Question

Which of the following is the first thing that happens when a signal molecule acts on a target cell?
A. A transcription factor acts on the DNA.
B. The signal molecule binds to RNA.
C. A new protein is made in the target cells.
D. The signal molecule binds to a receptor.

Answer 1

Hint: Cells that are susceptible to a hormone released are known as target cells. Three elements are required for target cell activation: hormone levels in the blood, receptor concentrations on target cells, and hormone-receptor affinity.


Complete step by step solution:
Endocrine signaling involves the secretion of signaling molecules (hormones) by specialized endocrine cells, which are then transported through the bloodstream to act on target cells in distal body regions.
As a broad term for molecules that particularly attach to other molecules, "ligand" is frequently used to refer to signaling molecules (such as receptors). The message sent by a ligand is frequently transmitted inside the cell through a series of chemical messengers.
Multicellular organisms utilize four different types of chemical signaling: paracrine, endocrine, autocrine, and direct signaling through gap junctions. This class of signaling chemicals, which also includes thyroid hormone, vitamin D3, and retinoic acid, is best known for its members, the steroid hormones. Tiny proteins and small ions are examples of signaling molecules, which can also be hydrophobic, water-soluble, or even a gas. The plasma membrane is permeable to hydrophobic signaling molecules (ligands), which can then attach to interior receptors. Any signal molecule binding to a receptor is required to be its initial action.
So, option (D) is correct.

Note:
By attaching to receptor proteins with remarkable selectivity, hormones control cellular target functions. The existence and functionality of particular and selective hormone receptors as well as the molecules in the downstream signaling pathway play a significant role in determining whether a peripheral cell is hormonally sensitive.