Here's why:

* Receptors are proteins that bind to specific signal molecules (also known as ligands) with high affinity.

* The three-dimensional shape of the receptor is crucial for this binding. It acts like a lock and key, where the receptor (lock) is designed to fit only a specific signal molecule (key).

* This complementary shape ensures that the signal molecule binds to the correct receptor and triggers the appropriate cellular response.

Examples of receptors:

* Neurotransmitter receptors in the nervous system: These receptors bind to neurotransmitters like acetylcholine or dopamine, mediating communication between neurons.

* Hormone receptors: These receptors bind to hormones like insulin or estrogen, regulating various physiological processes.

* Immune receptors: These receptors recognize antigens (foreign substances) and initiate immune responses.

Let me know if you'd like to know more about specific types of receptors or their functions!