Types of cell signals:
1) ligand has local effects, short lived (paracrine signals) early development
2) ligand has distant effects, ligand is a hormone (endocrine signals) hormone travels in blodd, hormone travels in xyloem, phloem in plants.
3) synaptic signaling (nerve cell involved) nervous system
4) direct contact (cell right next to eachother)
If receptor is internal - it has to pass membrane (lipid soluble, estrogen, testosterone, vitamin D