10 THE PLANT CELL WALL 



Among useful optical methods of analysis other than those 

 considered, electron microscopy ranks high. High-voltage electron 

 beams possess very short wavelengths, of the order of 0.005 m\i, 

 when compared with light and even hard (high-voltage) X-rays. 

 Accordingly, the resolving power of the electron microscope is 

 far greater than that of the light microscope. Electron microscopy 

 has provided eloquent proof of the reticular structure of various 

 gels as inferred from indirect measurements. 



A quite different physical characterization of complex sub- 

 stances is represented by measurement of their hydrodynamic 

 properties. When solutions of polymers are forced through fine 

 orifices (capillary viscometers) or subjected to a torque (cylinder 

 viscometer) their response to shearing forces reflects their size 

 shape and molecular interactions. In the ideal hydrophilic sol, 

 viscosity results from the interaction of single molecules with 

 solvent, hence a molecule consisting of repeating units will interact 

 with solvent in relation to its degree of polymerization. Very great 

 size (length) or insufficiently diluted solutions lead to interactions 

 among solute molecules as well which complicate interpretation of 

 viscous behavior. Such interactions lead to particles which no 

 longer form a true sol consisting of single molecules, and, finally 

 to gelation. The molecules in colloidal solutions can also be char- 

 acterized by their behavior in a gravitational or centrifugal force 



field. 



The displacement of particles (sedimentation) subjected to 

 enormous angular accelerations (10 5 Xg) in the ultracentrifuge 

 is determined by their effective particle weight. Ideally, the particles 

 of the sol consist of single molecules, hence, with suitable correction 

 for densities of particle and medium, sedimentation may be related 

 simply to molecular weight. 



When colloidal particles also carry a charge, they may be 

 displaced in solution by an applied electrical field. Electrokinetic 

 behavior, or electrophoresis, is characteristic of acidic wall polymers 

 such as pectic acids or agar, but not of cellulose or other neutral 

 polysaccharides. Electrophoresis depends upon hydrodynamic and 

 properties, hence the proper analysis of displacement in an electrical 

 field must take account of viscous behavior as well. 



