Final answer:
A receptor that binds to ligands at very low concentrations has a high affinity for the ligand, as indicated by a large equilibrium constant, k. This high affinity allows the receptor to be occupied by the ligand efficiently, facilitating quick cellular responses, such as turning off gene transcription, via different receptor types, including internal and cell-surface receptors.
Step-by-step explanation:
A receptor that binds to a ligand at very low concentrations is indicative of a high affinity for that ligand. This high affinity can be quantified by an equilibrium constant, k, in receptor-ligand binding dynamics. Large values of k indicate that the receptor has a high likelihood of being occupied by the ligand, even if the ligand is present at low concentrations. This is essential for maintaining a responsive state within a cell, allowing for efficient signal transduction upon ligand binding.
There are different types of receptors, such as internal receptors and cell-surface receptors. Internal receptors often bind to hydrophobic ligands and can directly affect transcription and translation by entering the nucleus. Conversely, cell-surface receptors tend to bind hydrophilic ligands and could either change conformation to allow ion passage or initiate a signaling cascade through second messengers.
In the scenario presented, when the water-soluble molecule, which is the ligand, binds to the receptor, it causes the cells to turn off transcription of a gene, thus implicating its role in a signaling pathway that can control gene expression. In conclusion, the properties of the receptor determine how the cell responds to the presence of ligands, with high-affinity receptors being sensitive to even minute ligand concentrations.