Final answer:
PIN proteins, or auxin efflux carrier proteins, transport the plant hormone auxin and determine its direction of movement. They are found in the plasma membrane of plant cells.
Step-by-step explanation:
Auxin efflux carrier proteins, also known as PIN proteins, play a crucial role in the movement of the plant hormone auxin within the plant. They function as transporters and are responsible for directing the flow of auxin from one part of the plant to another. They are located in the plasma membrane of plant cells, particularly in regions where active auxin transport occurs.