Final answer:
Protein hormones bind to surface receptors and activate proteins in the cell, while water-soluble hormones bind to surface receptors and trigger a signaling cascade involving second messengers.
Step-by-step explanation:
Protein hormones and water-soluble hormones work through different mechanisms. Protein hormones, which are large and soluble, cannot pass through the cell membrane, so they bind to surface cell-membrane receptors. This binding initiates a cell-signaling pathway involving G proteins, adenylyl cyclase, cyclic AMP, and protein kinases. These protein kinases then phosphorylate cytoplasmic proteins, activating proteins that carry out the changes specified by the hormone.
On the other hand, water-soluble hormones, which are unable to diffuse through the lipid bilayer of the cell membrane, bind to receptors located on the surface of the cell membrane. This binding triggers a signaling cascade that is carried out by a molecule called a second messenger. The second messenger initiates a cellular response that includes the production of proteins and enzymes as well as altered membrane permeability.