The passage tells us that "dystrophin localizes to the sarcolemma," so we know it is located at the membrane of the muscle fiber. We also know that its role is to structurally link the muscle fiber and the basal lamina. We can eliminate the choices for ion channel, signaling protein, and chemical receptor based on what we know about dystrophin's function. We are left with either fibrous protein or transmembrane protein. Though fibrous proteins also have structural roles, transmembrane protein is the best choice because we know that dystrophin is linking the muscle fiber to another structure, meaning that it must span the membrane.

Enzymes will increase the rate of a chemical reaction, but will not alter the equilibrium of a reaction. As a result, the amount of product is not affected by enzymes. The same amount of product will be made; it will just be made at a faster rate.

Feedback inhibition is a type of regulation in which an enzyme product blocks an earlier part of a metabolic reaction. This allows cells to regulate resources by signaling when enough product is made.

All upstream products, prior to the action of enzyme C, will start to increase in concentration because each enzyme is reversible. Directly, product 3 will build up and product 4 will decrease. This will lead product 5 to decrease because there is no 4 to make it. As 3 builds up, enzyme B will start working in reverse, converting it back into product 2, according to Le Chatelier's principle. Then, when 2 starts to build up, it will also be converted backwards into product 1.

Reversible binding in the active site is typical of a competitive inhibitor. Other kinds of inhibitors bind to the active site after the normal substrate has already bound, such as in uncompetitive inhibition. Alternatively, allosteric inhibitors can bind to sites other than the active site and induce a shape change that diminishes affinity for the typical substrate.

Only competitive inhibition can be overcome by the addition of more substrate. Competitive inhibitors work by binding to and blocking the enzyme's active site. If more substrate is added, it increases the chance that an enzyme molecule will bind to the substrate instead of the inhibitor. Noncompetitive inhibition is not affected by the amount of substrate, uncompetitive inhibition is not tested on the MCAT, and "substrate-sensitive" is not a type of inhibition.

Noncompetitive inhibitors do not bind at the active site of an enzyme. Instead, they bind at another position on the enzyme and alter its conformation. This passive approach to inhibition makes an enzyme unable to attach to the substrate at the active site. Only irreversible inhibitors bond covalently; both competitive and noncompetitive inhibitors bind noncovalently.

Whenever you read that an enzyme inhibitor has covalently bonded to an enzyme, you can conclude that it is an irreversible inhibitor. Both competitive and noncompetitive inhibitors bind noncovalently to the target enzyme.

Negative feedback provides the body with a method for shutting down a reaction once sufficient product has been created. Parathyroid hormone (PTH) is responsible for increasing blood calcium levels, but once the level is sufficient, the parathyroid glands detect the sufficient calcium level and no longer produce PTH. PTH works in coordination with calcitonin to maintain this balance via its negative feedback loop.

Positive feedback, in contrast, involves the exponential increase of a reaction upon detection. Very few examples of positive feedback exist in the body, though oxytocin follows this model during childbirth.

Competitive inhibitors block substrates by binding noncovalently to the active site on enzymes. This prevents the substrate from entering the active site.

Noncompetitive inhibitors, in contrast, will act on regions outside of the active site to prevent binding. Uncompetitive inhibition is a specialized form of noncomptitive inhibition in which the inhibitor binds to the enzyme complex after the substrate has entered the active site. Irreversible inhibitors form covalent bonds, and fall outside the common inhibitor classifications.