The isoelectric point is the pH where the amino acid solution is electrically neutral. In acidic conditions, the carboxylic acid and amino terminus will both be protonated. This results in a positive charge (due to the amine being protonated). In basic conditions, both ends are deprotonated, resulting in a negative charge.

If an amino acid is basic, that means that the pH must be above 7 in order to deprotonate the amine in the side chain. Only then will the amino acid be electrically neutral. All basic amino acids (three of them) have an isoelectric point above a pH of 7. All other amino acids have an isoelectric point at a pH below 7.

The amino acid is polar, so we do not need to worry about charges in the side chain. Since the amino acid is starting in a highly basic solution, we know that the amino acid is deprotonated at both termini. That is, the amino terminus is neutral and the carboxyl terminus is negative. This results in a net charge of -1 (at the carboxyl end). The first half equivalence point upon titration will be seen when half of the amino acids are neutral and half of the amino acids are negatively charged. The full first equivalence point will show all molecules with a neutral charge, while the second full equivalence point will show all molecules with a positive charge due to protonation of the amine.

Only basic amino acids have an isoelectric point above a pH of 7. All others will have an isoelectric point below a pH of 7.

Since the amino acid is polar, it will have an isoelectric point below a pH of 7. Since a pH of 1 is extremely acidic, we would expect both ends of the amino acid to be protonated at that pH. As a result, a pH of 5.4 is more appropriate when predicting the amino acid's isoelectric point.

Serine and threonine are classified as hydrophilic amino acids and contain hydroxyl (-OH) groups in their side chains. Tyrosine, although it is considered hydrophobic, does contain a hydrophilic hydroxyl group in its side chain. The answer is I, IV, and V, as all of these contain hydrophilic functional groups.

Phenylalanine:

Since the phenylalanine residue is at the C-terminus end of the molecule, only its carboxylic acid pK_a is relevant, as its amine is involved in a peptide bond with arginine. At a pH of 7 (well above the carboxylic acid pK_a of 2.58), the C-terminus carboxylic acid would be deprotonated and have a charge of .  

Arginine:

For the middle amino acid, arginine, the only relevant pK_a is that of its side chain since both its carboxylic acid and amino groups are involved in peptide bonds with neighboring amino acids. Since the side chain of arginine would be protonated at a pH of 7 (well below the sidechain pK_a of 12.48), this amino acid would have a charge of .  

Valine:

Finally, for valine, the relevant pK_a to consider is the NH₂ pK_a of 9.72. At pH 7, this would also be protonated, resulting in a charge of .  

The overall charge of the molecule at pH 7 would be .

One mole is base is required to deprotonate each carboxylic acid or amine, until the amino acid exists in its most deprotonated and basic form. First, two moles of base would be needed to deprotonate the two acid groups on glutamic acid (one depicted on each end). Next, a third mole would be needed to deprotonate the amine to its neutral state. Three total moles is the correct answer. 

The resonance of the two C–O bonds that result after deprotonation of an organic acid is the major contributor to anion stability. 

A depletion of amino acids would immediately impact protein and enzyme production in the cell. All proteins, including enzymes, are made from chains of amino acids. Lack of nutrients, such as amino acids, would eventually impact other processes listed.

Transmembrane proteins are embedded within the phospholipid bilayer of the cell membrane; therefore, the protein is exposed to the nonpolar fatty acid tails, the polar phospholipid heads, and the polar environments of the cytoplasm and extracellular space.

The questions asks which amino acids would be found facing the cytoplasm. Because the cytoplasm is polar, the amino acids interacting with the cytoplasm must also be polar. Glutamine, threonine, and tyrosine are all polar amino acids, making this the best answer. The other answers contain nonpolar amino acids (proline, leucine, cysteine, valine) causing these answers to be incorrect.

The question states that the membrane-spanning region of the transmembrane protein is stained red; therefore, you will find only hydrophobic amino acids in this region. Recall that an amino acid has a central carbon that has a hydrogen group, carboxylic acid group, amino group, and a side chain attached. The differences between amino acids arise from the different side chains.

A hydrophobic amino acid contains nonpolar side chains. Glycine and alanine contain  and  side chains, respectively. Both of these amino acids will have nonpolar properties and be found in the red region of the protein. Cysteine contains , which is a polar side chain, and will be found in the blue region.