Multicellularity requires communication between cells;
cells in multicellular organisms have a number of decisions to make:

Signalling routes:
Endocrine pathway

Signalling routes:
paracrine pathway

Signalling routes:
Neuronal pathway

Signalling routes:
contact-dependent

Examples of signalling factors

Signal transduction

2 things that can happen intracellularly in response to binding

Receptors

Receptors:
cell surface receptors

Receptors:
intracellular receptors

Major membrane receptor families

Receptors:
ion channels

Receptors:
G protein-coupled receptors
GPCRS

action of G-protein-coupled receptors

cAMP

Example of a G-protein coupled receptor pathway:
reaction of cells to adrenaline; the 'fight or flight' hormone

G-protein coupled receptor activation:
Receptor activation of protein kinase C (PKC)

Summary:
the 2 different G-protein coupled receptor pathways

Receptor kinase pathway