QUESTION 152 (5 marks)
Three amino acids are placed at the origin in separate lanes for electrophoresis in a buffer of pH 6.0. The positive electrode is on the left and the negative electrode is on the right. Use the pI values in the table. Ignore bulk movement of the buffer.
| Amino acid | pI |
|---|---|
| A | 3.2 |
| B | 6.0 |
| C | 9.8 |
(a) Predict the average net charge and migration direction of each amino acid. [3 marks]
(b) The experiment is repeated at pH 2.0. Predict the migration directions, then explain why direction alone cannot identify the three amino acids in this second experiment. [2 marks]
Practice marking scheme
Answer
At pH 6.0, A is negative and moves left, B has zero average net charge and has no net migration, and C is positive and moves right. At pH 2.0 all three move towards the negative electrode; direction alone cannot distinguish them.
Working
An amino acid is net negative above its pI and net positive below it. At its pI it has zero average net charge and no net migration in this ideal model. At pH 2.0 the buffer is below every supplied pI, so all three amino acids are net positive and migrate right. They may have different migration rates, but no rate data are supplied and common direction does not establish identity.
Marking criteria
- Predicts A is net negative and migrates to the positive left electrode. [1 mark]
- Predicts B has zero average net charge and no net migration. [1 mark]
- Predicts C is net positive and migrates to the negative right electrode. [1 mark]
- Predicts all three migrate towards the negative electrode at pH 2.0. [1 mark]
- Explains that their shared direction cannot distinguish identities without further data. [1 mark]
Practice question aligned to the current QCAA syllabus; review the worked solution and marking criteria.
View the QCAA syllabusCompare your working with the guide above.