How do you know how to ionize an Amino Acid?

My teacher said something about pK 2 and pK 9 is when they are ionized...i think. But what happens at those two pK values for the majority of amino acids? Like I know COO- would become COOH but when? and When does NH3+ turn into NH2? I've looked online so if someone could explain it in "normal english" that would be greatly appreciated!