CONCEPTS AND TERMS 



All the above points are provided for if we adopt the 

 hypothesis that facilitated diffusion involves movement through 

 a pore or slit composed of the polar groups of protein lamel- 

 lae . . . The junction between two protein lamellae will not 

 be a simple aqueous pore: it will be a region composed of polar 

 groups and including a good deal of water, as is the case with 

 protein crystals, and extensive hydrogen bonding between the 

 protein chains will give it a unique character. The properties of 

 this polar pore will include: 



(i) ready permeability to small hydrogen-bond-forming 

 molecules such as water and formamide. 



(ii) if the protein component is positively charged, e.g. if it 

 were haemoglobin, it would be selectively permeable to small 

 anions, and thus provide the facilitated diffusion mechanism in 

 red cells suggested by Davson. If negatively charged it would be 

 selectively permeable to small cations. 



(iii) to larger polar molecules the pore would be permeable 

 only if the structure and configuration of the molecule con- 

 formed to the structure of the pore. 



(iv) passage through such a pore need not occur by move- 

 ment of the penetrating molecule only. We can envisage the 

 protein components of such pores oscillating between different 

 configurations. Examples of such oscillations are found in re- 

 versibly denatured proteins. Such oscillations may assist in con- 

 veying molecules through the membrane. 



(v) a pore of this nature offers a basis for working out pos- 

 sible modes of action of hormones, such as insulin and "growth" 

 hormone, which are concerned in transfer processes. 



(vi) a pore of this character provides a mechanism which will 

 permit proteins to pass through plasma membranes. The possi- 

 bility of such passage would depend upon the specific configura- 

 tions of the proteins of the pore and of the permeating protein, 

 and a mechanism of this type may account for selective per- 

 meability to proteins of the type reported by Brambell and 

 Hemmings for the passage of antibodies through the intestinal 

 wall, etc. 



(vii) pores of this character would not only exert the selec- 

 tivity characteristic of facilitated diffusion, but would also be 

 susceptible to the action of enzyme poisons. . . . 



In short, a pore structure of this type appears to provide an 

 excellent working hypothesis for study in connection with facili- 

 tated diffusion. The components of the pore need not be entirely 

 restricted to protein, but might include nucleic acids, polysac- 

 charides, etc. This conception has the additional advantage that 

 by simple extension the mechanism of facilitated diffusion be- 

 comes a mechanism of active transport. Where movement of the 

 penetrating species is determined by the kinetic energy of alter- 

 native structures under the influence of thermal agitation, the 



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