why water soluble hormones have a receptor on the outer side of the membrane

Answer 1

Water soluble hormones, such as protein and peptide hormones, cannot pass through the cell membrane due to their hydrophilic nature. Therefore, in order to exert their effect on target cells, these hormones bind to receptor proteins on the outer side of the cell membrane.

The receptors for water soluble hormones are typically transmembrane proteins that span the cell membrane. These receptors have an extracellular domain that binds specifically to the hormone molecule, allowing the hormone to be recognized and captured by the receptor. Once the hormone binds to the receptor, it triggers a series of intracellular signaling events that ultimately lead to a cellular response.

Having the receptor on the outer side of the membrane allows the hormone to interact directly with the receptor without needing to pass through the lipid bilayer. This is important as the hydrophilic nature of water soluble hormones prevents them from diffusing across the hydrophobic cell membrane. By binding to the receptor on the outside of the cell, they can initiate signaling pathways inside the target cell without having to cross the lipid barrier.

Overall, the presence of receptors on the outer side of the cell membrane allows water soluble hormones to interact with target cells and elicit their specific cellular responses, despite their inability to pass through the lipid bilayer.