Final answer:
If both glycine and alanine are present, they compete for the binding sites on carrier proteins, which can decrease the transport rate for either amino acid.
Step-by-step explanation:
When a carrier protein can transport two closely related substances, such as the amino acids glycine and alanine, the presence of both can affect the rate of transfer for either substance. Carrier proteins have a finite number of binding sites, and when these are occupied, the proteins are considered saturated. If glycine and alanine are both present in the environment of the carrier protein, they will compete for the available binding sites which can potentially decrease the rate of transport for either amino acid when compared to the rate when only one is present.
This competitive interaction is because each carrier protein is specific to a single solute or a single family of closely related solutes. The cell regulates these proteins through various means, such as allosteric effectors, which are molecules indicating the cell's status and need to import or export the solute. In this case, when glycine and alanine are both present and competing for transport, the overall transport efficiency for each amino acid may decrease since the availability of binding sites on the carrier proteins becomes limited.