Complex sentence: Membrane or Transmembrane receptors are specialized integral membrane proteins to which extracellular signal molecules (usually hormones or cell recognition molecules) attach, triggering changes in the function of the cell.
Split into simpler sentences: Membrane or Transmembrane receptors are specialized integral membrane proteins that take part in communication between the the cell and the outside world. Extracellular signal molecules (usually hormones or cell recognition molecules) attach to the receptor, triggering changes in the function of the cell.