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Title: Biophysics: concepts and mechanisms
Identifier: biophysicsconcep00case (find matches)
Year: 1962 (1960s)
Authors: Casey, Edmund Jeremiah, 1924-
Subjects: Biophysics
Publisher: New York, Reinhold Pub. Corp
Contributing Library: MBLWHOI Library
Digitizing Sponsor: MBLWHOI Library

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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
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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.

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