Cell-surface Signaling

Some hormones such as those that are lipid insoluble like oxytocin cannot diffuse through the cell membrane. Instead they must bind to receptors on the cell's surface. Upon binding the hormone, considered the first messenger, activates a signaling cascade.

For example when a G protein coupled receptor or GPCR is activated by a first messenger on the outside of the cell membrane on the inside an enzyme such as phospholipase C hydrolizes the membrane phospholipid PIP two into IP three and DAG, two different second messengers.

Once formed IP three moves from the cell membrane to the endoplasmic membrane where it binds to an IP three gated calcium channel releasing calcium ions, a third second messenger into the cytoplasm and inducing changes like muscle contraction.

In this way, a hormone can affect a cell that it does not even enter, activating a number of cellular processes depending on the ligands and receptors that are present.