Which membrane protein binds a solute and changes shape to move it across the membrane?

• A. Carrier proteins
• B. Receptors
• C. Glycoproteins
• D. Channel proteins

Answer: (A)

Carrier-mediated transport describes membrane proteins that bind a specific solute on one side of the membrane and then change their shape to move that solute to the other side. When the solute binds, the protein undergoes a conformational shift that flips it across the membrane and releases the solute on the opposite side. This process can occur passively as facilitated diffusion or actively when energy is used to move the solute against its gradient. It’s saturable because only a finite number of carriers exist, so increasing solute concentration speeds transport only up to the point where all carriers are occupied. Carriers are also specific, recognizing and transporting only particular solutes.

Receptors bind signaling molecules without transporting them across the membrane, glycoproteins are mainly carbohydrate-modified proteins involved in recognition or structure, and channel proteins form pores that allow substances (often ions or water) to diffuse through without the carrier’s conformational binding-and-shift mechanism. Hence, the described behavior best fits carrier proteins.