Cell signaling can be divided into 3 stages
1. Stage 1 Reception: refers to the binding of the signaling molecule (ligand) to the cell surface receptor or receptor inside the cell. In the case of hydrophilic ligands such as insulin, has receptor on the cell surface as it cannot cross plasma membrane. Hydrophobic ligands such as steroid hormones can cross the plasma membrane therefore receptor may be either in the cytoplasm or nucleus.
2. Stage 2 Transduction: The binding of ligand to the receptor causes a conformational change in the receptor that initiates a sequence of reactions activating many relay molecules finally transferring the signal to a target molecule leading to a change in cellular function. Target molecules are often transcription factors that can directly bind to DNA and can activate expression or suppression of genes. There are key intermediate signaling molecules called second messengers. Understand more: Diagram Quiz on Cell Signaling
3. Stage 3 Response: Finally, these signals (target molecule) trigger a cellular response in target cell like cell division, activation of enzymes, expression of some genes, opening of ion channels etc.
