Questions: You recently identified a molecule that you believe is a primary messenger for a signal transduction mechanism. All you know about this ligand is that it is hydrophilic. Where would you expect to find initial interaction of the molecule with its receptor? on the outer surface of the plasma membrane on the cytoplasmic side of the plasma membrane within the nucleus of the cell within the cytoplasm of the cell in endosome vesicles

The answer is the first one: on the outer surface of the plasma membrane.

Explanation for each option:

1. On the outer surface of the plasma membrane: This is the correct answer. Hydrophilic molecules cannot easily cross the hydrophobic lipid bilayer of the plasma membrane. Therefore, they typically bind to receptors located on the outer surface of the plasma membrane to initiate signal transduction.

2. On the cytoplasmic side of the plasma membrane: This is incorrect because hydrophilic molecules cannot pass through the hydrophobic core of the plasma membrane to reach the cytoplasmic side without the help of a receptor or transporter.

3. Within the nucleus of the cell: This is incorrect because hydrophilic molecules would not be able to cross the plasma membrane and the nuclear envelope to reach the nucleus without specific transport mechanisms.

4. Within the cytoplasm of the cell: This is incorrect because, as mentioned, hydrophilic molecules cannot easily cross the plasma membrane to enter the cytoplasm directly.

5. In endosome vesicles: This is incorrect because endosomes are involved in the internalization and processing of molecules after receptor binding, not the initial interaction of the ligand with its receptor.

Summary: The initial interaction of a hydrophilic molecule with its receptor would be expected to occur on the outer surface of the plasma membrane.