The cells of all organisms, prokaryotic and eukaryotic alike, are surrounded by a thin sheet called the cell membrane. This barrier keeps cellular materials in and foreign objects out. The membrane is key to the life of the cell. By regulating what gets into and out of the cell, the membrane maintains the proper chemical composition, which is crucial to the life processes the cell carries out.

The cell membrane is made up of two sheets of special fat molecules called phospholipids, placed on top of each other.

This arrangement is known as a phospholipid bilayer. Phospholipid molecules naturally arrange in bilayers because they have a unique structure. The long chains of carbon and hydrogen that form the tail of this molecule do not dissolve in water; they are said to be hydrophobic or “water fearing.” The hydrophilic phosphorous heads are attracted to water. Forming a bilayer satisfies the water preferences of both the “heads” and “tails” of phospholipids: the hydrophilic heads face the watery regions inside and outside the cell, and the hydrophobic tails face each other in a water-free junction. The bilayer forms spontaneously because this situation is so favorable.

Phospholipids form the fundamental structure of the cell membrane, but they are not the only substance found there. According to the fluid-mosaic model of the cell membrane, special proteins called membrane proteins float in the phospholipid bilayer like icebergs in a sea.

The sea of phospholipid molecules and gatekeeper membrane proteins is in constant motion. The membrane’s fluidity keeps the cell from fracturing when placed under strain.

The most important property of the cell membrane is its selective permeability: some substances can pass through it freely, but others cannot. Small and nonpolar (hydrophobic) molecules can freely pass through the membrane, but charged ions and large molecules such as proteins and sugars are barred passage. The selective permeability of the cell membrane allows a cell to maintain its internal composition at necessary levels.

Molecules that can pass freely through the membrane follow concentration gradients, moving from the higher concentration area to the region of lower concentration. These processes take no energy and are called passive transport. The molecules that cannot pass freely across the phospholipid bilayer can be carried across the membrane in various processes that require energy and are therefore called active transport.

There are three main types of passive transport: diffusion, facilitated diffusion, and osmosis. In fact, osmosis is simply the term given to the diffusion of water.

In the absence of other forces, substances dissolved in water move naturally from areas where they are abundant to areas where they are scarce—a process known as diffusion. If there is a higher concentration of carbon dioxide gas dissolved in the water inside the cell than in the water outside the cell, carbon dioxide will naturally flow out from the cell until its distribution is balanced, without any energy required from the cell.

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.

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.

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.

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 pinches off into a vesicle that is inside the cell.

There are two kinds of endocytosis: in phagocytosis the cell takes in large solid food particles that it then digests. In pinocytosis, the cell takes in drops of cellular fluid containing dissolved nutrients.