I'm having a hard time understanding why my slides in my biology course have two different representations of the "basic structure" of the amino acid:


enter image description here


enter image description here

The top one seem like what I would expect. The bottom one, it looks like the amine group gained a Hydrogen ion and the Carboxyl group lost one. Also, the center carbon gained an H. I honestly don't know if this description is even correct or not.

I'm extremely new to this level of biology, so very simple explanations are much appreciated. I just don't totally understand why there are two versions of the basic structure.

Also, I apologize if the tag is wrong, again, this isn't my major, so I'm very new to this material.

  • $\begingroup$ Ugly pictures in both cases :) $\endgroup$
    – Alchimista
    Jan 17, 2019 at 21:05

1 Answer 1


Your description is more or less correct, but so are both images. The bottom images shows the predominant form the amino acid would take at neutral pH (or any pH between ~2 and ~9.5). This is because the caroboxylic acid (-COOH) and amino (-NH2) groups are a weak acid and base and can lose or gain a proton (hydrogen cation), respectively. pH below the pKa of the functional group (~2 for carboxylic acid and ~9.5 for amino) favours protonation whereas pH above the pKa favours deprotonation. Also note that the "centre carbon" (the α-carbon) did not gain a proton in the second image, it's just drawn differently.

You could try this website for an overview of acid/base chemistry and perhaps these Wikipedia sections on zwitterions and isoelectric points with respect to amino acids, though there are many alternative sources of material available if you search for them. This question and answer may also be useful for you, at least eventually.

  • 1
    $\begingroup$ To expand slightly on this answer, this is valid for these sorts of chemical structures more generally in organic chemistry, too. When you see groups like these that act as weak acids/bases, you should think of them existing chemically in both forms, with the ratio dependent on the pH. People will switch back and forth between them freely because that's how the molecules actually exist, as well. $\endgroup$
    – Bryan Krause
    Jan 17, 2019 at 20:27
  • $\begingroup$ Thanks again for this, I now see the center H as just being drawn differently. I must've been overwhelmed enough to miss that. $\endgroup$ Jan 24, 2019 at 7:18
  • $\begingroup$ @joe_04_04 You’re welcome. $\endgroup$
    – canadianer
    Jan 24, 2019 at 7:53

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