Which of the following signal molecules does not interact with cell surface receptors?

(a) Insulin

(b) Gastrin

(c) Glucagon

(d) Testosterone

The correct answer is (d) Testosterone.

The key to this question is understanding the chemical nature of the signal molecules, which determines whether they can cross the cell membrane.

Cell Surface vs. Intracellular Receptors

• Cell Surface Receptors: These are used by signal molecules that are water-soluble (hydrophilic) and cannot pass through the hydrophobic lipid bilayer of the cell membrane. Most peptide hormones and neurotransmitters bind to surface receptors. Insulin, gastrin, and glucagon are all peptide hormones and therefore act on cell surface receptors.
• Intracellular Receptors: These are used by signal molecules that are lipid-soluble (hydrophobic). These molecules can easily diffuse across the cell membrane and bind to receptors located in the cytoplasm or nucleus.

Testosterone is a steroid hormone. Steroids are derived from cholesterol and are lipid-soluble, allowing them to enter the cell and interact directly with intracellular receptors to regulate gene expression.