The complexity of eukaryotes is massively multiplied with multicellularity. Most cells in a multicellular organism are in close, ongoing associations with their neighbors.
Tissues are cell masses joined by junctions, or by an extracellular matrix (ECM) of secreted glycosylated (carbohydrate-rich) proteins that create attachment bases for cells: holding tissue together without direct contact between neighboring cells.
Glycocalyx is a common glycoprotein ECM, produced by some bacteria as well as eukaryotic cells. Glycolcalyx forms a coating on the outside surface of a cell membrane. The slime on the outside of a fish is a glycolcalyx.
Glycocalyx plays various roles: cell recognition, cell adhesion, protection, and acting as a permeability barrier. Glycocalyx acts as one way for an organism to distinguish between its own healthy cells and those that are diseased, as well as assisting in recognizing invaders. Multicellular organisms expend tremendous time and energy building and maintaining molecular forests of glycocalyx.
ECM glycoproteins come in various shapes and sizes, but functionally they fall into 3 groups: transporters, ion channels, and receptors.
Transporters carry specific molecules, usually food, across cell membranes. Each type of molecule has its own jitney model.
Ion channels are gated communication pathways that form a selective signaling matrix. Communications across cell membranes are regulated by ion channels.
Receptors are the most diverse group of cell-surface glycoproteins. Each receptor is designed to respond to a specific signaling molecule. A signaling molecule binding to its receptor sets off a sequence of biochemical events which may regulate cell production, growth, or even death (apoptosis).
The various communications systems of multicellular organisms, such as the nervous and endocrine systems, are multifaceted matrices that use ECMs.