Surfactants, such as phospholipids, are molecules with a polar “head” and a nonpolar “tail”. When mixed in water, these molecules form micelles… microscopic bubbles with a water-soluble surface and a fat-soluble interior. Surfactants form micelles because it is one of the few stable configurations they can find in water.
Another stable configuration is the bilayer. A bilayer is two layers of surfactant molecules, with polar water molecules on both sides. The membrane that surrounds a cell consists of a phospholipid bilayer with proteins embedded in it. Because these phospholipids are in a liquid state, cell membranes are flexible and can grow and re-form. They also keep the contents of the cell inside the cell and most everything else out. Most water-soluble molecules, especially ions, are unable to cross the cell membrane because they would have to pass through the fat-soluble interior of the phospholipid bilayer.
Besides forming cell membranes, phospholipid bilayers also form membranes around organelles inside of the cell and structures called vesicles. Vesicles are basically small bubbles used by the cell to store various substances. Vacuoles are vesicles that contain mostly water. Lysosomes are vesicles that contain acidic digestive enzymes that are used to break down food particles and worn-out organelles. Food vacuoles are vesicles that contain food particles before they are broken down by lysosomes. And secretory vesicles contain waste materials that are to be excreted from the cell. If molecules need to be transported from one part of a cell to another, it is usually carried inside of a vesicle.
Because membranes are liquids, two vesicles are able to merge into a single large vesicle, and a large vesicle is able to divide into two smaller vesicles. This is how food particles get digested by lysosomes. A lysosome will merge with a food vacuole, releasing its digestive enzymes inside of the food vacuole. It is also how food particles can enter a cell (especially for unicellular organisms) and waste materials can exit the cell.
Cells would be unable to interact with their environments very much if cell membranes consisted solely of a phospholipid bilayer. A phospholipid bilayer is an excellent barrier, but molecules need some way to enter and exit a cell. Most transport through the cell membrane is controlled by proteins embedded in the phospholipid bilayer.
Proteins are long chains of amino acids chemically bonded together. Some amino acids, such as phenylalanine, are nonpolar molecules, and other amino acids, such as glutamine, are polar molecules. By building proteins with nonpolar amino acids in the middle and polar amino acids at both ends, a protein can be designed to sit across a phospholipid bilayer, providing a channel for molecules to pass into or out of the cell.
But cells cannot allow just any molecule to enter and leave through the cell membrane. Proteins use receptors to identify specific molecules and to control which molecules are transported in and out. These receptors function like the active sites on enzymes. They identify a molecule by matching its shape and dipole charges. Molecules that do not have the correct shape and dipole charges will not be transported by the protein. If a protein requires energy to transport a molecule, then the process is called “active” transport. If energy is not required, then it is called “passive” transport.
Proteins embedded in the cell membrane have a second important function: transmitting cell signals. Cells communicate with other cells across long distances by releasing chemicals called hormones. An example of a hormone is insulin. Insulin causes cells to take up glucose from the blood and to store it as glycogen. Excess glucose in the blood is toxic. Receptors on a cell’s membrane receive these hormone messages from the environment and respond by releasing chemicals inside of the cell that cause the cell to react.
Imagine that there is a chemical reaction in a cell that starts by chemically bonding molecule A to molecule B, and ends by producing molecule Z. When molecule Z is in short supply, the cell needs to produce more molecule Z. When there is too much molecule Z, the cell needs to stop producing molecule Z. Cells use feedback control loops to turn chemical reactions on and off. The most basic type of feedback control loop uses the product of a reaction to inhibit the enzyme used for one of the steps in the reaction.
An enzyme is used to catalyze the chemical bonding of molecule A to molecule B. But if molecule Z attaches to the enzyme first, the active site for molecule B is physically blocked, and the reaction is unable to proceed.
Most enzymes have an active site where the chemical reaction occurs, and then a secondary site where the enzyme can be inhibited. As the amount of molecule Z builds up, more and more of the enzymes will be inhibited and the chemical reaction will be blocked. But as the amount of molecule Z is used up, more and more of the enzymes will become available and the chemical reaction will proceed. The enzyme uses the presence of molecule Z as feedback to control the rate of the reaction.