They may support either the inner or the outer surface. They may be contained partially within one of the surfaces, or they may extend completely through the lipid bilayer. Some of these proteins are highly specialized molecules that capture specific substances from the bloodstream and transport them through the lipid barrier. Those proteins that extend through the membrane may actually transport substances through pores within themselves. Other proteins may combine with a substance at one membrane surface, diffuse across the lipid bilayer, and dissociate from the substance at the other surface.
Transport is proportional to blood concentration.
If one amino acid is proportionally higher in concentration it is preferentially transported through the membrane. How simple it would be if brain concentrations could be increased just by the ingestion of pharmaceutical serotonin or, for that matter, any of the many foods which contain serotonin. Unfortunately, this is not the case. In the bloodstream, at physiological PHI the serotonin molecule exists in the ionized form. An electrical charge from the hydroxy group on the indole ring creates polarity in the molecule.