Which of the following proteins is responsible for transporting substances across a cell membrane?

Carrier proteins are integral membrane proteins that facilitate the transport of substances across a cell membrane. They bind to specific molecules or ions on one side of the membrane and undergo conformational changes that allow them to release these substances on the other side. This process can be active, requiring energy, or passive, depending on the concentration gradient of the substance being transported.

This ability to selectively transport various substances, such as sugars, amino acids, and ions, is crucial for maintaining cellular homeostasis and communications. In contrast, enzymatic proteins primarily catalyze biochemical reactions, transport proteins often refer broadly to any protein that assists in moving molecules, and receptor proteins are tasked with recognizing and binding to specific signaling molecules, initiating cellular responses. Thus, carrier proteins are the precise answer to the question, emphasizing their role in facilitating transport across membranes.