140 



BIG MOLECULES 



ular reaction, like reactions of other randomly coiled molecules in solution, 

 is made just that much more difficult. Some very fine X-ray diffraction work 

 has been done On crystalline DNA, but even in crystalline form it may as- 

 sume several structural arrangements, depending upon the humidity. 



Molecular Structure of Living Membranes 



There are two main subjects of interest in membrane biophysics: the 

 structure of the membrane, and its penetration by small and large molecules 

 and ions. They are closely interrelated. Thus there exist, in the human 

 body, membranes which have every degree of specialization — frorn the quite 

 nonspecific mosaic membrane of the small intestine to the highly specific 

 membrane of nerve cells which not only can distinguish sodium ion from 

 potassium ion (a trick which analytical chemists have only recently learned 

 to do) but even change the rate at which it lets them through! We confine 

 ourselves here to considerations of structure only. Penetration is discussed 

 in Chapter 10. 



From analytical and electron microscopic work, it has been found (Danielli 

 and many others) over the past twenty-five years that most living mem- 

 branes*** are laminar, composed of at least three, sometimes five, layers. 

 The heart of the membrane is a bimolecular layer of lipid, flanked by thin 

 layers of protein, or cellulose, or both (Figure 6-8 (a)). The cellulose, if pres- 



cellulose 

 ond/or 

 protein layers 



bimolecular 

 fatty acid 

 layer 



Figure 6-8. Schematic Representation of Layers in the Living Membrane. For 

 many membranes the total thickness is about 75A. (a) Note the position of the 

 defect or pore, (b) Plan view of lipid film. 



ent, seems to be there simply for structural reasons — to make the membrane 

 mechanically strong. The protein layer can also provide strength. However, 

 various metal ions and water form complexes with the protein, and some 

 protein of most membranes has enzyme activity, a property which is cur- 



*** For example, the cell wall, the endoplasmic reticulum within the cell, etc. 



