Cell membranes are semi-permeable, because they are made up of lipids, and some proteins. The lipids are called phospholipids, which are similar in structure to fats, but a phosphate group is attached to the third carbon of the glycerol molecule. As a result, a phospholipid has a hydrophilic "head" and a hydrophobic "tail." A double layer, otherwise known as a bilayer, forms the fundamental structure of the plasma membran of organell membranes. Integral protens protrude into the inner part of the cell as well as the outside. Proteins in the membrane act like tunnes, allowing certain molecules (or very small ones) into and out of the cell. The entire membrane is always changing. Things can diffuse into the membrane, such as water (through osmosis). The membrane can block the passage of things into or out of the cell by peripheral proteins. Therefore it is semi-permiable. Meaning, basically, things can get into and out of it, but they can also be blocked.