, the critical bond- 

 ing forces are visualized as being electrostatic cross links between 

 polyuronide chains, whose free carboxyl groups have formed salts 

 with Ca+2 (Figure lA). 



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