What do drugs typically target in order to exert their effects?

Drugs typically exert their effects by targeting specific cellular receptors. These receptors are proteins located on the surface of cells or within cells that bind to different substances, including neurotransmitters, hormones, and drugs. When a drug binds to its target receptor, it can trigger a variety of biochemical responses within the cell, leading to changes in function, signaling pathways, enzyme activity, or gene expression. This mechanism is fundamental to how many drugs achieve their therapeutic effects, as the interaction with specific receptors allows for a targeted approach, enhancing efficacy and minimizing side effects.

While cells' external membranes play a role in how substances enter or interact with cells, they are not typically the direct targets for drug action. Circulating hormones can influence drug activity, but they are not the primary targets of most drugs. Plasma proteins are involved in drug transport and metabolism but are not the specific targets through which drugs exert their physiological effects. Thus, the correct answer highlights the importance of receptor specificity in pharmacology, reflecting the precision required for drug action.