Lipid bilayers are increasingly impermeable to molecules as they signalling molecule. The most extreme form of short-distance signalling
increase in size or hydrophobicity. Transport mechanisms are therefore is contact-dependent (juxtacrine) signalling, where one cell responds to
required to carry essential polar molecules, including ions, nutrients, transmembrane proteins of an adjacent cell that bind to surface recep-
nucleotides and metabolites of various kinds, across the plasma mem- tors in the responding cell membrane. Contact-dependent signalling
brane and into or out of membrane-bound intracellular compartments. also includes cellular responses to integrins on the cell surface binding
Transport is facilitated by a variety of membrane transport proteins, to elements of the extracellular matrix. Juxtacrine signalling is impor-
each with specificity for a particular class of molecule, e.g. sugars. Trans- tant during development and in immune responses. These different
port proteins fall mainly into two major classes: channel proteins and types of intercellular signalling mechanism are illustrated in Figure 1.7.
carrier proteins. For further reading on cell signalling pathways, see Kierszenbaum and
Channel proteins form aqueous pores in the membrane, which open Tres (2012).