Molecular Mechanisms of Transport Across Membranes

A protein engaged in the transport of a specific solute across a membrane must have two properties (1) the ability to bind the solute and (2) the ability to carry out a vectorial process, i.e., a directional process which delivers the solute from one side of the membrane to the other. 

Question What are some simple means by which the vectorial process might occur  

The transport protein could behave as a mobile carrier e.g., a large, transmembrane protein could rotate, as in Fig. 6-17(a), or a small protein could traverse the membrane as in Fig. 6-17(6). 

The transport protein could constitute a pore or channel as in Fig. 6-17(c). 

Artificial systems permitting the transport of cations have been inserted into natural membranes and into artificial vesicles. These are known as ionophores, and there are two types, exemplified by gramicidin A and valinomycin. 

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