Hydrophilic signaling molecules are unable to pass through the lipid bilayer of the cell membrane. In hydrophilic signal transduction, the hydrophilic signaling molecules bind to receptors on the cell surface, which allow the ligands to pass through to the aqueous cell environment. 

There are two types of cell-surface receptors: 

1. Ligated gated channels, which open with signaling molecules binds to them
2. G-coupled protein receptors, which alter their shape slightly when the signal molecule binds to them.