cell-surface receptor
A cell-surface receptor is like a doorbell mounted on the outside of a house. Some hormones — the water-soluble ones — cannot pass through the greasy, fatty cell membrane, so they never get inside. Instead they press the doorbell from outside, and the receptor relays the message to whoever is in the house.
These receptors span the cell membrane, with a part facing outward that binds the hormone and a part facing inward that talks to the cell's interior. When the hormone binds outside, the receptor changes shape and activates inner machinery — often producing a second messenger or switching on an enzyme. Peptide and protein hormones, plus catecholamines like adrenaline, all act through this route because they are hydrophilic.
Because the signal must be passed across the membrane rather than carried in directly, surface-receptor responses tend to be fast — seconds to minutes — and are well suited to rapid adjustments. The trade-off is that the message has to be relayed and amplified inside, which adds steps that can be regulated, sped up, or shut down.
Adrenaline binds beta-adrenergic receptors on the surface of heart muscle cells; it never enters the cell, yet within seconds the heart beats faster and harder.
A hormone acting from outside the cell, with effects in seconds.
Two major surface-receptor types dominate endocrinology: G protein-coupled receptors and receptor tyrosine kinases. Both bind hormone outside and trigger different internal cascades.