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The plasma membrane of a cell is a boundary between the cell and its environment. One of the functions of the plasma membrane is to allow a steady supply of nutrients to come into the cell no matter what the external conditions are. One process that helps the cell to transport materials is selective permeability. This is a process which allows the plasma membrane of a cell to allow some molecules into the cell while keeping others out. Think about a screen in a window and how it allows air and light in but insects and other critters out of your house. If membranes show selective permeability, what might happen if a funnel is filled with starch molecules, and a selectively permeable membrane is stretched across the mouth of the funnel and the funnel is inverted into a beaker filled with iodine molecules? Starch molecules are too large to diffuse across the membrane; iodine molecules are not. If iodine reacts with starch, the starch turns blue-black. Suppose you observe the setup for several hours. In which direction did the iodine move? What is your evidence? In which direction did the starch move? What is your evidence?