General Considerations of Membranes

Abstract

It has long been believed that a membrane separates the contents of cells from the extracellular environment. With the advent of electron microscopy, this membrane, called the plasma membrane, was for the first time visualized and in addition it became evident that the internal structure of cells also contained complex systems of membranes (1) which are components of what often are called organelles (e.g., mitochondria, endoplasmic reticulum, Golgi complex, lysosomes, etc.) (Fig.1). In recent years it has become possible to isolate in relatively pure form many of these membranous organelles from homogenates of tissues by means of differential and density gradient ultracentrifugation. From studies utilizing these isolated membranes the biochemical nature and the functions of many of these organelles have been established.