By: Eliana Zhang
The plasma membrane is essential to a cell’s structure and function. It serves as a selective barrier, separating a cell’s contents and environment while still allowing appropriate molecules to be shuttled in and out when necessary. The phospholipid part of the plasma membrane -- which is composed of fatty acids, glycerol, and a phosphate -- forms a bilayer. Hydrophilic (hydro meaning water, philic meaning attracting) and polar heads project outward into the watery environment, and hydrophobic (hydro meaning water, phobic meaning fearing) tails turn inward. Naturally, this forms a boundary for large, polar molecules. Proteins embedded in the plasma membrane allow molecules to be transported in and out of the cell with varying degrees of efficiency, but this is not a fail-safe process; even though these protein transport channels work most of the time, they can create huge problems when they fail, and illnesses can arise.
One of the most well-known of these illnesses is type 2 diabetes. When you consume sugar, your pancreas, a small organ near your side, secretes insulin into your bloodstream, where it travels to cells and binds to membrane receptor proteins. This binding signals the cell to release other carrier proteins, which bring the glucose into the cell because glucose is too large to diffuse efficiently through the plasma membrane on its own. Subsequently, blood sugar levels drop, and the pancreas releases less insulin. Patients with type 2 diabetes generally face two problems; their pancreases do not produce enough insulin, and the insulin boost results in too few carrier proteins. Glucose accumulates in the blood and is eventually expelled in urine. As blood sugar levels rise, the pancreas releases even more insulin. Eventually, the insulin-producing beta cells become impaired and cannot make enough insulin for the body’s demand. Symptoms of type 2 diabetes develop slowly and can include increased thirst, frequent urination, fatigue, blurred vision, and slow-healing sores. When someone suspects they may have type 2 diabetes, they may visit their doctor to get a urinalysis and blood test done -- these diagnostics gauge sugar levels in urine and insulin levels in the blood. So far, there is no cure to type 2 diabetes, but patients can control it by switching to a healthier lifestyle in order to control their blood sugar level. If diet and exercise are not enough, people may need diabetes medication or insulin therapy.
Cystic fibrosis (CF) is another disease caused by abnormalities in plasma membrane proteins. Most CF patients are young and have experienced poor growth and repeated lung infections. This is mainly because of a defective CF gene and chloride ion (Cl-) channel proteins. Usually, chloride ions can pass easily through a plasma membrane channel protein. When their passage is not properly regulated, however, a thick mucus appears in the lungs and pancreas. The mucus can clog the lungs, causing breathing problems. It also creates an environment that harbors bacterial growth. As a result, frequent lung infections occur, which eventually damage the lungs and increase the risk of early death. Thick digestive fluids may also clog ducts leading from the pancreas to the small intestine. This prevents enzymes from reaching the small intestine, where they are needed to digest food. Patients with CF can experience digestive problems and stifled growth as a result. To diagnose CF in children, a doctor can order a test that measures the amount of salt (NaCl) in a child’s sweat; children with CF have abnormally high levels of salt in their sweat Treatments for CF include using a percussion vest to loosen excess mucus in the lungs and taking medications that target gene mutations.
Malfunctioning plasma membrane functions do not always cause fatal conditions, though; color blindness, for example, is usually harmless and caused by an aberrant membrane. Color vision depends on cone cells in the retina, the part of the eye that transmits messages to the brain in order to let us see. People with normal color vision have three types of cones that are each activated by different wavelengths of visible light: blue, green, and red. Vision with color requires a combination of these three types of cells. When a cone cell receives the wavelength of light it is sensitive to, it sends a signal to close sodium ion channels in its plasma membrane. Some, however, inherit a mutation that results in a lack of working red or green photopigment proteins within the cones. As a result, they have the inability to close sodium ion channels in their plasma membrane. These people have “red-green color blindness” and have difficulty distinguishing between these two colors. When both red and green photopigments malfunction, someone may lack color vision altogether and see a world in black and white. This is more uncommon.
In total, almost thirty genetic disorders have been attributed to malfunctions in plasma membranes. Mutations in embedded proteins are not always life-threatening, but they can cause many concerning diseases. Like life, mutations are unpredictable, but when they occur, people can adapt and change to successfully live with them.
What did you learn?
When does type 2 diabetes occur?
Two things must happen for a patient to have type 2 diabetes: their cells respond poorly to insulin, and they lack enough glucose-carrying proteins to absorb the glucose in their blood. Because insulin fails to trigger efficient glucose absorption, the glucose remains in their blood until it passes out as urine, and the pancreas continues to produce insulin.
Are malfunctions in the plasma membrane life-threatening?
They can lead to life threatening conditions sometimes, as in the case of type 2 diabetes and cystic fibrosis. However, there are treatments for those conditions, and also non-life-threatening conditions that can arise. Color-blindness is one of them. As malfunctions can occur in a variety of different proteins, the subsequent consequences can vary as well.