Final answer:
Chemical molecules bind at receptor sites on target cells to initiate cellular responses, either by changing membrane receptor conformation for ion transport or via G-protein interaction leading to signaling cascades within the cell.
Step-by-step explanation:
At receptor sites, chemical molecules either change the conformation allowing ions to pass through the plasma membrane, or interact with G-proteins causing a cascade of reactions inside the cell.
When a ligand binds to a receptor, it triggers a series of events, known as signal transduction, crucial for cellular communication. This binding can occur at two main types of receptor sites: intracellular hormone receptors or cell surface receptors. Intracellular receptors are usually bound by nonpolar, lipid-soluble ligands like steroids, and the resulting receptor-ligand complex interacts with DNA in the nucleus, influencing gene expression. Cell surface receptors, in contrast, are typically involved with polar ligands and can induce cellular responses through two main pathways.
Some membrane receptors alter their conformation to facilitate ion passage, such as Na+ and K+, across the plasma membrane, while others activate G-proteins within the cytoplasm, leading to further intracellular signaling. Both mechanisms are vital, and any disruption to these processes can result in a variety of diseases. Understanding how binding alters receptor conformation or interacts with G-proteins is fundamental to grasping how cells respond to external and internal stimuli, regulating a myriad of normal body processes.