What are cell membranes composed of? Why is it important for the cell membrane to be semi-permeable?

With few exceptions, cellular membranes - including plasma membranes and internal membranes - are made of glycerophospholipids, molecules composed of glycerol, a phosphate group, and two fatty acid chains. Glycerol is a three-carbon molecule that functions as the backbone of these membrane lipids

Cell membranes are semipermeable, which means molecules can move through them. This is pretty important for cells to survive. Osmosis is where solvent molecules (usually water) move from one side of a cell membrane to the other. This happens because the concentration of a solute is higher on one side.