intracellular receptors - protein receptors within the cell 

• signal molecules are usually lipid-soluble or very small in order to pass through the membrane
• gene regulating receptors - has binding site for DNA
  • inhibitor protein may prevent DNA from binding
  • either activates or suppresses certain genes after binding to DNA
  • response varies depending on the cell
  • lipid-soluble signal molecules tend to last longer than water-soluble signals
• regulators as enzymes - catalyzes reactions when activated
  • nitric oxide binds to guanylyl cyclase, catalyzes synthesis of GMP (messenger molecule that relaxes smooth muscle cells)

cell surface receptors - accounts for the majority of a cell's receptors 

• turns extracellular signals into intracellular ones
• water-soluble signals can't pass through the membrane, must bind w/ surface receptors
• chemically gated ion channels - allow ions through
  • opens only when a neurotransmitter binds to it
  • shape/charge of channel determines what type of ion goes through it
• enzymic receptors - activates an enzyme when binding to a signal molecule
  • protein kinases - enzymes that add phosphate groups to proteins
  • binds to signal molecule outside the school, enzyme activity occurs in the cytoplasm

G-protein linked receptors - uses GTP-binding protein to indirectly act on enzymes/ion channels 

• starts a diffusible signal within the cell
• has short duration
• G-protein changes shape, leaves receptor once signal molecule arrives
• GTP can start few events, turns into GDP+phosphate very quickly
• pathway shuts down if signals stop coming in
• threads back and forth across the membrane 7 times (7-pass transmembrane protein)
• more of these surface receptors than any other kind
• may have evolved from sensory receptors of prokaryotes
• Rodbell/Gilman - received Noble prize for work w/ G-proteins