What type of proteins are responsible for carrying polar molecules across the cell membrane?

Carrier proteins are specifically designed to facilitate the movement of polar molecules across the cell membrane. These proteins undergo conformational changes to bind specifically to the polar molecules they transport, enabling them to cross the hydrophobic lipid bilayer of the membrane. This process is crucial for the uptake of essential substances, such as glucose and amino acids, which cannot diffuse freely through the membrane due to their polarity.

Transport proteins is a broader term that encompasses various types of proteins involved in moving substances across the membrane, including channel proteins in addition to carrier proteins. Signal proteins, on the other hand, are primarily involved in communication within and between cells and do not serve the function of transporting molecules. Structural proteins provide support and shape to cells and tissues, but they do not facilitate the movement of substances across membranes. Hence, the specificity of carrier proteins for polar molecules makes them the correct answer for this question.