What type of signaling molecules are classified as proteins that do not cross the lipid bilayer and act on cell surface receptors?

Cytokines are classified as proteins that do not cross the lipid bilayer and instead act on cell surface receptors. They are a specific type of signaling molecule that is integral to the immune response and cell communication. Being water-soluble proteins, cytokines bind to receptors on the surface of target cells, triggering a series of intracellular signaling cascades that lead to various physiological responses.

In contrast, hormones can be either lipid-soluble or water-soluble, with lipid-soluble hormones able to pass through the cell membrane and bind to intracellular receptors. Neurotransmitters generally act at synapses and can be small molecules or peptides, but many of them also readily bind to receptors on the surface of cells (neurons or muscle cells). Growth factors are a subset of cytokines that specifically induce cell growth, proliferation, and differentiation, but they also do not cross the lipid bilayer and utilize surface receptors for their action.

Therefore, in this context, cytokines are the best fit for the description provided, as they are epitomized by their interaction with cell surface receptors without crossing the cell membrane.