zwitterion refers to a molecule that has both a positive and a negative charge but is still neutral over all (an amino acid has a positive charge on the nitrogen and a negative charge on the oxygen).
amphoteric refers to a molecule that can act as either a base or an acid, depending on what it is reacting with. (water is an example)
H2O + NH3 ---> OH- + NH4+ (here water acts as an acid)
H2O + HCl ---> H30 + Cl- (here water acts as a base)
Exactly, and to expand upon this in an MCAT relevant fashion an example of an amphoteric molecule that forms zwitterions in biological systems, we have amino acids.
Amino acids have an amino reside and a carboxylic acid residue:
H2NCHRCOOH
The amine reside and the carboxylic acid residue have different pKa values, so they'll be protonated and deprotonated at different pH's.
Lets say that the pKa of the amine residue is 10 and the pKa of the carboxylic acid residue is 3.
At pH > 10 the amine will not be mostly protonated, but the carboxylic acid will lose a proton. So, the overall charge of the molecule is negative.
H2NCHRCOO- ******** (see note below!!!!)
At 3<pH<10 the carboxylic acid residue will still lose protons, and the amine group will start to pick them up here. So, the carboxylic acid reside is a carboxylate and is negatively charged, and the amine residue is an ammonium and is positively charged, thus it is a zwitterion because it has two residues that are both charged but its overall charge is neutral ******** (see note below!!!!).
+H3NCHRCOO-
At pH<3 the carboxylic acid will no longer lose protons to the solution, so will be neutral, but the amine will be protonated and exist as an ammonium, so the overall charge of the molecule is now positive.
+H3NCHRCOOH ******** (see note below!!!!)
Oh yeah, and know that the pI (where it will exist as a zwitterion) is the average of the pKa of the acid and the pKa of the base residues.
******** Note: this moreso means that there are lots of zwitterions in this range between 3 and 10 pH, but there will be a predominant form that is not a zwitterion b/c there will still be proton losing and gaining going on. In reality, below the pI (6.5 in this case) the amino acid will be protonated and positively charged more often, and is therefore considered to be positive below the pI. Likewise, above the pI the amino acid will be deprotonated and negatively charged more often, and is therefore considered to be negatively charged.
Also note that at "exactly" pKa amine and pKa carboxylic acid those are half equivalent points, and for the amine the concentration of protonated amine is equal to the concentration of deprotonated amine. For the carbox pKa at that point the carboxylic acid concentration is equal to its conjugate base, the carboxylate anion. So, although I gave "protonated / deprotonated" etc examples, in reality its more like a "gradient" of how the amino acids are charged, and how many are zwitterions, etc, but at the levels I indicated you can determine with relatively easy about how many zwitterions will exist, or what form of the AA predominates.
