26.1 Introduction 26.2 Carriers and channels form water soluble paths through the membrane 26.3 Ion channels are selective 26.4 Neurotransmitters control channel activity 26.5 G proteins may activate or inhibit target proteins 26.6 G proteins function by dissociation of the trimer 26.7 Growth factor receptors are protein kinases 26.8 Receptors are activated by dimerization 26.9 Receptor kinases activate signal transduction pathways