The Plasma Membrane
- Describe the structure of the plasma membrane
- Explain the major features and properties of the plasma membrane
- Differentiate between materials that can and cannot diffuse through the lipid bilayer
- Identify specializations of the plasma membrane
Despite differences in structure and function, all living cells in multicellular organisms have a surrounding plasma membrane. As the outer layer of your skin separates your body from its environment, the plasma membrane) separates the inner contents of a cell from its exterior environment. This membrane provides a protective barrier around the cell and regulates which materials can pass in or out.
Structure and Composition of the Plasma Membrane
The plasma membrane is an extremely pliable structure composed of 2 layers of back-to-back phospholipids (a “bilayer”). Cholesterol is also present between the phospholipids, which contributes to the fluidity of the membrane. There are various proteins embedded within the membrane that have a variety of functions. A single phospholipid molecule has a phosphate group on one end, called the “head,” and two side-by-side chains of fatty acids that make up the lipid tails (Figure 3.2). The phosphate group is negatively charged, making the head polar and hydrophilic—or “water loving.” A hydrophilic molecule (or region of a molecule) is one that is attracted to water. The phosphate heads are thus attracted to the water molecules of both the extracellular and intracellular environments. The lipid tails, on the other hand, are uncharged, or nonpolar, and are hydrophobic—or “water fearing.” A hydrophobic molecule (or region of a molecule) repels and is repelled by water. Some lipid tails consist of saturated fatty acids and some contain unsaturated fatty acids. This combination adds to the fluidity of the tails that are constantly in motion. Phospholipids are thus amphipathic molecules. An amphipathic molecule is one that contains both a hydrophilic and a hydrophobic region. In fact, soap works to remove oil and grease stains because it has amphipathic properties. The hydrophilic portion can dissolve in water while the hydrophobic portion can trap grease in micelles that then can be washed away.
The plasma membrane consists of two adjacent layers of phospholipids. The lipid tails of one layer face the lipid tails of the other layer, meeting at the interface of the two layers. The phospholipid heads face outward, one layer exposed to the interior of the cell and one layer exposed to the exterior (Figure 3.3). Because the phosphate groups are polar and hydrophilic, they are attracted to water in the intracellular fluid. Intracellular fluid (ICF) is the fluid interior of the cell. The phosphate groups are also attracted to the extracellular fluid. Extracellular fluid (ECF) is the fluid environment outside the enclosure of the cell membrane. Interstitial fluid (IF) is the term given to extracellular fluid not contained within blood vessels. Because the lipid tails are hydrophobic, they meet in the inner region of the membrane, excluding watery intracellular and extracellular fluid from this space. The cell membrane has many proteins, as well as other lipids (such as cholesterol), that are associated with the phospholipid bilayer. An important feature of the membrane is that it remains fluid; the lipids and proteins in the cell membrane are not rigidly locked in place.
The structure of the plasma membrane gives it a characteristics of selective permeability. Selectively permeable means that the membrane allows some materials to pass while excluding others. The permeability of the plasma membrane is dependent upon size and solubility of the material passing through. Small gasses such as oxygen and carbon dioxide can easily pass through the membrane. Lipid soluble substances can also pass through the phospholipids. Water soluble (hydrophilic) substances such as glucose and charged molecules such as ions, however, are unable to pass through the lipid bilayer. These hydrophilic molecules and ions must use proteins within the membrane to pass into or out of the cell.
The lipid bilayer forms the basis of the plasma membrane, but it is peppered throughout with various proteins. Two different types of proteins that are commonly associated with the cell membrane are the integral proteins and peripheral protein (Figure 3.4). As its name suggests, an integral protein is a protein that is embedded in the membrane.
A channel or transport protein is an example of an integral protein that selectively allows particular materials, such as certain ions, to pass into or out of the cell. Another important group of integral proteins are cell recognition proteins, which serve to mark a cell’s identity so that it can be recognized by other cells. A receptor is a type of recognition protein that can selectively bind a specific molecule outside the cell, and this binding induces a chemical reaction within the cell. Receptors are like name tags for each cell that allows specific molecules to recognize it. Some integral membrane proteins are glycoproteins. A glycoprotein is a protein that has carbohydrate molecules attached, which extend into the extracellular matrix. The attached carbohydrate tags on glycoproteins aid in cell-to-cell recognition. The carbohydrates that extend from membrane proteins and even from some membrane lipids collectively form the glycocalyx. The glycocalyx is a fuzzy-appearing coating around the cell formed from glycoproteins and other carbohydrates attached to the cell membrane. The glycocalyx can have various roles. For example, it may have molecules that allow the cell to bind to another cell, it may contain receptors for hormones, or it might have enzymes to break down nutrients. The glycocalyces found in a person’s body are products of that person’s genetic makeup. They give each of the individual’s trillions of cells the “identity” of belonging in the person’s body. This identity is the primary way that a person’s immune defense cells “know” not to attack the person’s own body cells, but it also is the reason organs donated by another person might be rejected. Enzymes are also found embedded within the plasma membrane. Enzymes can be found as peripheral proteins typically found on the inner or outer surface of the lipid bilayer. These proteins typically perform a specific function for the cell. Some peripheral proteins on the surface of intestinal cells, for example, act as digestive enzymes to break down nutrients to sizes that can pass through the cells and into the bloodstream.
All plasma membranes share the characteristic of being selective permeable and containing various proteins. Some membranes, however, have components that are specialized for a specific purpose. Microvilli are finger-like projections on the surface of some cells. These projections increase surface are for absorption. Cells that line the small intestine contain microvilli. Junctions are another specialized group of proteins that connect to other cells. There are three main types of junctions: tight junctions, desmosomes, and gap junctions. Tight junctions are proteins that hold adjacent cells together very tightly so nothing can penetrate between them. Cells that line the digestive and urinary tract contains many tight junctions to ensure the contents within those hollow organs do not leak out into the outer layers or body cavity. Desmosomes are sometimes called anchoring junctions. These junctions hold cells together by fibers, which allows movement without separation. Cells that contain desmosomes are found within the muscle tissue and the skin. Gap junctions are open areas within the plasma membrane found between two adjacent cells. The proteins connect two cells while allowing chemicals to pass between the cells.
Diseases of the Cell: Cystic Fibrosis
Cystic fibrosis (CF) affects approximately 30,000 people in the United States, with about 1,000 new cases reported each year. The genetic disease is most well known for its damage to the lungs, causing breathing difficulties and chronic lung infections, but it also affects the liver, pancreas, and intestines. Only about 50 years ago, the prognosis for children born with CF was very grim—a life expectancy rarely over 10 years. Today, with advances in medical treatment, many CF patients live into their 30s. The symptoms of CF result from a malfunctioning membrane ion channel called the cystic fibrosis transmembrane conductance regulator, or CFTR. In healthy people, the CFTR protein is an integral membrane protein that transports Cl– ions out of the cell. In a person who has CF, the gene for the CFTR is mutated, thus, the cell manufactures a defective channel protein that typically is not incorporated into the membrane, but is instead degraded by the cell. The CFTR requires ATP in order to function, making its Cl– transport a form of active transport. This characteristic puzzled researchers for a long time because the Cl– ions are actually flowing down their concentration gradient when transported out of cells. Active transport generally pumps ions against their concentration gradient, but the CFTR presents an exception to this rule. In normal lung tissue, the movement of Cl– out of the cell maintains a Cl–-rich, negatively charged environment immediately outside of the cell. This is particularly important in the epithelial lining of the respiratory system. Respiratory epithelial cells secrete mucus, which serves to trap dust, bacteria, and other debris. A cilium (plural = cilia) is one of the hair-like appendages found on certain cells. Cilia on the epithelial cells move the mucus and its trapped particles up the airways away from the lungs and toward the outside. In order to be effectively moved upward, the mucus cannot be too viscous; rather it must have a thin, watery consistency. The transport of Cl– and the maintenance of an electronegative environment outside of the cell attract positive ions such as Na+ to the extracellular space. The accumulation of both Cl– and Na+ ions in the extracellular space creates solute-rich mucus, which has a low concentration of water molecules. As a result, through osmosis, water moves from cells and extracellular matrix into the mucus, “thinning” it out. This is how, in a normal respiratory system, the mucus is kept sufficiently watered-down to be propelled out of the respiratory system. If the CFTR channel is absent, Cl– ions are not transported out of the cell in adequate numbers, thus preventing them from drawing positive ions. The absence of ions in the secreted mucus results in the lack of a normal water concentration gradient. Thus, there is no osmotic pressure pulling water into the mucus. The resulting mucus is thick and sticky, and the ciliated epithelia cannot effectively remove it from the respiratory system. Passageways in the lungs become blocked with mucus, along with the debris it carries. Bacterial infections occur more easily because bacterial cells are not effectively carried away from the lungs.