How does the cell membrane control movement across it?

The cell membrane is a phospholipid bilayer, meaning there are two chains. Each chain has a serious of long non-polar tails (hydrophobic, hate water, non-permeable to polar molecules) with polar heads (hydrophilic, love water, permeable to polar molecules). The polar heads are on the outside of the membrane and the non-polar tails face the middle of the membrane. This structure prevents molecules from freely moving across the membrane. Embedded within the membrane are protein channels that are designed to specifically allow certain molecules through. Some of these channels open randomly, others require some kind of signal. They are selective by allowing molecules of certain size and/or charge (positive or negative) cross the membrane, also keeping in mind concentration gradients (molecules will move down their gradients). In some cases, molecules move against their gradients, but require energy to do it.How does the cell membrane control movement across it?
The cell membrane is composed of a bilayer of phospholipids. Latticed throughout this layer is a number of protein molecules and other fats (esp. cholesterol). The amount of cholesterol in the cell membrane is one factor that affects the permeability of the membrane (more cholesterol = less permeability). The proteins in the membrane are used especially for transporting larger molecules and ions that could not naturally diffuse through the lipid bilayer. Some proteins also act as pumps, actively transporting substances against their concentration gradient (i.e. sodium and potassium in neurons). Simple molecules like oxygen and carbon dioxide are able to diffuse straight through the lipid bilayer.How does the cell membrane control movement across it?
the cellmembrane is aslective membrane which control on materials that inter or get away from the cell also it has some bores on it