A cell has many duties to perform. One of its most important functions is to maintain a healthy environment within the cell. This requires controlling the intracellular concentrations of various molecules, such as ions, dissolved gases and biochemicals. A concentration gradient is a difference in the concentration of a substance across a region. In microbiology, the cell membrane creates concentration gradients.
The Cell Membrane
A cell membrane is composed of double layer of phospholipids, which are molecules containing a phosphate head and two lipid tails. The heads align along the inner and outer boundaries of the membrane, while the tails fill the space in between. The cell membrane has selective permeability -- the tails prevent large or charged molecules from diffusing through the cell membrane, while small and fat-soluble molecules can slip through. Selective permeability can create concentration gradients across the membrane that require special transmembrane proteins to overcome.
Small, nonpolar molecules can diffuse through a cell membrane based on the molecule’s concentration gradient. A nonpolar molecule has a relatively uniform and neutral electric charge throughout. For example, oxygen is nonpolar and diffuses freely across a cell membrane. Blood cells transport oxygen molecules to the spaces surrounding cells, creating a relatively high concentration of O2. A cell continuously metabolizes oxygen, creating a concentration gradient between the interior and exterior of the cell. O2 diffuses through the membrane because of this gradient. Water and carbon dioxide, though polar, are small enough to diffuse through the cell membrane unassisted.
Ion Channel Receptors
An ion is an atom or molecule with a different number of protons and electrons -- it carries an electric charge. Certain ions, including those of sodium, potassium and calcium, are important to the normal functioning of a cell. Lipids reject ions, but the cell membrane is peppered with proteins called ion channel receptors that help control ion concentrations within the cell. The sodium-potassium pump uses the cell’s energy molecule, adenosine triphosphate or ATP, to overcome the concentration gradient, allowing the movement of sodium out of the cell and potassium into the cell. Other pumps rely on electrodynamic forces rather than ATP to transport ions across the membrane.
Large molecules cannot diffuse through the lipids in the cell membrane. Carrier proteins within the membrane provide the ferry service, using either active transport or facilitated diffusion. Active transport requires the cell to use ATP to move the large molecule against the concentration gradient. Receptors within active transport proteins bind to specific passenger, and ATP allows the protein to translocate its passenger across the membrane. Facilitated diffusion doesn’t need biochemical energy from the cell. Carriers using facilitated diffusion act as gatekeepers that open and close based on concentration and electrical gradients.