Answer:
C.
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Explanation:
Simple diffusion is pretty much exactly what it sounds like – molecules move down their gradients through the membrane. Molecules that practice simple diffusion must be small and nonpolar*, in order to pass through the membrane. Simple diffusion can be disrupted if the diffusion distance is increased. If the alveoli in our lungs fill with fluid (pulmonary edema), the distance the gases must travel increases, and their transport decreases. Facilitated diffusion is diffusion that is helped along (facilitated by) a membrane transport channel. These channels are glycoproteins (proteins with carbohydrates attached) that allow molecules to pass through the membrane. These channels are almost always specific for either a certain molecule or a certain type of molecule (i.e. an ion channel), and so they are tightly linked to certain physiologic functions. For example, one such transporter channel, GLUT4, is incredibly important in diabetes. GLUT4 is a glucose transporter found in fat and skeletal muscle. Insulin triggers GLUT4 to insert into the membranes of these cells so that glucose can be taken in from the blood. Since this is a passive mechanism, the amount of sugar entering our cells is proportional to how much sugar we consume, up to the point that all our channels are being used (saturation). In type II diabetes mellitus, cells do not respond as well to the presence of insulin, and so do not insert GLUT4 into their membranes. This can lead to soaring blood glucose levels which can cause heart disease, stroke, and kidney
