Answer
The plasma membrane is a barrier between extracellular and intracellular fluids. This barrier allows some substances to enter or leave cytosol easily with various degrees of difficulty and some not at all. Because the membrane, so to speak, selects which solutes to admit, it is selectively permeable. This selectivity is based to a large degree on the hydrophobic nature of the lipid bilayer and on the integral proteins of the plasma membrane.
Work Step by Step
The plasma membrane allows some solutes to enter the cytosol easily by simple diffusion. Some of these solutes are nonpolar, uncharged
molecules like carbon dioxide, oxygen, and steroids. Small uncharged, non-poplar molecules like water and urea pass through the membrane with less ease, but ions and large uncharged polar molecules like glucose, fructose and galactose need the aid of special membrane proteins to be able to pass through the plasma membrane. Two types of proteins which function in the facilitated diffusion that allows membrane permeability of these molecules are the gated channel proteins and the carrier or transporter proteins. The former opens up channels or pores in the membrane that permit passage of the relevant moities; transporter, or carrier, proteins bind with the relevant solutes on one side of the membrane and deliver them to the fluid on the other side.
