Cell membrane receptor proteins, illustration. Membrane-spanning surface receptors (multicoloured ribbons) are proteins embedded in a cell membrane (here a layer of phospholipids). The receptors bind to chemicals and other structures in the extracellular fluid, with examples being antibodies, neurotransmitters, viruses and drugs. In many cases this triggers biochemical reactions and effects inside the cell in its cytoplasm, with receptors playing a key role in a cell's interaction with the external environment.