Final answer:
The pKa of aspartic acid's second carboxyl group increases after the first proton is lost, and a decrease in pH outside the cell can either increase or decrease the transport of amino acids into the cell, depending on the transport mechanism and amino acid charge.
Step-by-step explanation:
In aspartic acid, the presence of two carboxylic acid groups does not necessarily cause an increase in the pKa. Instead, for a polyprotic acid like aspartic acid, the acid strength decreases and the pKa values increase with the sequential loss of each proton. The second carboxyl group has a higher pK value after the first proton is lost because of increased electronic repulsion and stabilization of the remaining negative charge on the molecule.
When addressing the effect of pH on amino acid transport into cells: If the pH outside the cell decreases, making the environment more acidic, the transport of amino acids into the cell may either increase or decrease depending on the specific transport mechanism and how the amino acid charge state is affected by the pH change. For instance, amino acids that exist in a more favorable charged state for transport at lower pH levels may see increased uptake into the cell.