Final answer:
The binding of a signaling molecule to a cell surface receptor directly activates a second messenger, such as cyclic AMP (cAMP), which then activates protein kinases leading to changes in cellular activity.
Step-by-step explanation:
Binding of a signaling molecule to a cell surface receptor directly activates a second messenger, which in turn can activate a protein kinase.
This signaling pathway begins when a ligand, such as a hormone, binds to the receptor causing an allosteric change. This change can then initiate a phosphorylation cascade through second messengers like cyclic AMP (cAMP) or calcium ions (Ca²+), leading to further activation of protein kinases. These kinases typically add phosphate groups to specific proteins, including transcription factors, which then enter the nucleus and bind to DNA to regulate gene expression and cause changes in the cell's environment.
Such downstream cellular responses can result in altered metabolism or gene expression, exemplifying the power of cell signaling to control complex biological processes.