5. Which of the following signaling molecules enters the cell to initiate its action?
(1) Transferrin      (2) Insulin
(3) Glucagon       (4) Thyroxine

Overview of Signaling Molecules and Their Entry Modes

Cells communicate using signaling molecules that bind to receptors to initiate responses. Most signaling molecules like insulin, glucagon, and transferrin bind to surface receptors on the cell membrane without entering the cell. Their signals are transmitted inside through receptor-mediated cascades.

However, some signaling molecules can directly cross the plasma membrane to interact with intracellular or nuclear receptors. These molecules are typically small, lipid-soluble, or possess specific transport mechanisms allowing cell entry.

The Four Candidates Explained

1. Transferrin: A large glycoprotein responsible for iron transport in blood. It binds to transferrin receptors on the cell surface for receptor-mediated endocytosis but does not freely enter cells.

2. Insulin: A peptide hormone regulating glucose uptake. It binds to insulin receptors on the membrane and triggers intracellular pathways without entering the cell.

3. Glucagon: A peptide hormone that raises blood glucose levels by binding membrane receptors and activating signaling cascades. It does not enter the cell.

4. Thyroxine (T4): A small, lipophilic thyroid hormone derived from tyrosine. It can cross the cell membrane via specific transporters or passive diffusion and binds intracellular receptors in the nucleus to regulate gene expression.

Why Thyroxine Enters the Cell

Thyroxine’s lipid-soluble nature allows it to go beyond surface receptor binding. It enters target cells and interacts with thyroid hormone receptors located in the nucleus or cytoplasm, directly modulating transcription and protein synthesis. This mode is typical for steroid hormones and thyroid hormones, distinguishing them from peptide hormones.

Summary

Among the options provided, thyroxine uniquely enters the cell to initiate its biological action internally, unlike transferrin, insulin, and glucagon, which act exclusively via cell surface receptors.

Correct answer:

(4) Thyroxine