Final answer:
The correct option in the final answer is a. It is true that the ligand determines the intracellular response in cell signaling. The ligand-receptor interaction activates signal transduction pathways, resulting in a chain of phosphorylation events or the production of second messengers that lead to a specific cellular response.
Step-by-step explanation:
In cell signaling, it is true that the ligand also known as the first messenger, helps determine the intracellular response. When a ligand binds to a cell-surface receptor, signal transduction is initiated, leading to a series of intracellular events, likened to a signaling cascade. These events involve activation of receptors, production of second messengers, a change in enzyme activity, and the eventual regulation of cellular processes such as metabolism or gene expression.
The ligand-receptor interaction induces a conformational change in the receptor, which activates or inhibits downstream signaling proteins and pathways within the cell. Specific enzymes, like kinases, are often involved in this process, leading to the phosphorylation of downstream proteins, which changes their activity and results in a precise intracellular response. Different cells may have unique receptors or different intracellular signaling components for the same ligand, resulting in varied biological responses across cell types.
Some ligands are hydrophobic and can directly enter cells to interact with internal receptors. This interaction can directly influence the expression of certain genes, leading to the production of specific proteins. In contrast, for hydrophilic ligands that can't cross the cell membrane, the ligand-receptor complex triggers a signaling pathway within the cell that indirectly influences cellular functions.