What type of protein allows specific charged particles and polar substances to diffuse through the membrane?

The correct answer is channel protein. Channel proteins are integral membrane proteins that form pores or channels in the cellular membrane, allowing specific ions and polar molecules to pass through. These proteins facilitate the diffusion of charged particles such as sodium, potassium, calcium, and chloride ions, as well as polar molecules like water, through the lipid bilayer, which otherwise would not allow them to pass freely due to the hydrophobic nature of the membrane’s interior.

Channel proteins are selective, meaning that they typically allow only certain types of ions or molecules to diffuse through, which is essential for maintaining the cell's homeostasis, conducting electrical signals in nerve cells, and transporting nutrients and waste products. Their function relies on concentration gradients, allowing substances to move passively down their concentration gradient without the expenditure of energy.

Other types of proteins mentioned have distinct roles. Receptor proteins are involved in signaling by binding specific ligands, carrier proteins facilitate the transport of substances across membranes but often involve a change in the protein's shape to move the substance, and transport proteins is a broader category that can include both channel and carrier proteins. Channel proteins specifically highlight the pore-forming role critical for the passive diffusion of specific charged particles and polar substances.