The network of membranous compartments that forms part of many of the cell’s internal organelles and other functional components. It includes the membranes of the nucleus, endoplasmic reticulum, Golgi apparatus (or dictyosomes), plasma membrane, tonoplast, and outer (but not inner) membranes of the mitochondria and chloroplasts, which form a functional continuum within the cell. New membrane is constantly being made in the endoplasmic reticulum and transported in the form of vesicles containing cargo (e.g. secretory proteins) along microtubules to their target organelles or to the plasma membrane.