What makes cell membranes act like they are only SELECTIVELY permeable?
The phospholipid bilayer acts like a wall of fat surrounding the cell. This means that only those molecules that can dissolve in fat (are fat or lipid soluble) can enter and leave cells by going through the lipid bilayer portion of the cell membrane. Some molecules are more fat-soluble than others. For example, ethanol (the alcohol found in beer and wine) is relatively fat-soluble and moves quickly through the phospholipid bilayer into and out of cells. [This is why a breath test after ingesting too much alcohol detects how much alcohol one has consumed. The ethanol quickly moves through the cells in the stomach wall directly into the bloodstream (and then is carried via the blood to the upper airways of the lungs where it moves across the cells in the airway walls and is exhaled). Most consumed foods and liquids need to be processed in the mouth or stomach and then must enter the small intestine before they can be digested and absorbed into the bloodstream.] The nitroglycerine heart p
