Beginning around the turn of the last century, a number of scientists began working to define the nature of cell’s boundary layer. In the 1930's it was noted that small, water soluble molecules entered cells faster than predicted based on the assumption that the membrane acts like a simple hydrophobic barrier - an assumption known as Overton's Law. Collander et al., postulated that membranes were more than simple hydrophobic barriers, specifically that they contained features that enabled them to act as highly selective molecular sieves. Most of these are features are proteins (never fear, we are getting closer to a more thorough discussion of proteins) that can act as channels, carriers, and pores. If we think about crossing the membrane as a reaction, then the activation energy of this reaction can be quite high for highly hydrophilic and larger molecules, we will need a catalyst to reduce it to that the reaction can proceed. There are two generic types of membrane permeability catalysts: carriers and channels.
Carrier proteins are membrane proteins that shuttle back and forth across the membrane. They bind to specific hydrophilic molecules when they are located in the hydrophilic region of the membrane, hold on to the bound molecule as they traverse the hydrophobic region of the membrane, and then release their “cargo” when they again reach the hydrophilic region of the membrane. Both the movements of carrier and cargo across the membrane, and the release of transported molecules, are driven by thermal motion (collisions with other molecules), so no other energy source is necessary. We can write this class of reactions as: