We have seen in chapter 4 that passive diffusion of ions and small molecules across membranes occurs most efficiently when facilitated by pores, channels and carriers. Furthermore, in chapter 6 we discussed that certain gated structures had to exist for controlling the membrane permeability to ions during the generation and propagation of action potentials. These are now known as ion channels, and their presence and biological role has been convincingly revealed in electrophysiological studies even before high resolution structural studies became available. By contrast to channels and pores, ion pumps transport specific ions against concentration gradients across the membrane, and they do so by consuming energy produced in cellular processes.
The rate of channel-facilitated diffusion is 107–108 ions/s, more than 1,000 times higher than the rate of ion transport by ionic pumps. This rate is almost as high as that of free diffusion of ions in an aqueous solution. However, ion channels are not simple pipelines; instead, they possess a complex structure, which is sensitive to environmental changes. Channels present roughly two conformational states: open and close. Depending on the specific role of a channel and its location, transition between these two states (called gating or activation) may be triggered by various factors as summarized next.
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(2008). Structure and Function of Molecular Machines. In: Integrated Molecular and Cellular Biophysics. Springer, Dordrecht. https://doi.org/10.1007/978-1-4020-8268-9_7
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