All of the 20 common amino acids have at least pka values

Amino acids are weak acids that have at least 2 pka values. They have two titratable protons that composed of alpha carboxyl (-COOH) and alpha amino (-NH3+), but some amino acids have a third titratable protons with ionizable groups in the R side chain.  In addition, the structure of amino acids in peptides and proteins are determined by the charges of the titratable group and by the R group. 

b is the correct answer

the solute can move "downhill," from regions of higher to lower concentration, relying on the specificity of the protein carrier to pass through the membrane. this process is called passive transport or facilitated diffusion, and does not require energy.