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|Chapter Three: Cell Structures|
There are many ways in which a material can pass through the membrane. One such method is simple diffusion, which only occurs for small, nonpolar molecules (for example CO2 or O2). These molecules are small enough to squeeze between the phosphate heads of the phospholipids. Small polar molecules can also pass through, but usually the nonpolar fatty acids in the membrane repel them. The rate at which the small nonpolar molecules pass through is based upon the difference in concentration of that molecule on either side of the membrane.
The second method by which molecules can move through the cell membrane is by passive transport. Passive transport allows highly polar molecules to move through the fatty acid layer which would normally not permit them to. One form of passive transport utilizes protein channels, whereby protein molecules in the membrane form a tunnel through which polar molecules may diffuse without ever coming in contact with the fatty acids. A second type of passive transport is known as facilitated diffusion. In this process, proteins called carrier proteins bond with the molecule on one side of the membrane, move through the membrane, and then release it on the other side. Like enzymes, carrier proteins usually bond with a specific molecule, but little else is known about them.
In simple diffusion and passive transport, the cell did not expend any energy since the processes occurred naturally by diffusion and the free movement of proteins in the cell membrane. Another type of transport, called active transport, requires an input of energy by the cell. For example, to prevent too much water from entering the cell and causing it to burst, some cells have special structures called contractile vacuoles (discussed later in this chapter) which pump the water out.