Nonpolar and small polar molecules can pass through the cell membrane, so they diffuse across it in response to concentration gradients. Carbon dioxide and oxygen are two molecules that undergo this simple diffusion through the membrane.

 

    The simple diffusion of water is known as osmosis. Because water is a small polar molecule, it undergoes simple diffusion. SAT II Biology problems on osmosis can be tricky: water moves from areas where it is in high concentration to areas where it is in low concentration. Remember, however, that water is found in low concentrations in places where there are many dissolved substances, called solutes. Therefore, water moves from places where there are few dissolved substances (known as hypotonic solutions) to places where there are many dissolved substances (hypertonic solutions). An isotonic solution is one in which the concentration is the same as that found inside a cell, meaning osmotic pressure in both sides is equal.

 

    Immersing cells in unusually hypotonic or hypertonic solutions can be disastrous: water can rush into cells in hypotonic conditions, causing them to fill up so fast that they burst. To combat this possibility, many cells that need to survive in freshwater environments possess contractile vacuoles to pump out excess water.

 

    Facilitated Diffusion

 

    Certain compounds important to the functioning of the cell, such as ions, cannot enter the cell through simple diffusion because they cannot pass through the cell membrane. As with water, these substances “want” to enter the cell if the concentration gradient demands it. For that reason, cells have developed a way for such compounds to bypass the cell membrane and flow into the cell on the basis of concentration. The cell has protein channels through the phospholipid membrane. The channels can open and close based on protein membranes. When closed, nothing can get through. When open, the protein channels allow compounds to pass through along the concentration gradient, which is diffusion.

 

    Active Transport

 

    Quite often, cells have to transport a substance across the cell membrane against the normal concentration gradient. In these cases, cells use another class of membrane proteins. Instead of relying on diffusion, these proteins actively pump compounds in the direction the cell wants them to go, a process that requires energy. Cells can turn active transport on and off as needed.

 

    Endocytosis and Exocytosis

 

    Cells use yet another type of transport to move large particles through the cell membrane. In exocytosis, waste products that need to be removed from the cell are placed in vesicles that then fuse with the cell membrane, releasing their contents into the space outside the cell. Endocytosis is the opposite of exocytosis: the cell membrane engulfs a substance the cell needs to import and then off into a vesicle that is inside the cell.