Explanation

Cell membranes possess certain pores, which are called porin. These are present in the outer membrane of mitochondria, plastids and in some bacteria. These large proteins pores provide passage for small sized proteins. They are also called transport proteins, which can be divided into two types; carrier proteins and channel proteins. Carrier proteins bind to the particular substances, while channel proteins allow movement of an appropriate sized solute to be transported and this in turn is delivered to the outer side of the membrane by carrier proteins