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Amino Acid Quiz
Ten-question rounds on the 20 standard amino acids: names, codes, structures, classes, charge at a chosen pH and pI. Missed items come back later in the round.
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How to use this quiz
Amino acids are the most reliable source of biochemistry points on the MCAT because every question rests on the same few facts: the name and code of each residue, what its side chain looks like, and what charge it carries in a given solution. This quiz drills each of those in short rounds. Pick a single mode to focus, or leave it on mixed to practice recognizing a residue from any angle.
The structure mode draws each residue as a zwitterion, with a positive ammonium group and a negative carboxylate. The charge mode asks for the net charge of the free amino acid at a pH from 1 to 14. The tool computes every answer from the pKa values with the Henderson-Hasselbalch relation, adding up one term per ionizable group. The pI mode asks for the pH where the net charge is zero. For an amino acid with a neutral side chain, that is the average of the two backbone pKa values. For an acidic one, average the two lowest pKa values. For a basic one, average the two highest.
Questions you miss return a few questions later, and items you keep missing are drawn more often, so a round spends its time where you are weakest. Classes follow a common five-group scheme (nonpolar, aromatic, polar, acidic, basic). Textbooks disagree about a few residues such as Tyr, His, Cys and Gly, so follow the scheme in your own course.
Want the explanation behind the numbers? Read Amino Acid pKa Values and Charge at pH 7.4, then practice the arithmetic in the pKa and charge trainer.
Questions
Which pKa values does the quiz use?
Typical textbook values: about 2 for the alpha carboxyl group, 9 to 10 for the alpha amino group, and side-chain values of 3.9 (Asp), 4.1 (Glu), 6.0 (His), 8.3 (Cys), 10.1 (Tyr), 10.5 (Lys) and 12.5 (Arg). Sources differ by a few tenths, so use the numbers your course or passage gives you.
How is charge at a chosen pH calculated?
For each ionizable group the tool computes the fraction charged with the Henderson-Hasselbalch relation, then sums the charges. Acids count as -1 when deprotonated and bases as +1 when protonated. The answer is rounded to the nearest whole number only when the sum is clearly near an integer.
How is the isoelectric point (pI) found?
The pI is the pH where the net charge is zero. The tool finds it numerically, and the explanation shows the shortcut: average the two pKa values on either side of the neutral form.
Does the quiz save my scores or collect personal data?
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