Found in both plant and animal cells, the cell membrane is the outside wall of a cell. In plant cells, it is a second wall, and is found just inside the main cell wall. The cell membranes found in animal cells contain a chemical called cholesterol. This chemical makes the membrane harder. Plant cells do not need cholesterol because they have a cell wall, as a result, their cell membranes are softer.
In order for a cell to remain healthy, the cell needs to be able to bring in food, and get rid of waste. Look closely at this picture. You will notice that the cell membrane has small openings, or doorways. These openings allow the cells to move materials in and out of the cell.
Think of your home, or school. On the outside of the building are walls. These walls protect the building from danger and from weather. At various locations on the building are small openings that allow food, people, electricity, and water to enter. There are also openings that allow things to leave, including waste. Without these openings, the building would not be much use.