What is the main characteristic of carrier proteins in cellular transport?

Carrier proteins are crucial components in the transport of substances across cellular membranes. Their primary role is to facilitate the movement of specific molecules, such as ions or larger polar molecules, that cannot easily diffuse through the lipid bilayer of the cell membrane due to their size or polarity. Unlike simple diffusion, which allows some substances to pass freely, carrier proteins undergo a conformational change to bind to the substance they are transporting. This process allows for selective and regulated movement of molecules into and out of the cell, making carrier proteins essential for maintaining homeostasis.

In contrast, the other options mention functions that do not align with the primary role of carrier proteins. While some transport mechanisms, like active transport, do indeed require energy, not all carrier protein functions do. Structural support is typically provided by other cellular components, like the cytoskeleton, rather than by carrier proteins. Additionally, signal transduction is generally handled by different types of proteins, such as receptors, which are responsible for transmitting signals from outside the cell to the inside, rather than transporting substances across membranes. Thus, the characteristic most representative of carrier proteins is their function in transporting substances across membranes.