The Plasma Membrane's main job is to allow and not allow other molecules into the cell. The video here will go in more detail and describe the process by which transport occurs.
Now that you know how things move in and out of the cell we can talk about why this important. Your body is always fighting to maintain the correct internal conditions, if these conditions change too dramatically it could be fatal. For example when your body is in extreme cold you can get hypothermia. But no need to worry the body has processes which can help, like sweating when you get hot to cool you down. This internal condition or balance is called homeostasis and each individual cell is trying to maintain it.
From a cell to maintain homeostasis four things need to be correct, the temperature, the pH and glucose levels, and the amount of water. First cells maintain the correct temperature by communicating with other cells by chemical signals sent by the cells membrane. To maintain pH and glucose levels the cell membrane can only allow certain molecules to pass through or to be transported by proteins. And lastly water levels, are dependent on the solution around the cell because water can pass without trouble through the cell membrane. Water always more from an area with a high concentration of water to an area with a low concentration. If the cell is in a hypotonic solution (a solution with less solutes) the cell will flow into the cell and eventually the cell will explode. The opposite called hypertonic will shrivel the cell. If a solution has equal solutes to the cell then water will flow in and out equally, maintaining homeostasis.
From a cell to maintain homeostasis four things need to be correct, the temperature, the pH and glucose levels, and the amount of water. First cells maintain the correct temperature by communicating with other cells by chemical signals sent by the cells membrane. To maintain pH and glucose levels the cell membrane can only allow certain molecules to pass through or to be transported by proteins. And lastly water levels, are dependent on the solution around the cell because water can pass without trouble through the cell membrane. Water always more from an area with a high concentration of water to an area with a low concentration. If the cell is in a hypotonic solution (a solution with less solutes) the cell will flow into the cell and eventually the cell will explode. The opposite called hypertonic will shrivel the cell. If a solution has equal solutes to the cell then water will flow in and out equally, maintaining homeostasis.